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Science with the X-ray Imaging and Spectroscopy Mission (XRISM)

This white paper was prepared by members of the XRISM Science Team for the benefit of the general astronomical community. This paper describes the capabilities of XRISM, offers a sampling of the many science topics that the mission will address, and discusses the synergies of XRISM with the plethora of planned and existing facilities in the 2020s and beyond.


Introduction
The X-ray Imaging and Spectroscopy Mission (XRISM), formerly known as the X-ray Astronomy Recovery Mission (XARM), is a JAXA-NASA collaborative mission with ESA participation. XRISM will offer non-dispersive, high-resolution X-ray spectroscopy in the soft X-ray bandpass (∼ 0.3 − 12 keV), while offering complementary CCD imaging resolution over a wide field of view. The primary purpose of the mission is to recover the science that was lost after the Astro-H/Hitomi mission failed in 2016, approximately one month after launch. Four science categories have been defined for XRISM: (1) Structure formation of the Universe and evolution of clusters of galaxies; (2) Circulation history of baryonic matter in the Universe; (3) Transport and circulation of energy in the Universe; (4) New science with unprecedented high-resolution X-ray spectroscopy. A full description of the mission is provided in an SPIE proceeding 1 ; the reader is referred there for more detail. XRISM represents a revolutionary leap forward in X-ray spectroscopy. With a spectral resolution 20-40 times better than the CCD instruments that are used on Chandra, XMM-Newton, and Suzaku, as well as a substantially increased collecting area and bandpass over the grating instruments on those missions, XRISM represents a new tool for studying the universe, and will enable science never before possible. XRISM will do this using two instruments: • Resolve, a soft X-ray spectrometer with a constant < 7 eV FWHM spectral resolution over the entire bandpass. Resolve has a field of view of 2.9 × 2.9 over an array consisting of a 6 × 6 pixel X-ray microcalorimeter (pixels are 30" in size). The array has an operating temperature of 50 mK, and must be cooled by a multi-stage adiabatic demagnetization refrigerator. This system can operate in both cryogen and cryogen-free modes, meaning that the mission can continue to operate after the exhaustion of the liquid helium cryogen supply. The mission requirements for Resolve are given in Figure 1, along with the measured values from the in-flight performance of Hitomi (all of which exceeded requirements).
• Xtend, a soft X-ray imager providing simultaneous coverage of the Resolve field and the surroundings over a 38 × 38 field of view. Xtend will provide CCD-quality imaging spectroscopy Science with XRISM similar to that available on Suzaku (energy resolution < 250 eV at 6 keV), with ∼ 1 spatial resolution.
In Figure 2, we show the Hitomi SXS instrument, mounted in the calorimeter spectrometer insert. The XRISM instrument is currently in production, but will be identical to this. The pixels in the detector measure the heat imparted by incoming X-ray photons (only a fraction of a degree, hence the need for the detector to be cooled to such a low temperature). From this temperature change, the energy of the incoming X-ray photon can be calculated. XRISM is particularly well-suited to the astrophysical analysis of extended objects, such as galaxy clusters and supernova remnants. Because the light is not dispersed, spatial information about these sources is not lost as it often is in observations with the gratings on Chandra or XMM-Newton. Resolve features an exceptionally low background of ∼ 1 count day −1 10 eV −1 . As an example of this, in the Hitomi spectrum of the Perseus Cluster shown in Figure 3, there are ∼ 21, 000 counts in the Fe Kα complex shown; of these, only 16 counts are due to the background.
XRISM is planned for launch in 2022 from Tanegashima Space Center, aboard a JAXA HII-A rocket to an orbital altitude of 550 km and an inclination of 31 degrees. Two identical X-ray Mirror Assemblies (one for Resolve and one for Xtend), each with 203 nested circular foils in both the primary and secondary mirrors, focus X-rays onto the focal plane detectors at the base of the spacecraft, 5.6 meters away. The mirrors have an angular resolution of order 1 .
In this white paper, we give a brief introduction to the science that will be enabled by XRISM. This work is by no means exhaustive. A more full description of the scientific topics presented here can be found in a collection of 17 white papers written in 2014, prior to the launch of Hitomi 1 . This white paper serves as a summary of those more detailed white papers, with the important Science with XRISM inclusion of science learned from Hitomi and a more timely description of XRISM in the landscape of astronomy in the 2020s.

GALAXY AND CLUSTER EVOLUTION
(Primary editors: Irina Zhuravleva, Erin Kara, Chris Done) One of the biggest questions in modern astrophysics and cosmology is how the largest structures in the Universe form and evolve with time. As a result of enormous efforts in observational and theoretical astrophysics, we now understand what processes are playing an important role in the evolution of galaxies, groups, and clusters. However, the physics of the involved processes and the dynamical and chemical structure of the largest objects and their environments are still not well understood.
Most of the visible matter in the Universe is in the form of hot, X-ray emitting gas within massive galaxies, groups, and clusters of galaxies as well as in the interstellar and intergalactic gas between them. X-ray observations have provided spectacular high-resolution images of these objects. These data revealed a wealth of structures associated with various complex physical processes that take place in these environments. With XRISM, we will be able to resolve individual spectral lines and trace gas motions through the Doppler broadening and line energy shifts in many galaxies and clusters of galaxies. We will robustly measure the chemical composition, multi-temperature structure, and ionization state of the hot gas. Right: spectrum from the Perseus core (white square in the left panel) observed with Hitomi's microcalorimeter (black) and Suzaku's CCD imaging spectrometer (red). XRISM will provide similar high-resolution spectra in the 0.3 − 12 keV band for extended X-ray sources 3 .
In this chapter, we briefly review the key XRISM science related to the evolution of galaxies and clusters, physics of active galactic nuclei (AGN) and AGN feedback as well as the chemical composition and evolution of large-scale structure. A detailed description and simulations of astrophysical sources can be found in the Astro-H white paper 4 .

Dynamics of the most energetic events in the Universe
Clusters of galaxies -the largest and most massive structures in the Universe -are formed hierarchically via a sequence of mergers and accretion of surrounding matter, including smaller clusters and galaxy groups. These processes drive shocks in the intracluster medium (ICM), trigger bulk and turbulent motions of the gas, and channel energy into magnetic fields and relativistic particles 5;6 . Science with XRISM 2.1 Dynamics of the most energetic events in the Universe On smaller scales, an interaction between the energy generated by the accretion of matter onto a supermassive black hole (SMBH) and gas in the host galaxy (AGN feedback) takes place. Even a small fraction of the energy produced in the vicinity of a black hole and transferred to the gas has a profound effect on the host galaxy and cluster evolution 7;6;8 . XRISM will probe the kinematics of such complex and energetic processes for the first time, transforming our "static," single-frame view of these processes into a full dynamic picture.
Dynamics of violent mergers. Mergers of galaxy clusters are the most energetic events in the Universe since the Big Bang, with the total kinetic energy of colliding clusters reaching 10 65 ergs 9 . The hot intracluster gas that fills the gravitational potential wells of clusters is strongly affected by these collisions and can be traced with X-ray observations. Strong shocks, bulk motions of the gas, and turbulence dissipate energy from collisions, heating the gas to very high temperatures. Shocks have been robustly detected in a handful of clusters using high-resolution X-ray images 10;11;12;13;14;15;16 . However, the dominant energy conversion mechanism -via bulk motions of the gas and turbulence -has never been measured. Resolve will provide such measurements in many mergers at different stages thanks to its superb spectral resolution. Combining this velocity information with other existing data (X-ray, optical, sub-mm, radio) and numerical simulations, we will be able to reconstruct the geometries and dynamical stages of merging systems. Furthermore, the large field of view of Xtend (a factor of ∼ 2.2 larger than Chandra/ACIS-I) will provide complementary measurements of radial profiles of X-ray surface brightness, gas density, and temperature up to large radii.
Mergers are often accompanied by "cold fronts" (e.g., A2319, A3667, A754) that appear as sharp contact discontinuities in the X-ray surface brightness. Cold fronts arise either from subcluster stripping by ram pressure or from "sloshing" of the low-entropy gas in a disturbed gravitational potential well 17;18;10;19;20;21;22;23 . In the case of gas stripping, bulk motions of the subclusters are mostly in the plane of the sky, while in the sloshing case, there is a near-sonic flow along the line of sight inside the front. Resolve will measure both bulk and turbulent motions of the gas in such cold fronts, providing a solid basis for probing the microphysics of the ICM using cold fronts.
Many merging clusters host giant diffuse radio halos filled with relativistic particles. These particles emit synchrotron radiation in the cluster magnetic field 24 , rapidly losing their energy and mixing with the thermal plasma on length-scales well below the sizes of radio halos. Therefore, in order to explain the observed extent of these halos, particles have to be constantly accelerated in situ. One way to accelerate particles is by scattering on magnetosonic waves produced by turbulence 25 . Resolve observations will test the theories of cosmic-ray acceleration by tracing turbulence in regions within clusters that have strong radio emission.
Precise mass measurements and cluster cosmology. Knowing masses of galaxy clusters at different redshifts, we can trace the growth of structures over time and constrain cosmological models. This is usually done through measurements of cluster mass function, which decreases exponentially at the high-mass end. This exponential "tail" is very sensitive to the dark energy density parameter and provides a way to infer the role of dark energy in cosmic expansion 26;27 .
The precision of cluster cosmology depends on the accuracy of the mass measurements. One of the most important ways of measuring cluster masses is based on the assumption that the thermal pressure alone supports the hot gas against the cluster's gravity (hydrostatic equilibrium). However, numerical simulations show that the macroscopic motions of the gas may provide significant nonthermal pressure support, up to 25-30 % at r 200 28;29;30;31;32;33 . Hitomi observations of the Perseus core, which is dominated by the central massive galaxy NGC 1275, showed that the kinetic pressure is less than 10% of the thermal pressure on scaless of 60 kpc 3 . XRISM will continue Hitomi's legacy by measuring velocities of gas motions and the non-thermal pressure contribution with an accuracy of a few percent in many relaxed, bright galaxy clusters (Fig. 4). Velocity measurements up to Science with XRISM 2.2 The Physics of Black Holes and Active Galactic Nuclei ∼r 2500 , for even a small sample of nearby relaxed clusters, will significantly improve the constraints on dark energy and other cosmological parameters. Newton image of A2199 and 3 × 3 XRISM pointing regions (green boxes) up to r2500 (dashed circle). Middle: the input (solid line) and recovered (points) velocity dispersion in A2199. Black points: simultaneous fitting of the spectra from all annuli; red crosses: separately fitting the spectrum in each annulus. Errors are at 90% confidence. Adapted from 4 . Right: Measurements of gas velocity dispersion as a function of radius based on the mock data of a relaxed galaxy cluster from cosmological simulations. Curves: true velocity dispersion from simulations; points: recovered dispersion from the mock Resolve spectra. Adapted from 34 .
Physics of cool cores and mechanical AGN feedback. SMBHs residing in galaxy clusters 35 are preventing strong cooling and vigorous star formation in their host galaxies through feedback processes 36;37;7 . These processes include the inflation and rise of bubbles of relativistic particles, generation of shocks and sound waves, uplift of cooler, multi-phase gas, and turbulence and other motions of the gas 38;39;40;41;42;43;44;45;46;47;48 .
Resolve will perform comprehensive velocity diagnostics in the regions dominated by AGN feedback physics in the brightest cool-core clusters. Such measurements will address the role of turbulence in cooling-heating balance and metal mixing. In nearby objects like the Virgo cluster ( Figure 5), Resolve will measure the kinetic energy associated with AGN-injected bubbles and uplifted gas filaments.

