GCN Circular 31480
Subject
GRB 220117A: VLT/X-shooter optical afterglow and redshift
Date
2022-01-18T12:25:23Z (3 years ago)
From
Daniele B Malesani at Radboud U <d.malesani@astro.ru.nl>
J. Palmerio (CNRS, GEPI - Paris Observatory), D. B. Malesani (Radboud
Univ. and DAWN/NBI), J. P. U. Fynbo (DAWN/NBI), D. Xu (NAOC), L. Izzo
(DARK/NBI), S. Y. Fu (NAOC), Z. P. Zhu (NAOC, HUST), D. A. Kann
(IAA/CSIC), N. R. Tanvir (Univ. Leicester) report on behalf of the
Stargate collaboration:
We observed the field of GRB 220117A (Melandri et al., GCN 31466) using
the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our
first observations were carried out using the acquisition camera, using
the g, r and z filters (3x60 s in each band). Inside the XRT error
circle (Beardmore et al., GCN 31472), we detect a new source, not
visible in the archival Pan-STARRS images, which we identify as the GRB
afterglow. The source is very red, being detected strongly in z, only
weakly in r, and completely missing in g. Calibrated against nearby
stars from the Pan-STARRS catalog, we measure z = 20.81 +- 0.12 mag and
z - r ~ 2.1 mag (all AB magnitudes). The afterglow coordinates are
(J2000, 0.2" error):
RA = 06:06:17.409
Dec = -28:26:15.78
A spectrum was secured covering the wavelength range 3000-21000 AA, and
consisting of 4 exposures of 1200 s each. The observation mid time was
2022 Jan 18.15 UT (11.3 hr after the GRB).
In a preliminary reduction, continuum is detected in the red part of the
spectrum down to ~7260 AA. We interpret the break as due to Lyman alpha.
The absorption system is rather weak and, at the present time, we
identify with confidence only the Lyman-alpha trough and the Si II 1260
AA feature, which correspond to a redshift z = 4.961 for GRB 220117A.
We acknowledge excellent support from the ESO observing staff in
Paranal, in particular Michael Abdul-Masih, Diego Parraguez, and
Jonathan Smoker.