GRB 221009A/SN 2022xiw: A Supernova Obscured by a Gamma-Ray Burst Afterglow?

The connection between long-duration gamma-ray bursts (LGRBs) and broad-line Type Ic supernovae (SNe Ic-BL) has been established — the “GRB-SN connection” (e.g., Woosley & Bloom, 2006; Hjorth & Bloom, 2012; Cano et al., 2017). Since the discovery of GRB 980425/SN 1998bw (Galama et al., 1998), dozens of GRB-SNe have been observed, but relatively few were nearby (redshift $z<0.2$): GRB 980425/SN 1998bw ($z=0.00867$; Galama et al., 1998), GRB 030329/SN 2003dh ($z=0.16867$; Stanek et al., 2003), GRB 031203/SN 2003lw ($z=0.10536$; Malesani et al., 2004), GRB 060218/SN 2006aj ($z=0.03342$; Pian et al., 2006), GRB 100316D/SN 2010bh ($z=0.0592$; Chornock et al., 2010), GRB 130702A/SN 2013dx ($z=0.145$; D’Elia et al., 2015), GRB 161219B/SN 2016jca ($z=0.1475$; Tanvir et al., 2016; de Ugarte Postigo et al., 2016), GRB 171205A/SN 2017iuk ($z=0.0368$; Izzo et al., 2019), GRB 180728A/SN 2018fip ($z=0.117$; Izzo et al., 2018), and GRB 190829A/SN 2019oyw ($z=0.0785$; Valeev et al., 2019). However, some nearby LGRBs show no evidence of SN emission down to very deep limits: GRB 060505 ($z=0.089$; Fynbo et al., 2006), GRB 060614 ($z=0.1254$; Fynbo et al., 2006; Della Valle et al., 2006), and GRB 111005A ($z=0.01326$; Tanga et al., 2018; MichałowskI et al., 2018). Also, GRB 211211A ($z=0.0763$; Rastinejad et al., 2022; Troja et al., 2022) and the recent case of GRB 230307A ($z=0.065/3.87$; Levan et al., 2024, which in terms of flux is only second to GRB 221009A) have instead been associated with potential kilonova emission.

On 2022 October 9, at 13:16:59 (UTC dates are used herein), the Fermi Gamma-Ray Burst Monitor (GBM; Meegan et al., 2009) onboard the Fermi Gamma-ray Space Telescope (FGST) triggered an extraordinarily bright LGRB, GRB 221009A (Veres et al., 2022). About 53 min later, at 14:10:17, the Burst Alert Telescope (BAT; Barthelmy et al., 2005) onboard the Neil Gehrels Swift Observatory also triggered GRB 221009A, but it was named Swift J1913.1+1946 (Dichiara et al., 2022). The Swift X-Ray Telescope (XRT; Burrows et al., 2005) began observing the GRB at 14:13:09, 172 s after the BAT trigger, and the Ultraviolet and Optical Telescope (UVOT; Roming et al., 2005) began at 179 s (Williams et al., 2023). GRB 221009A is a nearby event, with a very low redshift $z=0.151$ (de Ugarte Postigo et al., 2022b; Castro-Tirado et al., 2022; Izzo et al., 2022). As GRB 221009A optical afterglow observations continued, photometric evidence for an SN appeared (Belkin et al., 2022a, b). A few days later, Maiorano et al. (2022) announced the results of follow-up spectroscopy of the afterglow of GRB 221009A, confirming the emerging contribution of SN 2022xiw (Postigo et al., 2022) as reported by de Ugarte Postigo et al. (2022a). Moreover, Fulton et al. (2023) presented extensive optical photometry of the afterglow of GRB 221009A, showing evidence for emission from an accompanying SN. Srinivasaragavan et al. (2023) also presented their optical photometry and announced that they found an SN component. However, Shrestha et al. (2023) did not find SN signatures in their imaging and spectroscopy, and Levan et al. (2023) failed to see significant evidence for SN emission in their James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A. But late-time JWST observations possibly suggest some specific SN spectral features associated with GRB 221009A. In particular, a close match with SNe Ic-BL suggests the presence of a typical GRB-SN in the spectrum (Blanchard et al., 2023, 2024).

Here we present our photometric follow-up observations of GRB 221009A. We analyze the optical afterglow of GRB 221009A by fitting the $R$-band light curve with a smooth broken power-law plus ${}^{56}\mathrm{Ni}$ model. In addition, we compare GRB 221009A with other GRB-SN events based on a GRB-associated SN sample. This paper is organized as follows. Section 2 presents our observations and data reduction, while Section 3 shows the analysis and results. Our conclusions and implications are presented in Section 4. Throughout, we adopt a concordance cosmology with parameters $H_{0}=69.3\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, $\Omega_{\mathrm{M}}=0.286$, $\Omega_{\mathrm{\Lambda}}=0.714$.

