GCN Circular 31324
Subject
GRB 211227A: A possible short GRB with extended emission at z = 0.228?
Event
Date
2021-12-28T12:22:53Z (4 years ago)
From
Daniele B Malesani at Radboud U <d.malesani@astro.ru.nl>
D. B. Malesani (Radboud Univ. and DAWN/NBI), L. Izzo (DARK/NBI), D. Xu
(NAOC/CAS), A. de Ugarte Postigo (Obs. de la Cote d'Azur), D. A. Kann
(IAA/CSIC), A. J. Levan (Radboud Univ.), Z.P. Zhu (NAOC, HUST), S.Y. Fu
(NAOC), report on behalf of the Stargate collabaration:
We observed the location of GRB 211227A (Beardmore et al., GCN 31316)
using the ESO VLT UT3 (Melipal) equipped with the X-shooter
spectrograph. Short images were secured with the acquisition camera in
the g, r, and z filters. No new object is detected inside the revised
XRT error circle (Goad et al., GCN 31322) down to an AB magnitude r >
24.7 (mean time Dec 28.191 UT).
The galaxy mentioned by Fu et al. (GCN 31320) is well detected in our
image. Its core lies 3.66" away of the center of the XRT circle (2.4"
90% error radius), though its wings extend towards it.
A spectrum of this object was secured, covering the wavelength range
3000-21000 AA, and consisting of 4 exposures of 600 s each. The
observation mid time was 2021 Dec 28.21 UT (5.60 hr after the GRB). We
detect several emission lines, which we interpret as due to [O II],
Hbeta, [O III] 5007, Halpha, [N II], [S II] all at a common redshift z =
0.228. Absorption features due to Ca II 3933/3968 and the Na I D doublet
5890/5896 are also visible at the same redshift. This value is
consistent with the SDSS photometric redshift. The galaxy is fairly
bright, with an absolute magnitude M_r ~ -21.3 (AB) using i = 18.84 (AB)
from SDSS.
We point out the similarity of the gamma-ray light curve of GRB 211227A
(e.g. https://gcn.gsfc.nasa.gov/notices_s/1091101/BA/) with that of GRB
060614 (e.g. Gehrels et al. 2006, Nature, 444, 1044), suggested by some
authors to be a merger event (e.g. Yang et al. 2015, Nat. Comm., 6,
7323). The location of the GRB, slightly offset from a bright, nearby
galaxy, would also be consistent with this interpretation. We raise the
possibility that GRB 211227A is a short GRB with extended emission.
However, a long GRB, either hosted by the z = 0.228 galaxy, or in the
background, remains a viable possibility.
We encourage further observations of this potentially interesting event
at all wavelengths.
We acknowledge excellent support from the ESO observing staff in
Paranal, in particular Israel Blanchard, Robert De Rosa, and Fuyan Bian.