GCN Circular 29077
Subject
GRB 201216C: VLT X-shooter spectroscopy and potential high redshift of a VHE-emitting GRB
Date
2020-12-17T22:12:41Z (4 years ago)
From
Alexander Kann at IAA-CSIC <kann@iaa.es>
J.-B. Vielfaure (APC, Paris University), L. Izzo (DARK/NBI), D. Xu
(NAOC), S. D. Vergani (GEPI, Observatoire de Paris), D. B. Malesani (DTU
Space), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), V. D'Elia
(ASI/SSDC, INAF/OAR), J. P. U. Fynbo (DAWN/NBI), D. A. Kann
(HETH/IAA-CSIC), A. J. Levan (Radboud U. Nijmegen), G. Pugliese (API,
Univ. Amsterdam), N. R. Tanvir (Univ. Leicester), D. Burgarella (AMU,
CNRS, CNES, LAM), and A. Rossi (INAF-OAS) report on behalf of the
Stargate Consortium:
We obtained spectroscopic observations of the optical counterpart (Izzo
et al., GCN #29066, Jelinek et al., GCN #29070) of the MAGIC-detected
(Blanch et al., GCN #29075) GRB 201216C (Beardmore et al., GCN #29061,
Malacaria et al., GCN #29073, Nadella et al., GCN #29074) with the ESO
Very Large Telescope UT 3 (Melipal) equipped with the X-shooter
spectrograph, covering the wavelength range 3200-22000 AA. Observations
started at 01:30 UT on 2020-12-17, 2.38 hr after the burst, and
consisted of 4 exposures of 600 s each.
The afterglow is well-detected in the stacked spectrum, but the
continuum is very red. As a consequence, the S/N drops dramatically from
the red to the blue end. We identify a doublet which we tentatively
match to Ca II H & K at z = 1.10. Unfortunately, no other lines are
detected to confirm this value, though most of them would fall in a
spectral region of poor S/N. We note that Ca II H&K absorption is
uncommon in intervening absorbers, making it likely this is the actual
redshift of the GRB, and that it does not lie at an even greater
distance (in accordance with the VHE detection).
A redshift of z = 1.1 would place this object among the most distant
known VHE sources. Using the Fermi GBM parameters (Malacaria et al., GCN
#29073), we derive a an observer-frame 10-1000 keV isotropic energy
release of E_iso = (4.71 +/- 0.16) * 10^53 erg.
Based on our grz photometry (Izzo et al., GCN 29066), we measure a
spectral slope beta_opt = 4.1 +- 0.2 (Fnu propto nu^-beta), which is an
unusually red value, suggesting significant extinction. This is
confirmed by the optical-to-X-ray spectral index, beta_OX ~ 0.1 which
indicates a very low optical/X-ray flux ratio, making this a bona fide
dark GRB.
We acknowledge excellent support from the ESO observing staff in
Paranal, in particular, Diego Parraguez, Bin Yang, and Zahed Wahhaj.