GCN Circular 29100
Subject
GRB 201221A: VLT/X-shooter afterglow discovery and spectroscopic redshift z = 5.7
Date
2020-12-21T13:16:00Z (3 years ago)
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.