The Physics of Black Holes and Active Galactic Nuclei
XRISM will revolutionize how we understand accreting SMBHs, starting from how cold gas flows in from the interstellar medium (ISM) of the galaxy, down to the inner accretion flow where matter is affected by the strong gravity of the black hole. This gravitational inflow leads to massive outflows, which may be powerful enough to prevent gas from cooling to form stars, thus affecting the evolution of the host galaxy. X-ray continuum emission is produced within a few gravitational radii of the central black hole and irradiates the remaining infalling or outflowing gas (e.g., the accretion disk, broad-line region clouds or the dusty torus), producing a plethora of emission features, known colloquially as the reflection spectrum. The Fe Kα line is the most commonly used diagnostic of the structure around the AGN because iron is an abundant element and this line grouping appears in a relatively isolated region of the spectrum. XRISM will provide an unprecedented view of this important spectral feature. The combination of improved sensitivity and better spectral resolution will enable XRISM results to be extended to large samples of objects.
Black hole spin through observations of broad, ionized reflection: Japan's ASCA satellite opened a new field in X-ray astronomy with the discovery of the broad Fe Kα line in MCG−6-30-15 50 . The implication of the broad line is immense, as it may provide a way of measuring an elusive fundamental parameter of a black hole: its spin 51 . As such, the broad Fe Kα line has ignited healthy debate and continued interest 52 . Rapid time variability, X-ray reverberation mapping 53;54 , and quasar microlensing 55 indicate that the broad Fe Kα line is produced by reprocessing off the inner accretion disk within ∼ 10R g , but the details of the inner disk radius and kinematics of the flow are still highly debated. By disentangling contributions from other spectral features (including narrow emission from the outer disk and/or torus, or absorption though ionized outflows along our line of sight), we can confidently probe properties of the inner accretion flow to make a clean estimate of the black hole spin. XRISM will give us a clear understanding of how much material is actually making it to the black hole and how much material is being removed through massive outflows (see the case study of MCG−6-30-15 in Figure 6). Furthermore, the combined use of Resolve and Xtend will allow for complementary measurements of the short light-travel time delays associated with the relativistically broadened Fe Kα emission line (known as X-ray reverberation).
Outer disk/torus structure through observations of narrow, neutral reflection: A narrow Fe line at 6.4 keV is nearly ubiquitous in AGN spectra. It is due to neutral Fe Kα fluorescence off circumnuclear optically-thick material, but its physical association with, for example, the dusty torus that may serve as a gas reservoir to the disk, or the outer edge of the accretion disk, is still not fully understood. Hitomi observed 1 AGN, the low-luminosity AGN NGC 1275 at the center of the Perseus Cluster, and found a narrow Fe Kα line, with a 500-1600 km s −1 width that suggests it is associated with a molecular torus extending tens to hundreds of parsecs 56 . XRISM will extend these studies to a large sample of AGN, spanning a range of mass accretion rates. This is important, as Seyfert galaxies observed with Chandra High-Energy Transmission Grating (HETG) suggest higher-velocity structure in the Fe Kα line, placing the Fe K emitting region within the optical broad line region 57;58 . Moreover, recent advances using 3rd-order spectra from Chandra HETG find that the narrow Fe Kα line in one Seyfert 1 galaxy NGC 4151 is, in Science with XRISM The ratio of the XMM pn spectrum to a powerlaw model fit between 2-4 keV and 7.5-10 keV. The pn spectrum can be equally well described by pure relativistic reflection with an inclination of 5 deg (as shown in blue) or by relativistic reflection with an inclination of 75 deg with additional emission and absorption, including an ultrafast outflow of vout ∼ 0.13c (as shown in red). Right: XRISM will easily distinguish between these two scenarios, as the narrow emission and absorption will be resolved.

Chemical Composition and Evolution in the Universe
fact, asymmetrically broadened, suggestive of Doppler shifts and weak gravitational effects 59 . This allows for determination of both the radius and inclination of the narrow Fe Kα emitting region. In this one tantalizing case, the outer and inner disk inclinations do not match, which might suggest a change in the vertical scale height of the disk at large radii due to radiation pressure. Interestingly, similar interpretations of changes in vertical disk structure have also been suggested 60 to explain intensive disk reverberation mapping of NGC 4151 61 .

Chemical Composition and Evolution in the Universe
The chemical make-up of the universe represents the sum of billions of past supernovae. In contrast to stellar systems, H II regions, or planetary nebulae, gas in the ICM is largely free of the effects of optical depth, extinction, and non-equilibrium ionization. In such a physical environment, robust measurements of chemical enrichment can be done with the high-resolution spectra from Resolve, providing valuable insights into the production of heavy elements by early stellar populations.
Chemical composition of bright nearby galaxies and galaxy clusters. All elements between C and Zn show emission lines between 0.1 and 10 keV in spectra of galaxy clusters. Resolve will measure the abundances of the α-elements and Fe with remarkable accuracy of a few percent in nearby cool-core clusters. These measurements alone already provide tight constraints on the mechanisms that produce these chemical elements 62;63;64;65 . With sufficiently long exposures, Resolve will detect weak lines from rare elements in the ICM, such as Na, Al, Cr, and Mn ( Figure 7). Al and Na are sensitive tracers of the metallicity of the underlying stellar population, while Cr and Mn can be used to probe the characteristics (including metallicity) of SNIa progenitors, resulting in a wealth of information about the chemical enrichment history of the ICM and the Universe.
Outside the regions dominated by the central brightest cluster galaxy, metallicity measurements will be important for constraining the chemical evolution driven by the remaining cluster member galaxies, which may have witnessed a different star formation history over cosmic time. By measuring metal abundances in galaxies and groups of galaxies, XRISM will further investigate the mass dependence of the chemical enrichment history 4 .
Hitomi observations of the Perseus core confirmed the power of high-resolution spectroscopy: the Cr/Fe, Mn/Fe and Ni/Fe ratios were measured accurately for the first time ( Figure 7). This  measurement revealed that these elements have a near-solar abundance ratio. Comparison with the nucleosynthesis calculations disfavoured the scenario of the SNIa progenitors being exclusively white dwarfs with masses below the Chandrasekhar limit 66 . Combining Hitomi measurements with XMM-Newton RGS grating spectra at lower energies yielded reliable measurements of the abundances of O, Ne, Mg, Si, S, Ar, Ca, Cr, Mn, Fe, and Ni, which were used to further test existing SN nucleosynthesis calculations. These results showed, in particular, that core-collapse supernova (CCSN) yield calculations with included neutrino physics improve the agreement with the observed pattern of α-elements 67 . Chemical evolution in high-redshift Universe. Tracing the star formation at different redshifts is directly connected to the chemical evolution of the Universe, so measuring abundances of different elements at high redshifts is crucial. Based on models of galaxy formation, the relative population of massive stars, and consequently the rate of CCSN, is thought to increase in the high-redshift Universe. The ratio of abundances of Fe and light alpha-elements like O, Mg, and Si can check this prediction. Current measurements do not show any clear evidences of the Fe/Mg evolution 68 , however, the observed scatter at high redshift is large.
It will be possible to observe high redshift clusters with XRISM with sufficiently long exposures (≥ 100 ks). Feasibility studies showed that the Fe abundance will be measured with a 5σ significance with a 100 ks exposure out to z = 1 clusters. Si abundance can be probed up to z = 0.6, while O only feasible for a few systems up to z = 0.3 − 0.4 4 .
Another way to probe high-z chemical composition is to study absorption lines imprinted by the circumgalactic and intergalactic media in the spectra of bright background beacons such as the X-ray afterglows of gamma-ray bursts (GRBs) and distant blazars. XRISM will resolve these absorption features, allowing a study of the intervening warm-hot intergalactic medium (WHIM). XRISM will also detect emission lines in GRB spectra, which are important for probing the high-z

