TITLE:   GCN CIRCULAR
NUMBER:  28764
SUBJECT: GRB 201024A: Redshift from GTC/OSIRIS
DATE:    20/10/24 04:28:55 GMT
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.