GCN Circular 31235
Subject
GRB 211211A - Gemini K-band detection
Date
2021-12-15T21:19:50Z (2 years ago)
From
Andrew Levan at U.of Leicester <a.levan@astro.ru.nl>
A.J. Levan (Radboud), J. Rastinejad (Northwestern), B. Gompertz (Birmingham), W. Fong (Northwestern), D. B. Malesani (Radboud), M. Nicholl (Birmingham) report for a larger collaboration:
���We obtained K-band observations of GRB 211211A (D���Ai et al., GCN 31202) with the Gemini-North Telescope and NIRI. Observations began at 14:40 UT on 15 Dec 2021 (~4 days after the GRB). A total of 900 s of exposure were obtained.
At the location of the afterglow identified by Zheng & Fillipenko (GCN 31203) we clearly detect a K-band source. Photometry is complicated by the narrow field of view, however, using the 2MASS source at RA, DEC(J2000)=14:09:08,86, 27:53:54.4 with a magnitude of K=12.8, we determine an afterglow magnitude of K = 20.5 +/- 0.1 (Vega) or 22.4 +/- 0.1 (AB). This magnitude is substantially brighter than expected for afterglow emission given the rapid decay observed in the i-band (de Ugarte Postigo et al., GCN 31229), and the relatively blue colours and spectra of the source at earlier times (Malesani et al., GCN 31221; Belles & D���Ai, GCN 31222).
The rapid fading and relative faintness in the optical apparently rules out a classical long-GRB at z = 0.076, since a supernova akin to those seen in long GRBs at this epoch would have i ~ 20, 4 magnitudes brighter than observed by de Ugarte Postigo et al. (GCN 31229). Alternatively, the prompt light curve appears to show some similarities with GRB 060614, another suggested short-GRB with extended emission (Gehrels et al. 2006 Nature 444 1044). If GRB 211211A is associated with a compact object merger, its large offset relative to the putative host at z = 0.076 may be expected. The absolute magnitude of the source seen in the K-band is -15.2 (AB), comparable to that of AT2017gfo at the same epoch. Hence, while it remains possible that GRB 211211A is a higher redshift event in chance projection with a low redshift galaxy, a compact binary merger at z=0.076 provides a good explanation of the galactic location, rapid optical fading and red colour of the source.
We thank the Gemini staff for the rapid execution of these observations."