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The X-ray Ridge Surrounding Sgr A* at the Galactic Center

We present the first detailed simulation of the interaction between the supernova explosion that produced Sgr A East and the wind-swept inner ~ 2-pc region at the Galactic center. The passage of the supernova ejecta through this medium produces an X-ray ridge ~ 9'' to 15'' to the NE of the supermassive black hole Sagittarius A* (Sgr A*). We show that the morphology and X-ray intensity of this feature match very well with recently obtained Chandra images, and we infer a supernova remnant age of less than 2,000 years. This young age--a factor 3--4 lower than previous estimates--arises from our inclusion of stellar wind effects in the initial (pre-explosion) conditions in the medium. The supernova does not clear out the central ~ 0.2-pc region around Sgr~A* and does not significantly alter the accretion rate onto the central black hole upon passage through the Galactic center.


Introduction
While Sagittarius A* (Sgr A*) dominates the gravitational dynamics in the central parsecs of the Galaxy, many other components are required to explain the wealth of detailed observations of this busy region. Even as we focus on the inner 3 parsecs, it is clear that other classes of object contribute to the overall radiative emission from the Galactic center (for a recent review, see Melia & Falcke 2001; see also Maeda et al. 2002). For example, the medium within which Sgr A* is embedded-bounded by the circumnuclear disk (CND, with inner radius ∼ 2-3 parsecs)-has a temperature ∼ 1.3 keV and emits a Chandra-detectable glow of diffuse X-rays (Baganoff et al. 2003). Rockefeller et al. (2004) have shown that this emission may be understood as the result of mutual interactions between the winds of Wolf-Rayet and O/B stars within ∼ 1 parsec of the supermassive black hole.
The CND is perhaps the shredded remains of a giant molecular cloud that passed by Sgr A*. The cavity within the inner ∼ 2-3 parsecs of this structure may itself have been created by the ablative influence of the cumulative wind outflow, which has by now produced a bubble of hot, ionized plasma. To make matters even more complex, observations of the supernova (SN) remnant Sgr A East suggest that its broadband emission arises from shock heating in a recent (< 10, 000-year-old) supernova (or other explosive outburst; see Mezger et al. 1989) originating within ∼ 3 parsecs of the black hole.
Any comprehensive model of this region must therefore include the effects of all these components: the supermassive black hole, the Wolf-Rayet and O/B winds, the dense CND and the expanding supernova remnant, which we now see predominantly as a radio-emitting shell. Although interacting winds can explain the bulk of the diffuse X-ray flux from the inner ∼ 2-3 parsecs, several other features seen in the Chandra X-ray image are not as easy to explain without the influence of some other interaction. In particular, a well-defined ridge of X-ray emission just outside the central region, to the NE of Sgr A* (see Figure 1), may be evidence of an ongoing collision between the SN ejecta and the cumulative Wolf-Rayet and O/B winds emanating from within the cavity (Maeda et al. 2002).
In this Letter, we model the expansion of the supernova, focusing on the effect this explosion has on the central few parsecs surrounding Sgr A*. We then directly compare the X-ray emission arising from the interaction zone with the actual Chandra image.
Interestingly, using the well-studied wind conditions at the Galactic center, we may also be able to place tighter constraints on the supernova explosion itself-both the released energy and the age of the remnant. With this knowledge, we can address several outstanding issues pertaining to the influence of this explosion on the morphology of the Galactic center: Did the supernova shock clear out the region surrounding the black hole, effectively shutting down what would otherwise have been a high accretion rate onto the black hole? Could the supernova have caused a brief increase in the accretion rate onto Sgr A*, producing a spike in X-ray emissivity that irradiated the X-ray-fluorescing Sgr B2 and other nearby molecular clouds some 300 years ago (see, e.g., Sunyaev & Churazov 1998;Fromerth & Melia 2001)?

