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Supernova SN 2020faa -- an iPTF14hls look-alike?

We present observations of SN 2020faa. This Type II supernova displays a luminous light curve that started to rebrighten from an initial decline. We investigate this in relation to the famous supernova iPTF14hls, which received a lot of attention and multiple interpretations in the literature, however whose nature and source of energy still remains unknown. We demonstrate the great similarity between SN 2020faa and iPTF14hls during the first 6 months, and use this comparison both to forecast the evolution of SN 2020faa and to reflect on the less well observed early evolution of iPTF14hls. We present and analyse our observational data, consisting mainly of optical light curves from the Zwicky Transient Facility in the gri bands as well as a sequence of optical spectra. We construct colour curves, a bolometric light curve, compare ejecta-velocity and Black-body radius evolutions for the two supernovae, as well as for more typical Type II supernovae. The light curves show a great similarity with those of iPTF14hls over the first 6 months, in luminosity, timescale and colours. Also the spectral evolution of SN 2020faa is that of a Type II supernova, although it probes earlier epochs than those available for iPTF14hls. The similar light curve behaviour is suggestive of SN 2020faa being a new iPTF14hls. We present these observations now to advocate follow-up observations, since most of the more striking evolution of supernova iPTF14hls came later, with light curve undulations and a spectacular longevity. On the other hand, for SN 2020faa we have better constraints on the explosion epoch than we had for iPTF14hls, and we have been able to spectroscopically monitor it from earlier phases than was done for the more famous sibling.


Introduction
The extraordinary supernova (SN) iPTF14hls was a Type II SN, first reported by Arcavi et al. (2017, hereafter A17) as having a long-lived (600+ d) and luminous light curve (LC) showing at least five episodes of rebrightening. Sollerman et al. (2019, hereafter S19) followed the SN until 1000 days when it finally faded from visibility.
The spectra of iPTF14hls were similar to those of other hydrogen-rich SNe, but evolved at a slower pace. A17 described a scenario where this could be the explosion of a very massive star that ejected a huge amount of mass prior to explosion. They connect such eruptions with the pulsational pair-instability mechanism.
Following the report of A17, a large number of interpretations were suggested for this unusual object. These covered a wide range of progenitors and powering mechanisms. For example, Chugai (2018) agreed on the massive ejection scenario, while Andrews & Smith (2018) argued for interaction with the circumstellar medium (CSM) as the source for the multiple re-brightenings in the LC, which was supported by S19. Dessart (2018) instead suggested a magnetar as the powering mechanism, whereas Soker & Gilkis (2017) advocated a commonenvelope jet. Wang et al. (2018) proposed a fall-back accretion model for iPTF14hls and Woosley (2018) discuss pros and cons of several of the above-mentioned models, and whether the event was indeed a final explosion. Moriya et al. (2019) interpret the phenomenon as due to a wind from a very massive star.
Taken together, this suite of publications demonstrate how extreme objects like iPTF14hls challenge most theoretical models and force us to expand the frameworks for transient phenomena. But iPTF14hls was a single specimen -until now.
In this paper, we present observations of SN 2020faa, a Type II SN that observationally appears to be similar to iPTF14hls during the first six months. We present LCs and spectra to highlight this similarity and also add information that was not available for iPTF14hls, like earlier spectroscopy and better constrains on the explosion epoch. In addition to the ground-based observations, we obtained several epochs of data with the Neil Article number, page 1 of 14 arXiv:2009.07270v1 [astro-ph.HE] 15 Sep 2020 A&A proofs: manuscript no. astroph Gehrels Swift Observatory (Swift, Gehrels et al. 2004). A main aim of this paper is to direct the attention of the community to this active transient, which may -or may not -evolve in the same extraordinary way as did iPTF14hls. If SN 2020faa become another iPTF14hls, we hope these observations, especially those at the early phases, will be complementary ingredients to iPTF14hls for the community to understand the physics of such peculiar long-lived transients.
The paper is structured as follows. In Sect. 2, we outline the observations and corresponding data reductions, including Sect. 2.1 where we present the detection and classification of SN 2020faa. The ground-based optical SN imaging observations and data reductions are presented in Sect. 2.2, in Sect. 2.3 we describe the Swift observations, a search for a pre-cursor is done in Sect. 2.4, the optical spectroscopic follow-up campaign is presented in Sect. 2.5, and a discussion of the host galaxy is provided in Sect. 2.6. An analysis and discussion of the results is given in Sect. 3 and this is summarised in Sect. 4.
For iPTF14hls, we follow A17 and adopt a redshift of z = 0.0344, corresponding to a luminosity distance of 156 Mpc. We correct all photometry for Milky Way (MW) extinction, E(B − V) = 0.014 mag, but make no correction for host-galaxy extinction. For SN 2020faa, we use z = 0.04106 (see below), corresponding to a luminosity distance of 187 Mpc (distance modulus 36.36 mag) using the same cosmology as A17. The MW extinction is E(B − V) = 0.022 mag, and also in this case we adopt no host galaxy extinction. We follow A17 and use the PTF discovery date as a reference epoch for all phases for iPTF14hls, while for SN 2020faa, we set the first ATLAS detection date as reference epoch.