Dark Matter Emission Lines
Using stacked spectra from dozens of galaxy clusters at different redshifts, both XMM-newton and Chandra detected a weak unidentified emission line at around 3.55 ±0.03 keV (cluster frame) 70 . A similar feature was found in XMM-Newton spectra of M31 and the Perseus cluster 71 , and several later works reported detections from the Milky Way. While the nature of this feature is not known, an intriguing possibility is that it could be the decay line from sterile neutrinos -a dark matter candidate -with a mass of 7.1 keV. All these results relied on the low spectral resolution of CCD spectrometers. The 70 result found a much stronger signal from the Perseus cluster compared to that from the stacked cluster sample. In 2016, Hitomi observations of the Perseus cluster did not detect the line at the high flux reported by 70 , though the statistics were limited 72 . While faint line detections in any single source using a relatively poor CCD resolution are susceptible to systematic and calibration uncertainties (and the Perseus cluster detection may have been an example of that), the stacked-cluster signal 70 is more robust with respect to the various instrumental effects. It has not been verified with high spectral resolution thus far. Combining all cluster observations with XRISM over several years will offer the chance to detect this possible signal from dark matter decay.

Missing Baryons and the WHIM
Current observations of hot gas in galaxy clusters, the Local Group, and the warm intergalactic medium (IGM) indicate that 20-40% of the baryons seen in the high-z Universe are ?missing? from our observations at z=0 73;74 . The missing baryons are suspected to occupy the filaments of gas between galaxies at temperatures > 10 6 K: also known as the WHIM. The dominant ions with emission and absorption features in the X-ray band are O VII, O VIII, and Ne IX. Measurements of the O VII resonant absorption line at 21.6Å arising from the WHIM have been reported, but all to < 5σ threshold. Additionally, there is confusion as to whether the absorption seen so far corresponds to the IGM or to material within the Local Group 73 . Recent work gave evidence for O VII absorption in the bright X-ray blazar 1ES 1553+113, from z=0.43 and 0.35, offering a tentative detection for the hot WHIM that accounts for 9-40% of the baryons 75 .
XRISM will study potential emission of the WHIM from the outskirts of more extended cluster regions (e.g., at distances more than two times the virial radius) in objects such as the Coma cluster 76 , Abell 85 77 , and Abell 2218 78;79 . XRISM will also measure the baryon fraction in gaspoor groups, big ellipticals or massive spirals, making it better able to characterize the thermal state and metal abundance of gas emission from lower mass structures in the Universe.

Absorption in the ISM
While only about 1% of the ISM is condensed into solid dust particles, dust imprints unique extinction features in the spectra of background X-ray sources. The extinction features of Si, Mg, O, and Fe have been studied in detail with current X-ray missions 80 of interstellar dust. Through high-resolution X-ray spectroscopy, it has become clear that the X-ray band offers a privileged point of view on the properties of interstellar dust.
The features caused by dust are due to the interaction between the incoming X-ray and the electrons inside the grain, which form an interference pattern, depending on the energy of electrons and the complexity of the compound. These features are not only sensitive to the chemical composition of the intervening dust, but potentially to the grain size 86 and its crystallinity 80;81 .
Due to UV radiation and cosmic rays, dust is believed to undergo a process of amorphization. According to infrared studies, no more than 2% of dust should survive in crystalline form. However, X-ray features are sensitive to short-range order in the grain, while the infrared features map long range order 87 . This allows X-rays to be more sensitive to the crystalline fraction in a complex structure, where amorphous and crystalline lattice coexist. Recent X-ray studies suggest that the crystalline fraction could be up to 12% 81 . The study of a single feature in the X-ray spectrum can reveal details about dust in different Galactic environments that are elusive at longer wavelengths.
XRISM will open a new parameter space with the investigation of the dense environments of the Galaxy (N H = 1 − 10 × 10 22 cm −2 ) near the Galactic Center or molecular clouds. Figure 8 shows a 50 ks simulation of the spectrum in the Si K edge region (E ∼ 1.84 keV), highlighting the impact of different grain composition and distributions. XRISM offers also the advantage of a negligible contamination by silicon in the detector itself. With XRISM we will be able to compare the spectral features of interstellar dust -especially for heavier elements such as Ca, S, and Fe -to material absorption measured in the lab. In addition, dust scattering produces a feature on the low-energy side of the absorption cross-section that results in an apparent spectral peak. The height of this peak directly measures the abundance of large grains (> 0.3 µm) and will allow XRISM to evaluate departures from the standard dust size distribution 88;89 .
In Figure 8 we also show the simulation of the Fe K edge at 7.1 keV 90 . The Fe K edge will be detected only for environments with N H > 8×10 22 cm −2 . Persistently bright sources with a high column density are not frequent in our Galaxy, so the limiting factor in an accurate study of the feature will be the signal to noise ratio 91 . XRISM will be able to distinguish between gas and dust absorption, offering a measure of depletion and abundance of iron in regions that are too absorbed to be observed at other wavelengths.
Large column densities will increase the opacities of less abundant elements, like S and Ca 92 . In every ISM environment, Ca is completely depleted. It is believed that Ca, together with Al, is at the core of silicate grains. The silicate themselves would create a shielding mantle against Science with XRISM 3.3 Dust Scattering and Haloes radiation in the ISM, keeping the Ca and Al in dust form in every environment. XRISM will be able to identify absorption by silicates containing Ca and Al even if absorption by this element produces a transmission of only 1-2%. Small angle forward scattering of X-ray light by interstellar dust grains leads to the phenomenon of dust scattering halos, an extended X-ray image spanning 10-20 arcminutes from a bright central source 93;94;95 . When a compact object exhibits a bright X-ray outburst, this phenomenon manifests as bright rings of X-ray light that grow in angular size with time, referred to as dust ring echoes. The time and spectral information available with dust ring echoes allows for a deconvolution of the intervening dust location relative to the point source, the light curve of the outburst, and the dust grain size distribution 96;97 .

Dust Scattering and Haloes
Like the high-resolution absorption features discussed previously, the scattering cross-section will also exhibit highresolution signatures of dust composition. The spectrum of a dust scattering halo is also sensitive to the composition and elements bound up in dust grains of radius ∼ 0.3 − 3 µm, which cannot be probed at any other wavelength. This may be related to the "missing oxygen problem," which arises from the fact that along the most highly depleted sight-lines, more oxygen is observed to be missing from the gas phase than can be explained by its incorporation into silicates 98 . This could be explained if oxygen is bound up in large micron-scale ice grains, which also prevents them from being detected via infrared spectroscopy. Due to shielding, large dust grains are also hidden from X-ray absorption 99 . Thus, high-resolution spectra of X-ray scattering halos in combination with high-resolution absorption features from the spectrum of the central point source are necessary for a full picture of interstellar dust composition ( Figure 9).
Resolve offers the opportunity to study the properties of dust in individual clouds in the ISM. This arises from the fact that dust scattering halos and ring echo images exhibit asymmetries caused by either enhanced scattering from distant clouds 100 or enhanced absorption from foreground clouds 96 . The Xtend instrument can be used to track the evolution of dust ring echoes, providing essential timing information that will identify the location of the interstellar clouds. XRISM will be an essential observatory for tracking processes of dust destruction and growth across various phases of the ISM, and will provide complementary datasets to the infrared spectra obtained by JWST. One of the most interesting phenomena in the center of the Milky Way is the Galactic Center X-ray Emission (GCXE). The GCXE consists of diffuse plasma with a spectrum characterized by strong atomic lines (Si to Ni, especially Fe). Only 10-40% of the GCXE is resolved in point sources even by longexposure Chandra observations 102;103;104 . Recently, Suzaku 105 revealed that the spectrum of the GCXE cannot be reconstructed by the assembly of wellknown point sources (magnetic or nonmagnetic cataclysmic variable and active binaries). In addition, Chandra 104 shows the equivalent widths of Fe K lines of the integrated spectrum of point sources in the GC region, and they are different from those of the GCXE shown by Suzaku 106;105 . These facts indicate that the majority of the GCXE might come from truly diffuse hot plasma.