General Physical Principles
Our simulation uses the SNSPH smoothed particle hydrodynamics code (Fryer, Rockefeller, & Warren 2005a) to follow the supernova explosion as it crosses the Galactic center. The domain of solution is a cube, 6 pc on a side, centered on the black hole. Particles that move beyond this domain, or within 1.9 × 10 17 cm of the origin (effectively onto the black hole), are removed to simulate outflow (or inflow) conditions. Our initial (pre-explosion) conditions are taken from the structure of the wind-swept medium at the end of the simulation by Rockefeller et al. (2004). The initial particle distribution is constructed from a ∼ 1 million particle supernova explosion placed 2 pc due east (in right ascension) of the central supermassive black hole within a ∼ 6 million particle wind-filled Galactic center region.
We assume that the density structure within this domain of solution at the time of the supernova explosion is dominated by matter lost by the Wolf-Rayet and O/B stars, plus the dense CND surrounding the central black hole. The CND is mimicked by 200 spherical clumps (totaling 10 4 M ⊙ ), in a torus with a low filling factor surrounding the black hole. The winds from these stars (which we assume have not changed noticeably in the past 10, 000 years) have blown a bubble in the Galactic center that is probably at the edge of the 50 km s −1 molecular cloud (Mezger et al. 1989).
Note, however, that our initial conditions do not include the initial molecular cloud blown out by the stellar winds. There is evidence that the supernova shock has reached the boundary between the wind-blown bubble and this cloud (Yusef-Zadeh et al. 1999). We can not address these effects with this current set of simulations. We also do not include the effect of mass loss from the supernova progenitor itself. However, the density structure near the X-ray ridge and the central black hole is dominated by the ∼ 25 stars we do include (Rockefeller et al. 2004), and not by any outer stars or the surrounding molecular cloud.
The structure of the supernova ejecta is set using a spherically symmetric model from Hungerford, Fryer, & Rockefeller (2004). This 15 M ⊙ star is exploded with an energy of 1.5 × 10 51 erg; these properties are typical of a supernova explosion, both in composition and energy. We place the explosion into our domain of solution after the shock has moved out for 1 year, at which point the explosion material is still within 0.02 pc of the supernova site.

Calculations and Results
The wind-swept initial conditions of the Galactic center lead to an aspherical progression of the supernova shock. Plowing through diffuse regions with density ∼ 1 particle cm −3 , across the dense CND, and through central regions with densities > 10 4 particle cm −3 , the supernova shock is far from symmetric. The ejecta flow around the dense regions, taking the path of least resistance and producing shocks where they collide with and are decelerated by the dense material. Figure 2 shows the density profile and position of a set of tracer supernova particles 650 years after the launch of the supernova explosion. This is roughly the time of deepest penetration of the supernova ejecta into the region surrounding the supermassive black hole. The actual supernova shock has now moved beyond the Galactic Center and has already passed beyond the southern and eastern edges of our simulation grid. The ejecta do not sweep through the central region, nor do they significantly alter the density of the inner 0.2 pc surrounding the black hole. A more detailed discussion of the effects of the supernova shock on the central region, the implications for the black hole accretion rate, and the possible excitation of Sgr B2, will be presented in Fryer et al. (2005b). Mezger et al. (1989) estimated the age and energy of the supernova remnant by assuming the supernova was plowing through a 10 4 particle cm −3 molecular cloud. They calculated a supernova remnant age of 7, 500 years and, to fit the observed shock temperature, they required an explosion energy in excess of 10 52 erg. However, using the lower mean density in our wind-swept initial conditions allows us to account for the observed remnant characteristics with a more typical supernova explosion (∼ 10 51 erg). Our simulation also suggests that the supernova remnant is younger than the estimate of Mezger et al. (1989) (see Fryer et al. 2005b, for more details on the fitting). From the position of the shock, we can set a crude lower limit to the remnant's age at ∼ 1, 000 years, but a more precise answer may be determined by comparing our predicted X-ray ridge properties to the observations shown in Figure 1.
The column integrated X-ray emission from our models is shown in Figure 3 in a series of temporal snapshots. For very energetic shocks, the high compression ratio at the point of impact can lead to significant particle acceleration and consequent nonthermal emission, but by this time in the interaction, the dominant 2-10 keV emission mechanism is expected to be optically thin bremsstrahlung (see Rockefeller et al. 2004;Fryer et al. 2005b, for details). The supernova shock reaches the dense inner 0.2-0.3 pc of the Galactic Center in 160 years and sweeps around this area after 650 years. The X-ray emission is highest where the shock is strongest (a function of both shock velocity and density of the impacted region).
By 1, 740 years after the explosion, the supernova shock has swept beyond our simulation grid. However, over 95% of the mass in the supernova ejecta is moving at less than half the speed of the supernova shock front. This slow-moving material continues to impinge on the inner 0.2-0.3 pc of the Galaxy. It is the interaction between the stellar winds pushing against the slow-moving supernova ejecta that produces the X-ray ridge observed today (Fig. 1).
For direct comparison between our simulation and the properties evident in Figure 1, we restrict our attention to the region bounded by the 9 ′′ and 15 ′′ arcs in this image; this appears to be the scene of dominant interaction at the present time. The observed flux is accurately modeled with two components at different temperatures: a 5.6 keV component with a 2-10 keV flux of 3.92 × 10 −13 erg cm −2 s −1 and a 1 keV component with a 2-10 keV flux of 5.8×10 −13 erg cm −2 s −1 ; these correspond to 2-10 keV luminosities of 3.0×10 33 erg s −1 and 4.4 × 10 33 erg s −1 , respectively, at a distance of 8 kpc from the Galactic center.
In our simulation, both the location of, and the X-ray intensity from, this region vary with time, so we have essentially two important constraints on the comparison between theory and observation. We suspect that the 1 keV component arises from either foreground or background emission; this is supported by the fact that the luminosity-weighted temperature in this region of our simulation is 4.9 keV after 1, 740 years. From our simulation, we find that the column-integrated 2-10 keV luminosity from the interaction within the swath highlighted in Figure 1 is 3.6 × 10 33 erg s −1 after 1, 619 years and 2.7 × 10 33 erg s −1 after 1, 740 years. These values are within 20% of the observed luminosity of the 5.6 keV component, and the good match between theory and observation for both the morphology and the X-ray radiance of the ridge therefore provides us with compelling evidence that ∼ 1, 700 years must be a reasonable estimate for the remnant's age.
As the velocity of the impinging supernova ejecta decreases, the X-ray ridge moves out and dims. By 2, 560 years, the ridge will have moved out 30 ′′ and its flux should then be 3 times lower than its (current) 1, 740-year value.