The afterglow of GRB 221009A was observed by many ground-based telescopes including our GWAC telescopes. The GWAC system is an optical transient survey located at Xinglong Observatory, China; it includes two 60 cm optical telescopes (GWAC-F60A/B), as one of the main ground-based facilities of the Space-based Multi-band Astronomical Variable Objects Monitor (SVOM)¹¹1SVOM is a China–France satellite mission dedicated to the detection and study of GRBs. mission (Wei et al., 2016). GWAC-F60 began observing the afterglow of GRB 221009A at 13:16:59, 22.04 hr after the GBM trigger, with a set of $R$-band images. In addition, we performed follow-up observations on October 14 and 16, but the optical afterglow was not detected in the stacked image. LCOGT (Las Cumbres Observatory Global Telescope Network; Brown et al., 2013) began observing GRB 221009A about 7.73 hr after the GBM trigger; $R$-band images were obtained with the 1 m Sinistro instrument at the Teide Observatory on Tenerife and the 1 m Sinistro instrument at McDonald Observatory, Texas, USA. $B$, $V$, $R$, and $I$ images of GRB 221009A were also obtained with the 1 m Nickel telescope at Lick Observatory (Vidal et al., 2022), and additional $Clear$-band images were obtained with the Lick 0.76 m Katzman Automatic Imaging Telescope (KAIT; Filippenko et al., 2001).

Point-spread-function (PSF) photometry was performed using DAOPHOT (Stetson, 1987) from the IDL Astronomy Users Library²²2http://idlastro.gsfc.nasa.gov/. Several nearby stars were chosen from the Pan-STARRS1³³3http://archive.stsci.edu/panstarrs/search.php catalog for calibration; their magnitudes were transformed into the Landolt (Landolt, 1992) magnitudes using the empirical prescription presented by Eq. 6 of Tonry et al. (2012).

The photometry results are corrected for Galactic extinction with $E(B-V)=1.36$ mag (Schlafly & Finkbeiner, 2011) for analysis. Owing to large uncertainties, we do not make corrections for the extinction in the GRB host galaxy. We report the original photometry from LCOGT, GWAC-F60, KAIT, and Nickel follow-up observations in Table 1.

We collected additonal photometry data for our analysis from Williams et al. (2023), Shrestha et al. (2023), Laskar et al. (2023), Srinivasaragavan et al. (2023) and Gamma-ray-burst Coordinates Network (GCN) Circulars (Broens, 2022; Hu et al., 2022; Belkin et al., 2022d; de Wet et al., 2022; Xu et al., 2022; Brivio et al., 2022; Durbak et al., 2022; Paek et al., 2022; Romanov, 2022a; Chen et al., 2022; Kim et al., 2022; Groot et al., 2022; Romanov, 2022b; Belkin et al., 2022c; Watson et al., 2022; Strausbaugh & Cucchiara, 2022; Butler et al., 2022; Vinko et al., 2022; Mao et al., 2022; Zaznobin et al., 2022; Sasada et al., 2022; O’Connor et al., 2022a; Bikmaev et al., 2022a; O’Connor et al., 2022b; Bikmaev et al., 2022b; Schneider et al., 2022; D’Avanzo et al., 2022; Huber et al., 2022; Shresta et al., 2022; Izzo et al., 2022; Belkin et al., 2022a; Rajabov et al., 2022; Im et al., 2022; Ferro et al., 2022; Rossi et al., 2022; Gupta et al., 2022; Belkin et al., 2022b; Pellegrin et al., 2022; O’Connor et al., 2022c; Aguerre et al., 2022). XRT data were downloaded from the UK Swift Science Data Center at the University of Leicester (Evans et al., 2009) ⁴⁴4https://www.swift.ac.uk/xrt_curves/01126853/. Figure 1 shows the multiband light curves of the afterglow in both optical and X-ray bands.

We have presented multiwavelength observations of the extraordinary GRB 221009A, spanning about 19 days in time. A weak bump, possibly from a supernova, emerges from the declining afterglow at $\sim 11$ days in the $R$-band light curve. Here we summarize our results.

1. 1.

   We used a smooth broken power law plus ${}^{56}\mathrm{Ni}$ model to fit the $R$-band light curve. The best-fitting results reveal that the SN ejected a total mass of $M_{\mathrm{ej}}=3.70\,M_{\odot}$, a ${}^{56}\mathrm{Ni}$ mass of $M_{\mathrm{Ni}}=0.23\,M_{\odot}$, and a total kinetic energy of $E_{\mathrm{SN,K}}=2.35\times 10^{52}\,\mathrm{erg}$. We estimate the energy partition $\eta=0.11$. In addition, we used a single-decline-rate power law, $f(t)\propto t^{-1.56}$, to describe multiband light curves, and found no significant SN signal in all bands.

2. 2.

   We compared GRB 221009A with other GRB-SN events based on a GRB-associated SN sample, finding that the correlations among SN parameters of GRB 221009A are consistent with other GRB-SN events. It is noteworthy that both the $\gamma$-ray energy and SN kinetic energy are large. We set these SNe in our sample at the distance of GRB 221009A, and find that only SN 2003lw and SN 2011kl can be obviously detected in the afterglow of GRB 221009A.

