GCN Circular 28764
Subject
GRB 201024A: Redshift from GTC/OSIRIS
Event
Date
2020-10-24T04:28:55Z (6 years ago)
From
Alexander Kann at IAA-CSIC <kann@iaa.es>
A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann, M. Blazek,
J. F. Agui Fernandez, C. C. Thoene (all HETH/IAA-CSIC), S. Geier, and M.
Rivero (both GRANTECAN) report:
We observed the afterglow (Lipunov et al., GCN#28760, Marshall et al.,
GCN#28761, Fernandez-Garcia et al., GCN#28763) of GRB 201024A (Marshall
et al., GCN#28761) with the 10.4m GTC telescope, at Roque de los
Muchachos Observatory (La Palma, Spain) equipped with OSIRIS. The
observation started on 24 October 2020 at 03:28:47 UT (0.663 hrs after
the GRB onset) and consisted of 3 x 900 s with the R1000B grism,
covering the wavelength range between 3700 and 7800 AA. Observations
were taken under good conditions but high airmass.
At the afterglow position, a clear source is detected. We measure r' ~
19.3 mag (AB) vs. nearby PanSTARRS field stars. This indicates the
source has faded rapidly vs. the early MASTER/BOOTES detection, as
Lipunov et al., GCN#28760 and Fernandez-Garcia et al., GCN#28763 stated.
A preliminary reduction of the spectrum shows a clear continuum, with
MnII, FeII, FeII*, MgII, and MgI absorption lines, as well as the [OII]
doublet in emission, all at a common redshift of z = 0.999, which we
identify as the redshift of the GRB.
We furthermore note that there is an underlying galaxy at the OT
position visible in SDSS and PanSTARRS, at r ~ 21.15 mag. We also see a
second galaxy offset by a few arcsec in our finding chart.
We thank P. D'Avanzo and L. Izzo for pointing out the galaxy.