TITLE:   GCN CIRCULAR
NUMBER:  30487
SUBJECT: GRB 210722A: Redshift from OSIRIS/GTC
DATE:    21/07/23 17:50:29 GMT
FROM:    C. C. Thoene at HETH/IAA-CSIC  <christina.thoene@gmail.com>

C. C. Thoene (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC,
DARK/NBI),  D. A. Kann, M. Blazek, J. F. Agui Fernandez (all
HETH/IAA-CSIC), J. P. U Fynbo (DAWN/NBI), L. Izzo (DARK/NBI), N. R. Tanvir
(U. Leicester) and N. Castro (GTC) report:

We obtained spectroscopy of the afterglow of GRB 210722A (D’Avanzo et al.
GCN #30475, Page et al. GCN #30483, Zhu et al. GCN #30476) with OSIRIS
mounted on the 10.4 m GTC telescope at the Roque de los Muchachos
observatory (La Palma, Spain). Observations started at 05:02:09 UT with a
mid-time of 05:17:30 UT (8.38 hr after the burst) and consisted of 2x900s
spectra taken with grism R1000B, which covers the spectral range between
3700 and 7800 AA. We also obtained an acquisition image of 60s in r'-band
at 04:58:25 UT. Observing conditions were rather bad with a seeing of 1.7
arcsec and taken during morning twilight.

We detect several weak absorption lines, consistent with the MgII doublet,
FeII 2344, 2374, 2383, 2587 and 2600 at a common redshift of z=1.145. The
lines are in the weakest 1.7% of the average GRB absorption line strength
(de Ugarte Postigo et al. 2012, A&A 548, 11). The detection of the
afterglow in all UVOT filters (Siegel et al. GCN #30482) is consistent with
this redshift.

The acquisition image shows the afterglow to be r’ = 18.92 +/- 0.03
compared to 7 SDSS field stars.