The Galactic Center
With XRISM, we can measure the Doppler shift of and line broadening of the Fe XXV K line in the GCXE with accuracies of 45 km s −1 and 300 km s −1 , respectively. If the majority of the GCXE is diffuse plasma of kT ∼ 7 keV (T ∼ 8 × 10 7 K), then it is not gravitationally bound to the GC region, and thus the rotation curve of the GCXE can be different from that in the radio band. In the case of a point source origin, narrow line broadening is expected, in contrast to expanding diffuse plasma 101 (see Figure 10).
The non-thermal component characterized by the Fe I Kα line is thought to be an X-ray echo from molecular clouds (MCs) irradiated by past X-rays from Sgr A* 107 . The detection of the Compton shoulder of the Fe I Kα line by XRISM will constrain the positions and physical properties of the irradiated MCs 108 . On the other hand, Suzaku 109;105 found that some fraction of Fe I Kα in the Galactic disk region is not from the integration of faint point sources, but from low-energy (keV-MeV) cosmic rays (LECRs). It indicates that an LECR component would also exist in the GCXE. The high-resolution spectra of the GCXE might be able to resolve the unknown LECR components from the X-ray echo components and measure the amount in the GC region.

Charge Exchange in the Solar System and Beyond
With the first observations of comet Hyakutake with ROSAT in March 1996, astronomers were surprised to discover that the comet was 100 times brighter in X-rays than predicted 110 . It is now known 111 that the X-rays were produced primarily by charge exchange (CX) collisions between hot plasma ions from the solar wind and cold neutral atoms in the cometary atmosphere. This process is also observed when the solar wind interacts with neutral gas in the heliosphere 112 and planetary atmospheres 113;114 . CX can occur in any region in which a hot plasma interacts with a cold neutral gas, making it a useful diagnostic tool for studying astrophysical plasmas and the ISM. Heliospheric and exospheric foreground CX emission also poses an ?astrophysical nuisance?, as it contaminates all X-ray observations 115 . Foreground CX emission particularly affects diagnostics of the X-ray emission from the local bubble 116 . CX is also expected to contribute to the X-Science with XRISM ray spectra from astrophysical sources outside of our solar system, such as in galaxy clusters 117 , supernova remnants 118 , and star forming galaxies 119;120 . CX emission will be discussed further in Section 6.
XRISM will readily discriminate CX emission mechanisms from other processes, and provide better constraints for CX within our own solar system. Observations of cometary spectra will help constrain the neutral density in a cometary coma and the solar wind ion abundance, and observations of the Mars and Jovian atmospheres will place constraints on the importance of CX in these atmospheres, helping us understand the relative significance of other processes producing X-rays. The ability to measure the widths and positions of emission lines of multiple elements at high precision will allow XRISM to perform unique diagnostics of the optically-thin plasma in a multitude of astrophysical systems. Below we discuss several avenues for such studies, which fall into two broad classes -the dynamics based on line Doppler shifts and broadening, and the relative line fluxes. While Resolve is spending long exposures performing spectroscopic studies of the cluster bright central regions, Xtend, with its wide field of view and low instrument background, will obtain high-statistics maps of the plasma density, temperature, and abundances of main elements at much greater off-center distances, providing information on large-scale cluster dynamics that drive many of those spectroscopic phenomena.

Velocity Field of Intracluster Plasma
The hot gas in galaxy clusters is continuously stirred by mergers, feedback from AGN, motions of galaxies, magneto-thermal and plasma instabilities, etc. All these processes generate bulk and turbulent motions of the gas in the ICM. Turbulent motions cascade down towards dissipation scales, amplifying and tangling the ICM magnetic field 121;122 and accelerating cosmic rays 123;124;125 through the second-order Fermi process. Using high-resolution XRISM spectra, we will map the gas velocity field in galaxies, clusters, and other extended X-ray sources with the accuracy of < 100 km s −1 , bringing a new dimension in the study of astrophysical plasmas.
Resonance Scattering in Strong Emission Lines The same data used for the Doppler broadening measurements will provide complementary velocity constraints through the resonant scattering effect 126 . Resonant scattering reduces fluxes in some strong resonant lines in the cores of galaxies and clusters with respect to the fluxes in an optically-thin plasma. The degree of the suppression depends mainly on the amplitude of turbulent motions of the gas -the stronger the motion, the smaller the effect. Therefore, the suppression of lines can be used as a powerful independent probe of small-scale motions 127;128 . Such measurements have been successfully done in the innermost regions of galaxies and groups using high-resolution XMM-Newton RGS data 129;45;130 as well as Hitomi data 131 on Perseus ( Figure 11 Fe XXIII (E = 1.129 keV), Fe XXIV (1.168 keV) and Fe XXV (6.7 keV). In cool systems such as giant elliptical galaxies and groups (T ∼ 1 keV), the resonant scattering effect could be even stronger since the optical depth of the strongest Fe XVII line (0.87 keV) could be up to ∼ 8 − 10 ( Figure 11).
Power spectrum of turbulence For many astrophysical applications, it is important to understand spatial scales associated with the measured velocities of gas motions as well as statistical properties of turbulence (e.g., power spectrum or structure function). For instance, the turbulent heating rate, which is important for understanding energy transport in the ICM, is ∝ v 3 /l, where v is velocity associated with turbulent motions on a scale l. The slope of the power spectrum within inertial range tells us about the intrinsic properties of turbulence (e.g., hydrodynamic vs. magnetohydrodynamic, isotropic vs. anisotropic) and transport processes in the intracluster plasma 133 .
Measuring the power spectrum of turbulence from the observed characteristics (line widths and shifts) is not a trivial matter. Some ideas tailored for galaxy clusters have been proposed 134;23 . One of them utilizes a steep decline of the X-ray surface brightness with radius observed in cool-core clusters. In such clusters, the measured line broadening as a function of projected distance from the center would effectively probe velocities on different spatial scales (Figure 12, left panel). For clusters with flat surface brightness profiles, such as Coma, mapping of the projected velocity field provides the most direct way of obtaining the velocity power spectrum 134 . 23 showed that even with a sparse coverage of the Coma cluster, XRISM will be able to determine the injection scale of the turbulence (Figure 12). Although it is likely that these measurements will be done in a small sample of clusters, they will significantly advance the field and calibrate the density fluctuations -velocity relation that is currently used as an indirect probe for the turbulence power spectrum 135;136 .
Science with XRISM 4.2 Shocks in Clusters and Elsewhere

Shocks in Clusters and Elsewhere
Shock fronts are ubiquitous on all scales across the Universe, from the solar wind to supernova remnants (SNRs) to the intergalactic plasma. They convert kinetic energy into thermal energy of the medium in which they propagate and accelerate cosmic ray particles via the Fermi mechanism. The physics of shocks in magnetized astrophysical plasmas is uncertain and complex. XRISM will uncover interesting aspects of the shock physics in clusters and SNR. For clusters with large shock fronts, such as A754 and Coma, XRISM's angular resolution is sufficient to isolate the post-shock region and derive its spectrum. XRISM will characterize the small-scale turbulence that could be driven by the shock and look for transient deviations from thermal and ionization equilibrium caused by the shock passage, probing the energy conversion mechanisms and the various equilibration timescales in the plasma.

