[vsnet-grb-info 27552] GRB 210312B: Redshift from OSIRIS/GTC

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29655
SUBJECT: GRB 210312B: Redshift from OSIRIS/GTC
DATE:    21/03/13 13:13:31 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann at 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.