TITLE:   GCN CIRCULAR
NUMBER:  30182
SUBJECT: GRB 210610B: NOT optical observations and tentative redshift
DATE:    21/06/10 23:35:42 GMT
FROM:    Daniele B Malesani at DTU Space  <malesani@space.dtu.dk>

J. P. U. Fynbo (DAWN/NBI), L. Izzo (DARK/NBI), A. de Ugarte Postigo 
(HETH/IAA-CSIC), D. B. Malesani (DTU Space), T. Pursimo (NOT), report on 
behalf of a larger collaboration:

We observed the optical afterglow of GRB 210610B (Page et al., GCN 
30170; Kumar et al., GCN 30174; Rumyantsev et al., GCN 30175; Hu et al., 
GCN 30177; de Wet et al., GCN 30180; Romanov et al., GCN 30181) with the 
Nordic Optical Telescope (NOT) equipped with the ALFOSC imager.

The optical afterglow is well detected, and we measure for it the 
following coordinates (J2000, against the Gaia reference system):

RA = 16:15:40.38
Dec = +14:23:56.6

Our observation was carried out with a clear filter. Calibrating the 
photometry against the r-band magnitudes of nearby Pan-STARRS point 
sources, we get an AB magnitude r = 17.3 +- 0.1, at a mean epoch of 1.69 
hr after the GRB.

Two spectra by 600 s each were obtained using grism #4, covering the 
wavelength range 3600-9400 AA. The continuum is detected at high S/N. 
While there are no strong features, we can identify the Mg II doublet 
and Mg I at a redshift of z = 1.13. We propose this value as the 
tentative redshift of GRB 210610B.