Plasma Diagnostics
While our usual approach of analyzing an X-ray spectrum is brute-force fitting of the whole energy interval with a physical model that predicts all the lines and continuum, in many cases the quantities of interest can be separated and determined independently using certain emission lines sensitive to those quantities. This can be particularly helpful when deviations from thermal and ionization equilibrium are possible or when the model includes nonthermal emission. XRISM will provide us with these new plasma diagnostics.
When a plasma is in thermal equilibrium, all particle species (electrons, protons, and heavier ions) must have the same temperature, which means that ions of different atomic mass should have different thermal velocities. On the other hand, if the plasma has any bulk streams or turbulence, all ions participate in those motions at the same velocity. By measuring the line widths for different elements, we can disentangle the two broadening mechanisms and thus determine the ion temper-4.3 Plasma Diagnostics ature (Kitayama et al. 2014). Various interesting processes can result in temporary electron-ion nonequilibrium, such as a shock passage and waves in the plasma. An attempt has been made to determine T i in Perseus 137 , which showed T i = T e but with a large uncertainty. XRISM will make such measurements possible in many clusters and other astrophysical systems.  In a plasma that has been rapidly heated (e.g., by a shock), different ions take different amounts of time to arrive at the ionization state determined by the electron temperature, and for a short period, the plasma can be out of ionization equilibrium 139 . This effect, common in SNRs, manifests itself as an anomalous ratio of the abundances of different ions of the same element with respect to that expected for the measured T e under the equilibrium assumption -it would correspond to a different "ionization temperature." In clusters, the ionization equilibrium timescales are generally very short, so the detection of any such deviation can indicate a very recent heating event. For a system that is a mixture of plasmas at different temperatures (in 3D or in projection), lines from different elements "highlight" different temperature phases.
The relative population of different excitation states for the same ion reflects the temperature of free electrons in the plasma. Thus, the ratio of lines emitted by the same ion can give an independent measure of T e , complementary to that determined from the shape of the X-ray continuum. This is especially useful when there is a mixture of plasmas at different temperatures along the line of sight -as is often the case for extended sources. The Hitomi Perseus observation has been used to determine the ionization temperature and T e from the line ratios (the "excitation temperature") using several line pairs (Figure 13, 138 ). The excitation and ionization temperatures are consistent with a mixture of temperatures in a relatively narrow range. XRISM will make these temperature diagnostics available for many clusters.
The thermal plasma in galaxy clusters is permeated with magnetic fields and ultrarelativistic (γ ∼ 10 4 ) electrons, which reveal themselves via radio synchrotron radiation 140 . While these components contribute little to the ICM pressure, they can alter its microscopic properties quite significantly. The radio synchrotron measurements cannot disentangle the magnetic pressure from the density of the relativistic electrons, but Inverse Compton (IC) emission from the same electrons -upscattering of the Cosmic Microwave Background photons into the X-ray band -can 141 . The X-ray IC spectrum should be a power law that can be deduced from the radio synchrotron emission. Searches of such power-law emission from clusters have been numerous but unsuccessful, because its surface brightness is far lower than the thermal emission and because the ICM is multitemperature, so any "hard tail" in the residuals of a fit of the continuum by a thermal model can be interpreted as another thermal component 142 . The latter complication can be alleviated by deriving the temperature structure of the plasma from the spectral lines (see above) rather than Science with XRISM the continuum fits, because a hot thermal component that is needed to explain a hard continuum excess must also produce emission lines characteristic of that temperature.

Winds from Compact Galactic Sources
Winds are ubiquitous from accretion disks, and are detected in black hole and accreting neutron star binaries with low magnetic fields 143;144;145;146;147;148;149;150;151;152 , and some pulsars 153;154;155 . They are also detected in accreting white dwarf systems, predominantly in UV and EUV spectra 156;157;158;159;160 . Disk winds and outflows are more widely studied in AGN, where such flows can potentially control the star formation-powered growth of the host galaxy. However, understanding winds and jets from accretion onto stellar-mass compact objects can provide an independent angle on the nature of winds and jets from massive black holes 161 .
Indeed, stellar-mass compact objects offer some unique opportunities. Whereas an AGN lifetime may typically be ∼ 10 7 years 162 , mass accretion rates can change by orders of magnitude over just weeks and months in stellar-mass compact objects 163;164 . Moreover, although the kinetic power of winds from stellar-mass compact objects can be modest, their role and impact may be anything but. They may be a manifestation of the processes that drive mass and angular momentum transfer within the accretion disk, enabling observational tests of analytical and numerical accretion disk theory 165;166 . In some cases, winds (and nova episodes in white dwarf binaries) may expel more gas than the system is able to accrete, altering the evolution of the binary system and compact object 148;149 . High-resolution X-ray spectroscopy is the key to learning how these winds are launched (which also determines how much energy, momentum and mass they carry), and how they connect to the accretion flow and its relativistic radio jet 167;168;169;170 .
Disk-dominated states Winds are seen when the accretion flow is dominated by an accretion disk: the high/soft state in black hole X-ray binaries, the "banana branch" in typical neutron star X-ray binaries, and outbursting or dwarf nova white dwarf systems 144;147;148;150 . Winds largely disappear when the source makes a transition to spectrally harder states and/or lower Eddington fractions. Typically, a radio jet appears as the source makes this transition 171 . This general anticorrelation of the jet and the wind has led to some speculation that these flows are fundamentally part of the same processes, with the state change leading to a re-organization of the magnetic field from powering the jet to powering the wind 172;173 . Alternatively, the response of the wind could instead be linked to the change in the illuminating spectrum at the state transition, with overionization leading to a lack of atomic transitions. Indeed, these factors are not mutually exclusive and may act concurrently.
Non-magnetic wind models are either radiation-driven (for L ≥ L Edd ), thermally driven, or UV line driven. The disks around white dwarfs are bright in UV, and the corresponding winds have a modest ionization; UV line driving is likely the dominant process by which winds are expelled from these disks. In contrast, black hole X-ray binary and neutron star X-ray binary disks are too hot for UV line driving to be important, and the relative importance of thermal and magnetic processes in driving winds in these systems remains unclear. The ability of high-resolution spectroscopy to accurately measure velocities, to separate distinct wind zones, and to measure launching radii will be crucial to resolving their nature.
The excellent spectral resolution of Resolve will make it possible to separate complexes of lines, and to examine the velocity profile of specific strong lines. Density-sensitive lines from intermediate  charge states 157;145;174;175 will be accessible to Resolve, by virtue of its broad pass band. When the density of a wind component is measured directly, the absorption radius can be derived via the photoionization parameter. Independently, velocity broadening of the emission component in accretion disk P Cygni profiles can be used to measure wind production radii 165;166 (see Figure 14).
The relative importance of emission and absorption features can determine the geometry of disk winds, and trace the scale height of winds as a function of Eddington fraction. Narrow emission lines can potentially reveal the scale height of the outer disk, the extent to which it is flared, and how such structure and ionization may relate to sharp flux dips 176 . Line broadening can reveal the characteristic radius, while line flux is related to the fraction of the sky that is subtended by the outer disk relative to the source.
Non-thermal X-ray states There is a major spectral transition seen in all types of disk accreting systems, where the disk emission drops dramatically and the non-thermal X-ray emission increases. In black hole X-ray binaries this is called the "low/hard" state; in neutron stars, it is the "island" or "extreme island" state; in dwarf novae, an equivalent state is called quiescence (note, however, in this case the X-rays are optically-thin thermal emission, not non-thermal). In all systems, this transition correlates with stronger radio emission from the jet 171;163 . There is still considerable debate concerning the origin and nature of this transition, but one model is that the inner, geometrically-thin, optically-thick accretion disk transitions into a hot, geometrically-thick, optically-thin flow 177;178 . Some theoretical treatments suggest that an optically-thin accretion flow with a large scale height is more efficient at producing a jet because poloidal fields are easier to maintain in such a flow 179;180 .
Exactly why winds appear to be quenched or diminished in non-thermal states is a complex problem. Clearly, changes in the accretion disk magnetic field topology, the geometry of the inner accretion flow, and the spectrum emitted by the inner accretion flow may be closely linked. The exact sequence of such changes is not clear; it may depend on, e.g., whether the mass accretion rate is increasing or decreasing, or a number of other factors. Disk reflection and wind studies with Resolve will allow substantial progress. Winds might persist, but at lower column densities and Science with XRISM 5.2 Surface Redshifts from Massive White Dwarfs much higher ionizations; the unprecedented sensitivity and resolution of Resolve can detect such winds. Resolve will also make it possible to separately trace the evolution of outflows, the inner disk, and the outer disk, potentially revealing the importance of geometric changes in diminishing outflows.

Surface Redshifts from Massive White Dwarfs
As noted in the Astro-H white paper on white dwarfs 181 , a microcalorimeter may be able to directly measure the redshift from the surface of a white dwarf. The anticipated shift is small (a few parts in ten thousand); however, the centroid of a strong line can be measured to an accuracy that is much sharper than a single spectral bin, particularly with the inclusion of the calibration pixel.
The motivations for this measurement are simple, but profound. It is not clear how effectively white dwarfs can gain mass through accretion; it may be that nova episodes -with temporary nuclear burning on the white dwarf surface -serve to expel more gas than they accrete between such episodes. Finding a massive white dwarf in a binary would clearly signal that accretion does add mass and can thereby lead to SNe Ia, with consequences for elemental seeding within galaxies and the use of SNe Ia as standard candles for cosmology.
In principle, the measurement is simple: the redshift of an Fe Kα line excited on the surface of the white dwarf determines M/R; since the equation of state is known, this can be translated to M . In practice, there are several difficulties. The Fe Kα line is actually a doublet, separated by 13 eV, and it may be the case that a range of charge states are observed; Fe I-XVII lines all lie within a range of just 30 eV. The motions of the bodies within the binary system and the rotation of the white dwarf may add complications. Finally, lines may not be excited only on the surface of the white dwarf, though lines from the surface should be stronger than lines from elsewhere in the system.