Detection and classification
The first detection of SN 2020faa (a.k.a. ZTF20aatqesi) with the Palomar Schmidt 48-inch (P48) Samuel Oschin telescope was on 2020 March 28 (JD = 2458936.8005), as part of the Zwicky Transient Facility (ZTF) survey Graham et al. 2019). The object had then already been discovered and reported to the Transient Name Server (TNS 1 ) by the ATLAS collaboration (Tonry et al. 2020) with a discovery date of 2020 March 24 (JD discovery = 2458933.104) at 18.28 mag in the cyan band, and a reported last non-detection (> 18.57) 14 days before discovery in the orange band.
The first ZTF detection was made in the g band, with a hostsubtracted magnitude of 18.40 ± 0.09 mag, at the J2000.0 coordinates α = 14 h 47 m 09.50 s , δ = +72 • 44 11.5 . The first rband detection came in 3.6 hours later at 18.50 ± 0.10. The non-detections from ZTF include a g-band non-detection from 15 days before discovery, but this is a shallow global limit (> 17.46), whereas the one at 17 days before discovery is deeper at > 19.37 mag. The constraints on the time of explosion for SN 2020faa are thus not fantastic, but in comparison with the very large uncertainty for iPTF14hls (∼ 100 days) they are quite useful.
SN 2020faa is positioned on the edge on spiral galaxy WISEA J144709.05+724415.5 which did not have a reported redshift in the NED 2 catalog, although the CLU catalog (Cook et al. 2019) has it listed as CLU J144709.1+724414 at the same redshift as our spectroscopy provides below. The SN together with the host galaxy and the field of view is shown in Fig. 1. SN 2020faa was classified as a Type II SN (Perley et al. 2020) based on a spectrum obtained on 2020 April 6 with the Liverpool telescope (LT) equipped with the SPRAT (SPectrograph for the Rapid Acquisition of Transients) spectrograph. That spectrum revealed broad Hα and Hβ in emission, the blue edge being shifted by ∼ 9000 km s −1 with respect to the narrow emission line from the galaxy that provided the redshift z = 0.041 consistent with CLU as mentioned above. The LT spectrum confirmed the tentative redshift and classification deduced from our first spectrum, obtained with the Palomar 60inch telescope (P60; Cenko et al. 2006) equipped with the Spectral Energy Distribution Machine (SEDM; Blagorodnova et al. 2018). That first spectrum was taken already on March 31, but the quality was not good enough to warrant a secure classification.

Optical photometry
Following the discovery, we obtained regular follow-up photometry during the slowly declining phase in g, r and i bands with the ZTF camera (Dekany et al. 2020) on the P48. This first decline lasted for ∼ 50 days, and no further attention was given to the SN during this time.
Later on, after rebrightening started, we also obtained a few epochs of triggered photometry in gri with the SEDM on the P60. The LCs from the P48 come from the ZTF pipeline (Masci et al. 2019). Photometry from the P60 were produced with the image-subtraction pipeline described in Fremling et al. (2016), with template images from the Sloan Digital Sky Survey (SDSS; Ahn et al. 2014). This pipeline produces PSF magnitudes, calibrated against SDSS stars in the field. All magnitudes are reported in the AB system.
The reddening corrections are applied using the Cardelli et al. (1989) extinction law with R V = 3.1. No further host galaxy extinction has been applied, since there is no sign of any Na i d absorption in our spectra. The LCs are shown in Fig. 2.
After the initial decline of about 50 days (this is past discovery in the observer's frame), SN 2020faa started to slowly brighten again. This continued for about 70 days and happened in all three bands. Once this was realized in late May 2020, a more intense follow-up could be activated, in particular with regular spectroscopic observations (Sect. 2.5.) We used a Gaussian Processing (GP) algorithm 3 to interpolate the photometric measurements and found that the peak happened at m peak r = 17.49 ± 0.01 after t rise r = 114.51 ± 0.10 rest frame days, via scipy. f ind_peaks. In the g and i bands the photometric behavior follows the same trend, and peaked at m peak g = 17.83 after t rise g = 114.70 days as well as m peak i = 17.58 after t rise i = 119.70 rest frame days.