Owing to the limited dataset and the bright afterglow, it is difficult to study the SN signals and give good constraints. Focusing on our fitting results, on the one hand, we estimate the absolute AB mag of the SN peak to be $M_{R}=-19.21$ mag, close to the value of $M_{r}=-19.4\pm 0.3$ mag that Fulton et al. (2023) measured, and also consistent with the limit $M_{r}>-19.5$ mag that is given by Shrestha et al. (2023). On the other hand, de Ugarte Postigo et al. (2022a) announced spectroscopic detection of emerging SN features $\sim 8$ days after the burst. Maiorano et al. (2022) also reported spectroscopic confirmation of an SN in LBT spectra at $t\approx 8.56$ days. Our peak time of $(1+z)t_{\mathrm{peak}}=12.73$ days is close to these values. In general, at the peak brightness of the SN, its spectral features are most obvious. Shrestha et al. (2023) suggested that the absolute magnitude of the associated SN would have to be brighter than $M_{r}=-20.66$ mag after correcting for the extinction of $E(B-V)=1.32$ mag, for them to detect SN bumps during that time period. But from our light-curve analysis, we suggest a weak bump emerges from the declining afterglow during that time period, and it is most visible at $t\approx 11$ days.

In addition, Shrestha et al. (2023) discussed that it is possible an associated SN was below their detection limit if they consider the high extinction. Although Levan et al. (2023) reported that they do not see significant evidence for SN emission in their observations, they do not discard the possibility that an event somewhat less luminous than SN 1998bw (and perhaps somewhat faster evolving or bluer) could simply have evaded detection in their observations. Levan et al. (2023) also reported that the optical to mid-infrared (0.6–12 $\mu$m) SED shows little evidence for variability from early to late times (0.5–55 days), but late-time JWST observations show that the G140M+G235M spectrum significantly differs from a power-law continuum 13 days after the burst (Blanchard et al., 2023). The late-time spectroscopy and photometry are well described by an SN and power-law afterglow. The close match with SNe Ic-BL in particular demonstrates the presence of a typical GRB-SN in the spectrum. These observations provide the first clear detection of an SN associated with GRB 221009A (Blanchard et al., 2024). Moreover, Srinivasaragavan et al. (2023) also announced that they found moderate evidence for the presence of an additional component arising from an associated SN and found that it must be substantially fainter than SN 1998bw (see more details in the review by Srinivasaragavan et al., 2023). Thus, our $R$-band light light is likely to contain contributions from an SN (named SN 2022xiw; Postigo et al., 2022).

Both the light-curve analysis and comparative analysis imply that the SN is not faint. However, why can we not obviously detect the SN signal? Shrestha et al. (2023) mentioned that the nondetection of an SN from GRB 221009A may be because most of the energy is carried by the relativistic jet, not the bulk ejecta. But we estimated the energy partition $\eta$ to be only 0.032, consistent with that inferred for most other GRB-SNe. Therefore, we suggest that the energy partition is not the primary reason why we cannot detect significant SN signal.

When we set the redshift of SNe in our sample to be the same as that of GRB 221009A, we find that most of them are obscured by the afterglow of GRB 221009A. Only two SNe can be obviously detected: SN 2003lw with $t_{\mathrm{peak}}=21.5\pm 3.5\,\mathrm{days}$, and SN 2011kl with $M_{\mathrm{peak}}=-19.8\pm 0.1$ days. So, we suggest that an SN can be detected in the GRB afterglow in GRB-SN events either if it appears very late (when the afterglow has faded) or is very luminous. In the case of GRB 221009A, the extraordinarily bright afterglow is likely a reason why the SN was not detected by several observers.

In conclusion, we suggest that SN 2022xiw is possibly associated with GRB 221009A, but the SN emission is largely obscured by the afterglow of GRB 221009A. Our analysis shows that the SN is bright, but the GRB afterglow is even brighter.

This work is supported by the National Natural Science Foundation of China (grant Nos. 12373042, U1938201, 12133003 and 12273005), China Manned Spaced Project (CMS-CSST-2021-B11), the Bagui Scholars Programme (W.X.-G.) and the Guangxi Science Foundation the National (grant No. 2023GXNSFDA026007). A.V.F.’s group at U.C. Berkeley is grateful for financial assistance from the Christopher R. Redlich Fund, Gary and Cynthia Bengier, Clark and Sharon Winslow, Alan Eustace (W.Z. is a Bengier-Winslow-Eustace Specialist in Astronomy), and many other donors. KAIT and its ongoing operation were made possible by donations from Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the U.S. NSF, the University of California, the Sylvia & Jim Katzman Foundation, and the TABASGO Foundation. We thank the staff at Lick Observatory for their assistance. Research at Lick Observatory is partially supported by a generous gift from Google.