High-mass X-ray Binaries
The compact objects in most high-mass X-ray binaries likely accrete by gravitationally focusing the wind driven by their massive companion star. Close to the compact object, the wind may be corralled into a small accretion disk. The exact geometry of the flow within the system is complex; the accreting gas may be shocked, photoionized, and collisionally ionized at different points. The flow is not axisymmetric, and may change considerably with source state. In pulsars such as Vela X-1, the pulse can act as a lighthouse that sweeps across the accretion flow, helping to reveal its nature. This effect, combined with the orbit of the binary, especially in eclipsing sources 182 , can potentially be utilized to study the wind geometry in detail.
These systems also afford the opportunity to study the winds of massive stars 183 . In particular, sharp flux dips in the light curves of high-mass X-ray binaries likely signal obscuration by dense clumps that are characteristic of UV line-driven winds from massive stars, giving us a unique chance to directly probe these clumps. Most current data only suggest that the obscuration events are the result of "grey" absorption, without a specific energy dependence. Indeed, it is not clear if the dips are due to a single cloud with a fairly uniform density, or a collection of very dense clumps with a low volume filling factor.
The exquisite velocity resolution enabled by XRISM will separate lines produced close to the compact object from those excited in the larger stellar wind, and will detect velocity shifts as a function of phase. Resolve will also measure the ionization, density and temperature of the accreting material, enabling detailed comparisons to hydrodynamic simulations. It may be possible to detect lines and variability within the strongest flux dips, and to thereby determine the nature of the Science with XRISM 5.4 Ultra-luminous X-ray Sources absorbers that cause the dips, improving our view of UV line-driven winds from massive stars.

Ultra-luminous X-ray Sources
Ultra-luminous X-ray sources (or, ULXs) can be defined as off-nuclear point sources in nearby galaxies with implied luminosities above the Eddington limit for a 10 solar mass black hole. In recent years, a growing number of ULXs appear to be strong pulsars, likely accreting from massive stellar companions 184;185;186;187 . It is likely that the strong magnetic fields play a role, and therefore it is not clear whether such sources are truly highly super-Eddington, or less extremely super-Eddington (or Eddington-limited) but subject to geometric and relativistic beaming. Careful observations of mildly super-Eddington pulsars such as LMC X-4 may enable a better understanding of accretion in ULXs (see Figure 15). In any super-Eddington source, radiation is coupled to gas and is -by definition -defeating gravity, so very strong outflows are expected. This is likely the case in the famous Galactic X-ray binary SS 433 188;189 . In two ULXs, there is now tentative evidence of fast, variable outflows in very deep observations with XMM-Newton 190 . Resolve observations that are triggered on specific flux states or behaviors in a subset of ULXs may be able to make wind detections and to advance studies of the accretion flow in these enigmatic sources.
With Resolve, we will begin to study whether outflows have a single velocity, or a range of velocities, as well as how and where they are launched. In a subset of the deepest ULX spectra obtained with XMM-Newton, there is also tentative evidence of emission lines. It may be possible with XRISM to examine whether these lines are excited in the wind of a massive companion star, if they are tied to a young supernova remnant, or if they are better attributed to local star formation.
Science with XRISM

Magnetic Cataclysmic Variables
When a neutron star or white dwarf has a magnetic field strong enough to channel the accretion flow, it is directed towards the magnetic poles. Even with the power of present-day computers, simplifying assumptions are necessarily involved in MHD simulations of accretion flows in the presence of strong magnetic fields. Details of a wide range of phenomena -from spin up/down of the compact object to ejection of a wind (including in the propeller regime) -depend on this interaction. Magnetic cataclysmic variables (CVs) of the intermediate polar (IP) type usually have a partial accretion disk, and a well-developed theory of X-ray emission 191 that can be used to measure a key parameter: the range of radii (∆r) over which disk matter connects to the field lines. Since accretion follows field lines that are close to dipolar, ∆r determines the area on the white dwarf surface over which accretion occurs.
We anticipate that we can use the triplet ratio of the He-like Fe lines to measure the density, expected to be in the 10 16−18 cm −3 range 181 . Combined with existing multiwavelength data, which allow us to infer other parameters such as the inner disk radius and the total accretion rate, the density measurement will allow us to infer ∆r.
The He-like Fe lines of some magnetic CVs are extraordinary strong, and may indicate that resonant scattering plays a role 192 . Since resonant scattering is dependent upon geometry, we can expect to see the resonant component and other components of the line change as a function of the white dwarf spin phase.

THE LIVES AND DEATHS OF STARS
(Primary editors: Paul Plucinsky, Lía Corrales)

Star Formation and the Cold/Hot ISM
How molecular cloud materials accrete onto pre-stellar cores is a fundamental question in star formation. Optical, infrared, and radio observations now image accretion disks around young stellar objects 193 , but they have not resolved the innermost regions where matter finally accretes onto the cores. Mass accretion at the innermost region is probably induced by interaction of large scale magnetic fields of the core and the innermost disk. Collimated jets seen in their outskirts are suspected to be byproducts of this innermost accretion process.
X-ray emission from protostars is heavily obscured by thick (N H ≥ 10 22 cm −2 ) circumstellar medium (CSM) below 2 keV where various emission lines are. However, their X-ray spectra often show strong Fe K emission lines at ∼6.7 keV as well as Fe fluorescence at 6.4 keV, which provide powerful tools to diagnose plasma conditions and dynamics of the innermost region. Freefall and Keplerian velocities near the core are a few hundred km s −1 , and hot plasmas are also expected to flow around these velocities. Unfortunately, existing high-resolution grating spectrometers are insufficient to resolve such velocity shifts. XRISM will resolve plasma motions near the core for the first time.
XRISM observations of protostars will reveal: i) geometry of X-ray plasmas around the core from Doppler shift or broadening measurements of highly ionized Fe emission lines, ii) plasma condition (e.g., equilibrium state) from multiple Fe emission lines, iii) dynamics of the innermost accretion disk from Doppler shifts or broadening of the fluorescent Fe K line, iv) jet formation mechanism from Doppler shifts of emission lines during X-ray flares.
Two nearby molecular clouds, ρ Oph (d ∼ 165 pc) and R CrA (d ∼ 130pc), hold protostars that are bright enough in X-rays for detailed spectroscopy with XRISM. The other stellar classes Science with XRISM 6.1 Star Formation and the Cold/Hot ISM important for mass accretion study are FU Ori/EX Lup type stars 194 , which are young stars experiencing episodic mass accretion outbursts. The events occur rarely, but if such an outburst occurs in a nearby young star, Resolve should be able to collect valuable information on mass accretion activity. Some massive star forming regions are accompanied by diffuse hot X-ray plasma with kT ∼0.1-0.8 keV (RCW38: 195 , M17: 196;197 , Carinae nebula: 198;199;200 , Orion nebula: 201 , see Figure 16). The plasma apparently fills cavities of the parent molecular cloud and interacts with cold, thick molecular clouds at the contact surface. The origin of the diffuse plasma is not understood. One hypothesis is that the plasma consists of multiple SNRs; however, none of them have a spherical shape like conventional SNRs or are accompanied by compact objects except for a few cases 202;203 . The other hypothesis is that the diffuse plasma is thermalized by collision of winds from massive stars in the field. However, it is controversial if massive stars in the field can supply enough energy to thermalize the observed X-ray plasmas.
Science with XRISM

Supernovae and their Remnants
The keys to solving this problem should be in the elemental abundance distribution and the dynamics of the plasmas. X-ray CCD spectra of the Carina nebula demonstrated strong spatial variation of the Fe L emission lines 198;199;200 . If this is caused by Fe abundance variation, the X-ray plasmas are likely produced by supernova explosions. Meanwhile, supernova ejecta fly much faster than massive stellar winds. This difference may be seen in broadenings of emission line unless the plasmas are fully relaxed.
XRISM observations of the diffuse X-ray plasmas in massive star forming regions will reveal: i) plasma elemental abundances and their spatial variation by resolving individual emission lines, ii) plasma dynamics from Doppler shifts or broadening of these emission lines, iii) the presence of charge exchange lines 200 , which traces interaction between hot plasma and cold molecular gases. A discovery of turbulence in the X-ray plasma may also yield greater insight into the physics of particle acceleration in the star forming region.