UVOT photometry
A series of ultraviolet (UV) and optical photometry observations were obtained with the UV Optical Telescope onboard the Swift observatory (UVOT; Gehrels et al. 2004;Roming et al. 2005).
Our first Swift/UVOT observation was performed on 2020 July 03 (JD = 2459034.4226) and provided detections in all the bands. However, upon inspection it is difficult to assess to what extent the emission is actually from the SN itself, or if it is dif- Fig. 1 A gri-colour composite image of SN 2020faa and its environment, as observed with the P48 telescope on 2020 April 5, eight days after the first ZTF detection. The g-band image subtraction is shown in the top panel.
fuse emission from the surroundings. We need to await template subtracted images to get reliable photometry.

X-rays
With Swift we also used the onboard X-Ray Telescope (XRT; Burrows et al. 2005). We analysed all data with the onlinetools of the UK Swift team 4 that use the methods described in Evans et al. (2007Evans et al. ( , 2009 and the software package HEASoft 5 version 6.26.1 to search for X-ray emission at the location of SN 2020faa. Combining the five epochs taken in July 2020 amounts to a total XRT exposure time of ∼ 11000 s (3 hr), and provides a 3σ upper limit of 0.001 count s −1 between 0.3 and 10 keV. If we assume a power-law spectrum with a photon index of Γ = 2 and a Galactic hydrogen column density of 2.65 × 10 20 cm nitude of −15.6) and we can rule out similar outbursts for 50% of the time assuming that an outburst lasts for at least one week.

Optical spectroscopy
Spectroscopic follow-up was conducted with SEDM mounted on the P60. Further spectra were obtained with the Nordic Optical Telescope (NOT) using the Alhambra Faint Object Spectrograph (ALFOSC). A log of the spectral observations is provided in Table 1, which includes 14 epochs of spectroscopy. SEDM spectra were reduced using the pipeline described by Rigault et al. (2019) and the spectra from La Palma were reduced using standard pipelines. The spectra were finally absolute calibrated against the r-band magnitudes using the GP interpolated measured magnitudes and then corrected for MW extinction. All spectral data and corresponding information will be made available via WISeREP 6 (Yaron & Gal-Yam 2012).
We measured the brightness of the host in a consistent way from the far-ultraviolet to the mid-infrared (i.e., measuring the total flux and preserving the instrinsic galaxy colours) using LAMBDAR 8 (Lambda Adaptive Multi-Band Deblending Algorithm in R; Wright et al. 2016) and the methods described in Schulze et al. (2020). Table 2 gives the measurements in the different bands.

Spectral energy distribution modelling
We modelled the spectral energy distribution with the software package prospector version 0.3 (Leja et al. 2017). Prospector uses the Flexible Stellar Population Synthesis (FSPS) code (Conroy et al. 2009) to generate the underlying physical model and python-fsps (Foreman-Mackey et al. 2014) to interface with FSPS in python. The FSPS code also accounts for the contribution from the diffuse gas (e.g., HII regions) based on the Cloudy models from Byler et al. (2017). We assumed a Chabrier initial mass function (Chabrier 2003) and approximated the star formation history (SFH) by a linearly increasing SFH at early times followed by an exponential decline at late times. The model was attenuated with the Calzetti et al. (2000) model. Finally, we use the dynamic nested sampling package dynesty (Speagle 2020) to sample the posterior probability function.

