TITLE:   GCN CIRCULAR
NUMBER:  29100
SUBJECT: GRB 201221A: VLT/X-shooter afterglow discovery and spectroscopic redshift z = 5.7
DATE:    20/12/21 13:16:00 GMT
FROM:    Daniele B Malesani at DTU Space  <malesani@space.dtu.dk>

D. B. Malesani (DTU Space), J.-B. Vielfaure (APC, Paris University), L. 
Izzo (DARK/NBI), S. D. Vergani (GEPI, Observatoire de Paris), D. 
Burgarella (AMU, CNRS, CNES, LAM), V. D'Elia (ASI/SSDC), A. de Ugarte 
Postigo (HETH/IAA-CSIC, DARK/NBI), J. P. U. Fynbo (DAWN/NBI), A. J. 
Levan (Radboud Univ.), B. Milvang-Jensen (DAWN/NBI), D. A. Kann 
(HETH/IAA-CSIC), E. Palazzi (INAF/OAS), G. Pugliese (API, Univ. 
Amsterdam), N. R. Tanvir (Univ. Leicester), K. Wiersema (Univ. Warwick), 
report on behalf of the Stargate consortium:

We observed the field of GRB 201221A (Page et al., GCN 29096) using the 
ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.

Using the acquisition camera, multi-band imaging was secured. Within the 
XRT error circle (Evans, GCN 29097), no source is detected in an r-band 
image (taken at mean epoch of 42 min after the GRB) down to limiting 
magnitude r > 23.5 (AB). However, a source is clearly visible in the 
z-band image with z = 19.76 +- 0.04 AB (mean time 35 min after the GRB). 
The corresponding color r-z > 3.7 is exceptionally red. The coordinates 
of the object are (J2000, 0.3" error):

RA = 14:17:55.26
Dec = -45:24:58.0

Spectroscopy of the z-band source was executed covering the wavelength 
range 3000-25000 AA. Unfortunately due to visibility constraints, only 
two spectra by 600 s each were secured, plus two extra exposures during 
twilight which have lower quality. We report on the analysis of the 
first set only, which has a mean time of 1.65 hr after the GRB.

Continuum is well detected in the infrared arm, as well as in the red 
part of the visible arm. A clear break is detected around 8150 AA, 
which, if interpreted as the Lyman break, corresponds to z = 5.70. Weak 
continuum is detected blueward of the break, until it vanishes 
completely at the corresponding location of the Lyman limit. We thus 
conclude that z = 5.70 is the redshift of GRB 201221A.

Further analysis is undergoing.

We acknowledge support from the ESO observing staff at Paranal, in 
particular Zahed Wahhaj.