Supernovae and their Remnants
The high-resolution spectra that will be acquired by XRISM promise to revolutionize the spectral analysis of extended objects like SNRs, much as the spectra from the gratings instruments on Chandra and XMM-Newton have done for point-like sources. An example of the type of spectrum that XRISM can obtain from an SNR is shown in Figure 17, showing a simulated spectrum of a Type Ia remnant. The various line complexes are highlighted, and broadening of the lines by several thousand km s −1 is included. We briefly describe here some of the science that will be enabled by this new capability. The reader is referred to the more comprehensive reviews written for Astro-H 204;205;206 for a more complete list. Figure 17: A sample spectrum that one might expect from a young Type Ia SNR, such as Tycho or Kepler's SNR.
Lines from various elements or groups are highlighted. Lines are broadened due to the extremely high temperatures that result from gas being shocked by a 5,000 km s −1 shock wave. XRISM will resolve the widths of these lines in SNRs, leading to a direct measurement of the plasma temperature.
Science with XRISM

Supernovae and their Remnants
Type Ia SNRs Type Ia SNe are generally believed to form through two scenarios: the singledegenerate (SD) channel composed of a white dwarf (WD) and a stellar companion and the doubledegenerate (DD) channel composed of two WDs 207 . Evidence has grown that both scenarios operate in nature but it is not clear if one channel is the dominant mechanism nor is it clear what the precise explosion mechanisms are for these systems.
Theoretical work on the explosion mechanisms and the nucleosynthetic yields 208;209 indicate that the amount of neutron-rich species ( 58 Ni, 56 Fe, 54 Fe, and 55 Mn) produced is sensitive to the central density of the WD, the process referred to as neutron-rich nuclear statistical equilibrium (n-NSE). The precise amount of these neutron-rich species and the intermediate mass elements (IME) also depends on the details of the explosion. These different nucleosynthetic yields offer the possibility that X-ray spectroscopy of the remnant of a Type Ia SNe can distinguish a progenitor of a WD near the Chandrasekhar mass (SD scenario) from one with a lower central density (DD scenario). The Ni/Fe and Mn/Fe ratios in the Suzaku spectra of 3C 397 imply a SD progenitor, assuming that these ratios can only be achieved with high neutronization in the ejecta 210 . The ratio of Ca to S was used as a diagnostic of high neutronization in the galactic remnants Tycho, Kepler, 3C 397 and G337.2-0.7 and the Large Magellanic Cloud remnant N103B to argue that the metallicity of the progenitor could not explain the high neutronization 211 .
The high-resolution spectra from Resolve of Type Ia SNRs will allow an accurate determination of the fluxes of all the lines that contribute in the Cr Kα, Mn Kα, Fe Kα, and Ni Kα line complex bandpasses, allowing the ratios of these elements to be determined with unprecedented precision. In addition, Resolve spectra will cover the entire energy range from the IMEs to the Fe peak elements with the highest spectral resolution yet achieved. These observations promise to be the definitive measurements of these abundances that will constrain the origin (SD vs. DD) and explosion mechanism for these objects.
Core-Collapse SNRs The explosion mechanism for CCSNe is still debated, with one common theme that spherical symmetry must be broken at some point to revive the shock which consistently stalls in simulations. But where and how this asymmetry is produced is still unknown. A key observational constraint needed is the 3D structure in space and velocity of the ejecta from the innermost layers of the star. Pioneering work on the ejecta distribution of the brightest Galactic SNR Cas A has been done using Chandra data 212;213 . Both these studies show a highly asymmetric ejecta distribution and significant overturning of the ejecta with high-Z material at larger radii than low-Z material. This distribution can be contrasted with that of Tycho, a Type Ia SNR, which shows a more symmetric distribution 214;215 .
The results to date have large uncertainties given the spectral resolution of the current generation of detectors. For example, the studies with Chandra rely on the unresolved Si XII He-like triplet. Assumptions must be made about the temperature and ionization state to derive the centroid of the line complex. With the resolution of XRISM, these assumptions will no longer be necessary. In fact, the resolution of the calorimeter is so high that for many lines of sight through the centers of remnants like Cas A and Tycho, it will be possible to separate redshifted material on the far side of the shell from blueshifted material on the near side of the shell.
The Hitomi results from the LMC SNR N132D from just 3.7 ks of data offer a preview of what will be possible with XRISM. In this short observation 216 , the Fe Kα complex was shown to be consistent with red-shifted emission with a velocity of ∼ 800 km s −1 , while the S emission is consistent with the local ISM velocity in the LMC, indicating a highly asymmetric ejecta distribution. However, this result is model-dependent and significant at only the 1.6σ level.
Plasma Physics in SNRs The processes which heat particles in shock waves is one of the most active areas of research in shock physics. It is not clear if the particles behind the shock will assume a temperature proportional to the mass of the particle, or if partial equilibration amongst Science with XRISM 6.2 Supernovae and their Remnants the electrons, protons, and ions occurs (and if so, on what timescales). A recent result on SN 1987A 217 argues that the X-ray spectra are consistent with mass-proportional heating behind the shock front for the ions of Ne, Mg, Si, and Fe. This analysis compared the widths of the lines as determined by the HETG on Chandra to the predictions from 3D hydrodynamical models (see Figure 18). The ratio of the ion temperature to the proton temperature is shown as a function of Z in Figure 19 (the analysis for Fe is based solely on the Fe L lines). XRISM will measure the widths of these lines to higher precision and allow more elements, such as Fe Kα, to be measured. The SN1987A analysis also assumes that the plasma conditions are similar for the entire remnant. For nearby galactic remnants, Resolve will measure the line broadening of ions as a function of position behind the shock, disentangling the effects of thermal broadening and bulk motions from mass-proportional heating. Just as the importance and prevalence of radiative recombination emission was not appreciated until Suzaku provided moderate-resolution spectra with low background and large collecting area 218;219 , spectroscopy of SNRs will change dramatically as new and unexpected features are discovered in highresolution spectra. An example of this is CX emission in SNRs, expected to be significant when high-temperature ions interact with neutral material. The best example of CX emission in a SNR to date comes from the XMM-Newton RGS spectrum of a dense knot in the Cygnus Loop 220 . Figure 20 shows the highresolution spectrum from the RGS and the moderate-resolution spectrum from the pn. This example demonstrates how an acceptable fit to the CCD spectra could be achieved Science with XRISM 6.3 Exoplanets while excluding the CX emission but such a model fails to fit the high-resolution data of the RGS.
Another important effect which has been mostly ignored in spectral fitting of SNRs is resonance scattering, which can alter the ratio of the forbidden and resonance lines. If such an effect is ignored in the spectral fitting, the fitted parameters of the model will be systematically biased. This is one possible explanation for the so-called "low-abundance problem" in which the ISM abundances determined from fitting the forward shock emission in a SNR are systematically lower than those determined by other means. The key to investigating this apparent discrepancy is to resolve the He-like triplets and develop a model which represents the high-resolution data. The Resolve spectra will transform the field by providing the data that demand that these processes are included in the spectral models, significantly improving our understanding of SNRs.

Exoplanets
It has become increasingly apparent − through studies of our own solar system and hot Jupiters around other stars − that magnetic and high-energy activity from planet host stars have a marked effect on planet atmospheres and habitability. X-ray observations of exoplanet host stars provide a marker of magnetic activity along with X-ray and UV flux, which affect exoplanet atmospheres through photoionization, photo-induced chemical reactions, and substantial mass loss 221;222 . With the recent launch of TESS, we expect many Earth-sized planets to be discovered around sun-like stars out to 100 pc, or M dwarf stars out to 10 pc. This proximity means that exoplanet host stars are likely to be X-ray bright enough for high signal-to-noise observations with XRISM, opening the door for contributions to exoplanet science.
XRISM could also provide a new avenue of research by observing planetary transits from X-ray bright exoplanet host stars. X-ray transits reveal the full extent of an exoplanet atmosphere and probe evaporation, because the X-ray absorption cross-sections of atmospheric gases are larger than the optical. Only one exoplanet transit has ever been observed in the X-ray band − that of the hot Jupiter HD 189733b − which showed a 6% transit depth, a factor of three deeper than the optical transit 223 . Xtend could also probe the atmospheres of hot Jupiter planets by measuring the depths of the X-ray transits, while Resolve can be used to search for high-resolution X-ray absorption features from volatile elements like C, N, and O 224 . Such measurements would be valuable for understanding atmospheric evaporation and measuring the composition of hot Jupiter atmospheres, which can then be extended to the effects of high energy irradiation and coronal mass ejection on Earth-like planets. With a suite of new observatories launching in the early 2020s, XRISM will benefit from a rich landscape, and will add depth to many of the main science drivers of those future observatories (see Figure 21). In this chapter, we briefly highlight some of the science topics where XRISM, together with space-and ground-based observatories, will make important breakthroughs, starting from the nano-parsec scales around black holes and zooming out to mega-parsec scales in clusters of galaxies. We emphasize that the synergistic science outlined in the following sub-sections is by no means exhaustive, and encourage further collaborative efforts.