Host galaxy spectrum
To obtain a spectrum of the host galaxy, we aligned the slit along the host galaxy in our latest NOT observation. In this way we could also extract a spectrum from the region around the SN position in order to measure the host galaxy emission lines. This was done to carry out an abundance analysis based on the strong line methods.

Light curves
The g-, r-and i-band LCs of our SN are displayed in Fig. 2. The general behaviour of the LCs was already discussed in Sect. 2.2, and the main characteristic is of course the slow evolution with the initial decline followed by the late rise over several months. In the figure we have also included the most restrictive upper limits as triangles (5σ), while the arrows on top of the figure illustrate epochs of spectroscopy. The GP interpolation is also shown, which is used for absolute calibrating the spectra. For the GP, we perform time series forecasting for the joint multi-band fluxes with their corresponding central wavelengths, in order to include colour information. Here, we use a flat mean function and a stationary kernel Matern 3/2 for the flux form. GP is also used elsewhere in this work to interpolate data, and we then use the flux model from Villar et al. (2019) for the mean function.
In Fig. 4 we show the g-, r-and i-band LCs in absolute magnitudes together with the LCs of iPTF14hls from S19. The bottom part has an inset highlighting the first 200 days, which zooms in on the evolution of SN 2020faa. The magnitudes in Fig. 4 are in the AB system and have been corrected for distance modulus and MW extinction, and are plotted versus rest frame days past discovery.
The inset shows the remarkable similarity in absolute magnitude and timescale of the two SNe, whereas the full figure might be seen as a prediction for the future evolution of SN 2020faa. We will continue to follow the SN at best effort with ZTF, but report on these results already now to encourage also the community to keep an eye on the continued evolution of this transient. We note that with a declination of +72 degrees the source is well placed to be observed around the year from Northern observatories. No offset was applied to match the absolute magnitudes, they fall very well on top of each other anyway. Note that also no shift was applied in the time scales, we have plotted iPTF14hls since time of discovery, which supports a similar evolution also in this dimension. It is worth to note that the explosion date 9 for iPTF14hls was unconstrained by several months (A17), which made it more difficult to estimate for example total radiated energy for that SN. The comparison here makes it likely that iPTF14hls was not discovered very late after all.
Needless to say, the evolution is very different from that of normal SNe Type II, which was already demonstrated by A17 in the comparison of iPTF14hls to SN 1999em (see also Fig. 4). A Type II SN normally stays on a relatively flat plateau for about 100 days, and then quickly plummets to the radioactive decay tail. The rejuvenated long-timescale rise for SN 2020faa argues, as for iPTF14hls, that a different powering mechanism must be at play.
The colour evolution of SN 2020faa is shown in Fig. 5. We plot g − r in the upper panel and r − i in the lower panel, both corrected for MW extinction. In doing this, no interpolation was used. Given the excellent LC sampling we used only data where the pass-band magnitudes were closer in time than 0.1 days. Comparison is made with the colour evolution for iPTF14hls, but this SN was not covered at early phases. There is anyway evidence for similar colours, which argue against significant host extinction. We also compare the colours against a normal Type II SN 2013ej (Valenti et al. 2013), which is selected because it is also not suffering from host-galaxy reddening and has been well monitored in the gri photometric system. These photometric data were obtained via the Open Supernova Catalog. As shown, SN 2020faa is relatively bluer than the normal Type II SN in g−r colour space, whereas they are matching well in r − i. Fig. 4 Absolute magnitudes of SN 2020faa together with the LCs of iPTF14hls, as well as SNe 1999em and 2013ej. No scaling has been applied to match these SNe. The inset highlights the early evolution (exactly 200 days), which is where SN 2020faa demonstrates a striking similarity with the early iPTF14hls LCs.

