TITLE: GCN CIRCULAR NUMBER: 29450 SUBJECT: GRB 210210A: Redshift from OSIRIS/GTC DATE: 21/02/10 11:24:24 GMT FROM: Alexander Kann at IAA-CSIC A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann, C. C. Thoene, M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC), and D. Garcia (GRANTECAN) report: We observed the afterglow of GRB 210210A (Laha et al., GCN #29444, Lipunov et al., GCN #29447) with OSIRIS at the 10.4m Gran Telescopio Canarias (Roque de los Muchachos Observatory, La Palma, Canary Islands, Spain), starting at 2021-02-10 06:18:28.465 UT (4.300 hours after the trigger). The source is clearly detected. We obtained 3 x 900 s spectroscopy with the R1000B grism, covering the spectral range from 3700 to 7800 AA. From the 30 s image starting at 2021-02-10 06:14:20.350 (0.1763 days after the trigger), we measure r' = 18.84 +/- 0.03 mag (AB) vs. a nearby PanSTARRS star. We note this indicates a slow decay compared with the initial Swift detection (Laha et al., GCN #29444). Further photometric follow-up is encouraged. The spectrum shows a strong continuum over the full spectral range with several absorption lines. We identify these as FeII, MgII, and MgI, at a mean redshift of z = 0.715. At this redshift, we also detect OII in emission, which allows us to identify this as the redshift of the GRB. We also find an intervening system in MgII and MgI at z = 0.481.