GCN Circular 29655
Subject
GRB 210312B: Redshift from OSIRIS/GTC
Event
Date
2021-03-13T13:13:31Z (4 years ago)
From
Alexander Kann at IAA-CSIC <kann@iaa.es>
D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC,
DARK/NBI), C. C. Thoene, M. Blazek, J. F. Agui Fernandez (all
HETH/IAA-CSIC), and R. Scarpa (GTC) report:
We observed the afterglow (Jelinek et al., GCN #29651; Lipunov et al.,
GCN #29652) of INTEGRAL GRB 210312B (Mereghetti et al., GCN #29650) with
OSIRIS at the 10.4m Gran Telescopio Canarias (Roque de los Muchachos
Observatory, La Palma, Canary Islands, Spain), starting at 2021-03-12
22:57:10.834 UT under adverse conditions (2" seeing, bad transparency
because of Calima). Following the finding chart exposure (Kann et al.,
GCN #29653), we obtained 2 x 900 s spectroscopic exposures with the
R1000B grism, covering the spectral range from 3700 to 7800 AA, before
the telescope was shut down due to worsening observing conditions. We
detect a faint trace in both images. We resumed observations after an
improvement in weather conditions at 2021-03-13 01:41:40.281 UT,
obtaining two further finding charts and 4 x 900 s spectra with the same
grism.
Using the same comparison star as Kann et al., GCN #29653, we find the
afterglow has faded to r' = 22.36 +/- 0.03 mag (AB) at 4.8316 hrs after
the GRB trigger.
Combining the first spectral exposure and the latter, for a total
exposure of 5 x 900 s, a trace is clearly detected redwards of 4300 AA,
as well as several absorption lines. We identify these as FeII, MgII,
and MgI, at a mean redshift of z = 1.069. At this redshift, we also
detect OII in emission, which allows us to identify this as the redshift
of the GRB.