GCN Circular 29077

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
Not found
 
 
), 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.