Spectra
The log of spectroscopic observations was provided in Table 1 and the sequence of spectra is shown in Fig. 6. Overall, these are spectra of a typical Type II SN. We compare these with spectra from iPTF14hls. Note that the rise of iPTF14hls was not picked up immediately and therefore the first spectrum of that SN was only obtained more than 100 days past first detection. We were faster for SN 2020faa, i.e. the first P60 spectrum was obtained 6 days past discovery, and we can measure the evolution of the expansion velocity from 65 days past discovery.
These velocities are shown in Fig. 7, where we compare to iPTF14hls and to SN 1999em following the methodology of A17, see their fig. 3. We measured the velocities for SN 2020faa using iraf to fit a Gaussian to the minimum of the absorption lines of the corresponding P-Cygni profiles. The difference in rest wavelength between the minimum of the best-fit Gaussian to the line location was translated to an expansion velocity. Uncertainties in the velocities were estimated by a random sampling on the Gaussian minimum 1000 times by ±5Å, as in A17. The time evolution of the velocities measured for Hα, Hβ and for Fe ii λ5169 match well with those of iPTF14hls at the common epochs, but also extend to earlier phases. The velocities for the comparison are taken from A17. The striking characteristic of the time evolution for iPTF14hls was the very flat velocity evolution. We do not know (yet) if SN 2020faa will follow such a flat evolution, or if iPTF14hls had a faster evolution in the first 100 days.
The velocities for the first two SN 2020faa spectra are not presented in Fig. 7. The first P60 does not have enough signal, and there is no P-Cygni profile in the LT spectrum. However, according to the widths of the emission components of Hα and Hβ, we estimate the expansion velocities are ∼9000 km s −1 , consistent with those inferred later from the P-Cygni minimum.
A striking characteristic of iPTF14hls was its very slow spectral evolution, with the spectral phase inferred from comparing to typical SNe II such as SN 1999em being a factor of 6 − 10 younger than the true phase. To check whether the same is true for SN 2020faa, we follow A17 and use the spectral comparison code superfit (Howell et al. 2005) to estimate the phase of each spectrum against a library of SN II templates. The results are shown in Fig. 8, with the estimated spectral age plotted versus actual age. The evolution is remarkably flat, and in particular all the spectra taken during the rebrightening phase to date (50 − 150 days past discovery) match SN templates with phases 7 − 30 days past peak light. This is similar to what was seen in A17 (compare their Extended Data fig. 4), and thus, slow spectral evolution is another property that iPTF14hls and SN 2020faa have in common.

Bolometric lightcurve
In order to estimate a total luminosity radiative output, we attempted to construct a bolometric LC. We follow a similar Black-body (BB) approximation approach as done for iPTF14hls by A17, and for the early evolution probed here we have better photometric colour coverage to pursue this.
The result is shown in Fig. 9. The red squares show the luminosity of iPTF14hls (from A17, their Extended Data fig. 2). There was only enough colour information to fully construct this luminosity for iPTF14hls at later epochs. For SN 2020faa, we can use the gri coverage to estimate the luminosity before this as well, and see that those estimates connect nicely at 150 days post discovery. Using this, we can estimate a maximum bolo- Fig. 5 Colour evolution of SN 2020faa shown in g − r (upper panel) and r − i (lower panel), binned in 3 days. The colours have been corrected for MW extinction and are plotted in rest frame days relative to epoch of discovery. For comparison we have also plotted colours for iPTF14hls and for one normal Type II SN 2013ej, whose epochs are also provided in rest frame days since discovery. metric luminosity for SN 2020faa of L bol = 1.12 × 10 43 erg s −1 at 120.6 rest frame days and a total radiated energy over the first 162.4 rest frame days (until the end of observations presented in this paper, i.e. 2020 September 06) of E rad = 1.0 × 10 50 erg. This can be compared to the total radiative output of iPTF14hls which was E rad = 3.59 × 10 50 erg over 1235 days (S19). In that paper, the early bolometric of iPTF14hls was reconstructed, and that comparison is also shown in Fig. 9. Within the uncertainties, these are quite similar, the S19 early bolometric luminosity was estimated from the r-band data and a constant bolometric correction. Already the first 160 days of SN 2020faa cannot easily be powered by the mechanism usually responsible for a Type II SN LC -radioactive decay. Using, L = 1.45 × 10 43 exp(− t τ Co )( M Ni M ) erg s −1 from Nadyozhin (2003) implies that we would require a solar mass of 56 Ni to account for the energy budget. This is already out of the scope for the traditionally considered neutrino explosion mechanism (e.g., Terreran et al. 2017).
From the BB approximation we also obtain the temperature and the evolution of the BB radius. The radius evolution was an important clue to the nature of iPTF14hls in A17 (their fig. 4), and we therefore show a very similar plot in Fig. 10. The radius thus obtained is directly compared to the values for iPTF14hls and SN 1999em. We here also include the radius estimated from the spectroscopic velocities, estimated from the P-Cygni minima of the Fe ii λ5169 line.
The figure shows that the BB radius of SN 2020faa at the earliest phases are similar and evolve similarly to those of SN 1999em, and approach the values of the radius for iPTF14hls at 140 days. The vt radius on the other hand are higher for SN 2020faa, just as they were for iPTF14hls. We can see that they smoothly attach to the values for iPTF14hls.