Conclusion
Our simulation of the passage of Sgr A East across the Galactic center has produced several new insights into the structure of the environment within ∼ 3 pc of Sgr A*. The front of the SNR flowed around the Galactic center ∼ 1, 100 years ago (650 years after the supernova explosion). The shock front pushed back the combined winds from the Wolf-Rayet and O/B stars, but did not get within ∼ 0.2 pc of the accreting supermassive black hole. The collision between the supernova ejecta and the central winds produces a ridge of X-rayemitting gas ∼ 9 ′′ -15 ′′ to the NE of the black hole. Comparing our simulations with the observations allows us to estimate the age of the supernova remnant to be ∼ 1, 700 years. We predict that this X-ray ridge will move out and dim with time. In ∼ 800 years, it should be roughly twice its current distance from Sgr A*, and a factor ∼ 3 dimmer.
The supernova did not significantly alter the accretion rate onto the black hole. We discuss this in greater detail in an upcoming paper (Fryer et al. 2005b). Our simulations demonstrate how rich in information are the X-ray observations of the Galactic Center. Although this region contains several complex flows that are difficult to simulate completely, the extensive body of data contains many features that we can employ to tightly constrain the calculations. As we continue to refine our models, we expect to uncover additional new features regarding the environment and physical processes at the Galactic center.
This work has barely scratched the surface of what we can learn about the Galactic center by carefully studying the gas dynamics in this region. In Fryer et al. (2005b), we will discuss in more detail the hydrodynamic evolution of the shock and calculate the emission as the shock hits the large molecular cloud to form Sgr A East. We will also report in detail the metallicity gradients expected within Sgr A East as a function of time and distance from Sgr A*, providing yet another observational signature that may be used to better constrain this remnant's age. Fig. 1.-Chandra image of the inner 1 ′ × 1 ′ of the Galaxy, centered on the location of the radio source Sgr A*. The (square) pixel size is 0. ′′ 492 × 0. ′′ 492, and the two arcs indicate distances of 9 ′′ and 15 ′′ from the center. The X-ray ridge, to the NE of Sgr A* and in the direction of Sgr A East, is bounded by these two arcs. Fig. 2.-Density contours of a 2-dimensional slice of the inner 3 pc surrounding Sgr A* 650 y after the launch of the supernova explosion. The contours take mean densities from a 0.2 pc slice centered on the radial distance of the supermassive black hole and correspond to number densities of 1 (blue), 10 (cyan), 100 (green), 1000 (red), and 10 4 (magenta) cm −3 . The supermassive black hole is located at 0, 0. Roughly at longitude = −0.89 pc, latitude=−1.47 pc is the origin of the supernova (at the same radial distance as the supermassive black hole). The circumnuclear disk is modeled with several dense spherical clumps with a low volume filling factor (magenta contours). The vectors show tracer particles of the supernova ejecta themselves (arrows denote velocity direction and length denotes velocity magnitude). Note that the explosion flows around Sgr A* and the dense central region along paths where the density is lowest. Fig. 3.-Contours of column-integrated 2-10 keV X-ray luminosity per 1 ′′ .2 × 1 ′′ .0 bin from our simulation, shown at four different snapshots in time (160, 650, 1740, and 2560 years after the supernova explosion). In order from blue to cyan to green to red, the luminosity densities indicated by the contours are 1.5×10 29 , 10 30 , 10 31 , 10 32 erg s −1 bin −1 . At 160 years, the shock from the supernova ejecta hitting the dense wind-dominated region surrounding Sgr A* produces an X-ray hot spot. The X-ray hot spot moves around Sgr A* as the head of the supernova shock sweeps around the central region (650 years). After 1740 years, the stellar wind material begins to reassert itself. The shock between it and the slower moving supernova ejecta forms an X-ray ridge with a 2-10 keV luminosity of roughly 3×10 33 erg s −1 . This is our best estimate for the current state of the Galactic center. With time, this ridge will move out and dim. By 2560 years after the launch of the explosion (800 years from now), the luminosity of the ridge will be a factor ∼ 3 dimmer, and the feature will lie nearly 30 ′′ away from Sgr A*.