Independent Measurements of Black Hole Spin
The strong gravitational field around a black hole can be completely parametrized by its mass and spin, which imprint distinct signatures on the inner accretion flow that emits in the X-ray band. The spin tells how black holes form, as a slowly rotating black hole can only reach the maximum spin allowed by general relativity by increasing its mass 225 . Therefore, measuring the distribution of black hole spin in supermassive systems reveals the extent to which they are grown through accretion or mergers 226 . On the other hand, stellar mass black holes do not accrete enough material to affect their spin, and therefore, measurements of black hole spin in these systems can constrain supernova mechanisms and the evolution of massive stars.
Several techniques to measure black hole spin have been used. XRISM provides the best energy resolution in the Fe Kα band, and thus will constrain the systematic uncertainties on the spin through broad line spectroscopy. XRISM will determine if other components, e.g., ionized winds, are imprinted on the broad Fe Kα emission line. IXPE will provide the first X-ray polarization measurements of black hole spin. In black hole X-ray binaries in the thermal-dominated state, a fraction of the thermal flux will be deflected by the strong-field gravity of the black hole and will scatter off of the disk. In highly spinning black holes, the thermal emission will be highly polarized 227 . Finally, observations with NICER can provide spin constraints through timing analysis, by measuring short timescale X-ray reverberation lags from reprocessed emission off the inner accretion disk 228 or through high-frequency quasi-periodic oscillations in black hole X-ray binaries that may be associated with the orbital timescale at the innermost stable circular orbit 229  larizations and arrival times) to put the tightest constraints on strong gravity in accreting black holes.

The Disk-Wind-Jet Connection in Galactic Black Holes
Accretion remains a central topic in modern astrophysics, and the variability timescales in X-ray binaries (ms to days/months) are particularly suited for studying the complex coupling between accretion and ejection. XRISM will measure the wind ionization, column density, and outflow velocity to derive the mass outflow rate and the associated mechanical power. Radio and IR facilities, such as the JVLA, ALMA and the VLT will measure the power output from jets. Comparing the power output from winds vs. jets, and their evolution during an accretion outburst 230 will determine the relation between these different ejection modes, and will establish if they are important for triggering state transitions 231 .
Observations of accreting black holes will test theoretical models that associate the X-ray corona with the base of a relativistic jet 232 by making detailed measurements of the X-ray reflection features. XRISM's exquisite spectral resolution will be instrumental in disentangling these reflection features from other processes, such as absorption through disk winds. Hard X-ray observations with NuSTAR will provide an important "lever-arm" for XRISM, by putting simultaneous constraints on the X-ray continuum and reflection above 10 keV.

Dust Scattering Halos and Molecular Clouds
Studies of dust scattering halos around X-ray binaries constitute a useful tool to investigate the distribution and properties of dust grains along the line of sight or to calculate distances via changes in the halo profile 233 . XRISM will allow a two-tiered approach to dust studies by using Xtend and Resolve to investigate dust scattering haloes and the X-ray absorption fine structure at 0.2−2 keV, respectively. Adding observations of CO with NOEMA or ALMA will help to constrain dust models by providing an accurate map of the existent molecular clouds along the line of sight 234;96;235 .

Following Extreme Transients in Time Domain Surveys
X-ray observations have played a major role in time domain astronomy: from the discovery of new galactic black holes, giant stellar flares, tidal disruption events (TDEs), and flares from Sgr A*, to the existence of super-Eddington accretors, the discovery of magnetars, and millisecond X-ray Science with XRISM 7.5 Galactic Outflows and the Startburst-AGN Connection pulsars. Time domain astronomy is a quickly advancing field. Even in the next few years, we will see major increases in the rate of transient discovery. In the optical, ZTF and LSST will discover thousands of transients per night. In the radio, SKA will find tens of transients per night. In Xrays, the eROSITA all-sky survey will provide a 30-fold improvement in sensitivity, discovering faint transients, and producing a baseline against which we will compare future observations. Finally, the Chinese Academy of Science's Einstein Probe (to launch in late 2022) will use "lobster-eye" optics to make a vast improvement in sensitivity for X-ray transient searches.
XRISM will follow-up on a wide variety of transients like TDEs, changing-look AGN, X-ray bright SNe, Galactic black hole outbursts, GRB afterglows, and perhaps nearby kilonovae. XRISM has a niche in time domain astronomy, and will be best suited for studying the detailed physics of bright, isolated transients that evolve on timescales of days to weeks. High-resolution spectroscopy has already proved important for transient science, with the discovery of highly ionized winds in the thermal state in black hole X-ray binaries 150 , along with the discovery of disk winds (even ultrafast outflows) in TDEs 236 . XRISM will have a ToO response time similar to other large observatories like XMM-Newton and Chandra.

Galactic Outflows and the Startburst-AGN Connection
The discovery of massive dusty and molecular components in multi-phase galactic outflows associated with starbursts and AGN over the past decade confirmed that they have the potential to regulate and perhaps halt star formation in their host galaxies, but we still do not know which types of outflows most effectively disrupt and remove star-forming gas 237 . Moreover, outflows can compress gas and trigger star formation, and it remains unclear whether starbursts trigger quasars or vice versa. The source of outflows in systems with both a starburst and an AGN is also an open question. Sensitive sub-mm facilities, such as ALMA and IRAM, as well as optical and near-IR integral field spectrographs, have illustrated the diversity of outflows 238 . The largest missing piece is measurements from hot and/or highly ionized gas, which contains most of the energy and much of the mass and metals 239 .
X-ray observations of outflowing gas on both small (<1 pc) and large (>10 kpc) scales will determine the genesis and ultimate impact of outflows. In particular, Fe XXV (6.7 keV) and Fe XXVI (6.96 keV) trace both the very hot gas driving starburst winds 240;241;242 and the relativistic disk winds driven by the AGN 243;244 . XRISM will provide the first velocity, temperature, and mass measurements in multiple hot starburst winds, and will be sensitive to weak and variable ultra-fast outflows in many AGNs. On larger scales, soft X-ray lines trace the expanding winds and their interaction with the ISM and CGM.
These measurements are important on their own, but must be understood in the context of observations of cooler gas. The combination of XRISM, JWST, ALMA, and ground-based optical and near-IR instruments (such as VLT/MUSE, Gemini/GMOS, TMT/IRIS, or ELT/HARMONI) will provide a detailed accounting of the physical state, bulk velocity, and mass of the neutral and ionized gas in nearby (d <200 Mpc) starburst and AGN outflows, enabling us to determine how much gas can actually escape the galaxy's gravity. In addition to the hottest gas, this will include cool H 2 detected in CO bands, warm H 2 detected in the 2.124 µm line, dusty outflows detected via PAH bands from 3−20 µm, warm ionized gas via Brγ, and hot, but cooling, gas from mid-IR forbidden lines, such as

AGN Feedback in Clusters of Galaxies
The hot atmospheres of many galaxy clusters are so luminous in X-ray emission that they should radiate away their thermal energy in < 1 Gyr. Uninhibited cooling should then fuel molecular gas and star formation at rates of hundreds to thousands of solar masses per year. However, observed molecular gas reservoirs and star formation rates are ten times lower than predicted from pure cooling. Instead, jets launched from the central SMBH are thought to regulate cooling 245;246 , but the details of the AGN feedback process are not well understood. XRISM will allow us to probe the physics of AGN feedback by measuring the velocity of bulk and turbulent motions of the hot phase of the ICM.
Molecular gas is another key component in the AGN feedback cycle. This is the reservoir for gas condensing out of the hot atmosphere, and the fuel source for accretion onto the central supermassive black hole. Together XRISM and ALMA will probe the dynamics and energetics of the hot phase and cold phases of the ICM, which is crucial for developing a complete model of AGN feedback.

Particle Acceleration in Large-Scale Plasmas
XRISM will provide the first direct measurements of the turbulent line broadening in clusters that host extended radio haloes. These will be crucial to verify existing, indirect hints that link the non-thermal component and the kinetic energy in the ICM 247;248 , opening a new window into quantitative studies of turbulent particle acceleration and magnetic field amplification on large scales. Progress in this field is currently driven at a rapid pace by sensitive radio surveys using LOFAR 249 and GMRT 250 , and will continue in the future with the upgraded LOFAR 2.0 and uGMRT, as well as MWA, MeerKAT, and the SKA. As these surveys reveal more and more of the faint non-thermal plasma and map its radio spectral index, the Resolve instrument will deliver the complementary imaging spectroscopy capabilities required to connect the properties of the relativistic electrons seen in the low-frequency radio band with detailed dynamics of the thermal gas permeating the cosmic web, thus revealing the intricate physics behind the Universe's largest particle accelerators.

Conclusion
The importance of high-resolution X-ray spectroscopy to the future of astronomy in general cannot be overstated. Despite the fact that Hitomi only operated for a matter of weeks in 2016, the results from that spacecraft are among the most highly-cited papers in the field of high-energy astronomy in the past several years. By recovering the science lost by the untimely demise of Hitomi, XRISM will continue its legacy into the next decade, paving the way for future X-ray missions, such as Athena, which will be equipped with even greater capabilities.
The checkout, calibration, and performance verification phases will last approximately nine months, after which XRISM will enter the general observer phase. Astronomers around the world will be able to propose to an annual announcement of opportunity for competitively-awarded time on the observatory. The science that is briefly outlined in this white paper is only the beginning of what will be possible given the capabilities that this observatory will provide.