Host galaxy
The results of the SED modeling (Sect. 2.6.2) of the host galaxy is displayed in Fig. 11. We obtain a good fit for a galaxy with a mass of 3.2×10 9 M and a star-formation rate of 0.6 M per year. This is a relatively regular host galaxy for a Type II SN. In Fig. 12 we compare the host mass with the distribution of host masses for SNe II from the PTF survey from Schulze et al. (2020). As can be seen, the host of SN 2020faa is a regular host galaxy in this respect, and is slightly more massive than the host of iPTF14hls, which is also illustrated in the figure.  Table 1. The epoch of the spectrum is provided to the right. For comparison we also show spectra of iPTF14hls in grey.

Host galaxy metallicity
Using the emission lines from Sect. 2.6.3, we can adopt the N2 scale of the Pettini & Pagel (2004) calibration using the flux ratio between [N ii] λ6583 and Hα. We found that 12 + log(O/H) = 8.50 ± 0.15. We also employed another metallicity diagnostic of Dopita et al. (2016)

Summary and Conclusions
We have presented SN 2020faa, a young sibling to the spectacular iPTF14hls. The first 150 days of the LC evolution is very different from a normal Type II supernova, and very similar to that of iPTF14hls. We therefore encourage continued monitoring of this transient to explore if it will evolve in a similar fashion, with LC undulations, longevity and a slow spectral evolution. From the observations already in hand, we can conclude that just as for iPTF14hls the energy budget is already too high to be driven by a standard radioactivity scenario. The plethora of other powering mechanisms needs to be dusted off again, to explain the evolution of SN 2020faa.
ZTF will continue operations as ZTFII, with more discoveries in sight. Several community brokers are already processing the data in real time and more activity is foreseen as we come closer to the era of the Vera Rubin telescope. The broker Alerce (Förster et al. 2020) is an example where a combination of computer filtering and human inspection already provides early alerts for infant SNe. We also need to keep an eye on SN LCs that behave in unusual and interesting ways also at later stages. This includes re-brightenings as for SN 2020faa here or due to late CSM interaction as in Sollerman et al. (2020), but could also be rapid declines or undulations, as in iPTF14hls. Hitherto most of these have been found by human scanners reacting to a 'funny' LC. This will unlikely be the case in the Rubin era. Type II SN best-fit phase (rest-frame days) SEDM LT NOT Fig. 8 Phases estimated by comparison to superfit templates are plotted versus rest frame days since first detection for SN 2020faa. The overall spectral evolution revealed by these comparisons is very slow and even at more than 100 days the best matches are with younger Type II SNe. This is similar to what was found by A17 for iPTF14hls, which continued to display slow evolution for 600+ days. Fig. 9 Luminosity of SN 2020faa after accounting for MW extinction, distance and integrating a BB fit to the gri photometry. A similar method was used for iPTF14hls which only had colour data past 150 days, and we can see that the early time emission of SN 2020faa nicely merges with the late time luminosity for iPTF14hls. The GP fit on the joint LCs of SN 2020faa and iPTF14hls is shown as a black dashed line. In green we show the luminosity estimate for iPTF14hls from S19 up to 300 days, which assumed a constant bolometric correction at early times. Fig. 10 Evolution of the the radius as a function of time for SN 2020faa (binned in 10 days), as compared to the extraordinary iPTF14hls and the regular Type II SN 1999em. This figure closely follows the presentation from A17, their fig. 4, and shows estimates for the radius evolution from two different methods for the three different SNe. A main theme in A17 was that for iPTF14hls, the radius evolution estimated from the BB approximation and the radius estimated from the spectroscopic velocities were different and diverged with time. Normalised distribution function SN 2020faa iPTF14hls Fig. 12 The host-galaxy mass of SN 2020faa and iPTF14hls in the context of host galaxies for SNe II from the PTF and iPTF survey (as presented by Schulze et al. 2020).