TITLE: GCN CIRCULAR NUMBER: 29799 SUBJECT: GRB 210406A: 10.4m GTC follow-up observation DATE: 21/04/11 21:00:25 GMT FROM: Youdong HU at IAA-CSIC Y.-D. Hu, E. Fernandez-Garcia, A. J. Castro-Tirado, M.D. Caballero-Garcia, M. A. Castro Tirado (IAA-CSIC), V. Sokolov (SAO-RAS), I. Carrasco and A. Castellon (UMA) and S. B. Pandey (ARIES), on behalf of a larger collaboration, report: Following the detection of GRB 210406A by INTEGRAL (Mereghetti et al. GCNC 29767), we triggered the 10.4m GTC telescope equipped with OSIRIS in La Palma (Spain) starting on Apr 10 at 20:47 UT (~4.15 d after trigger). The 1.5 arcmin error region (Mereghetti et al. GCNC 29767) was covered by the two OSIRIS CCDs with an exception of a 9.4 arcsec gap in between. For the optical candidate detected at Mondy Observatory (Belkin et al. GCNC 29775) a magnitude 22.45+-0.09 in r-band is measured. Since this value is consistent with the second epoch Mondy observation, a potential host galaxy cannot be excluded if this optical variability is finally confirmed although we notice that no X-ray afterglow has been reported by Swift/XRT at this position (D'Elia et al. GCNC 29785). In addition to this, close to the source #3 position (6.4 arcsec error region) reported by Swift/XRT (D'Elia et al. GCNC 29785) we found two objects on the outskirts of it. The first one is located at RA: 8:50:00.18 DEC: +76:31:50.50 (J2000) for which we measure 22.96+-0.12 mag in r-band and seems to be recorded in Pan-STARRS DR2. A fainter object is found at RA: 8:50:01.42 DEC: +76:31:41.02 (J2000) for which we measure a magnitude of 24.00+-0.26 in r-band. All magnitudes are calibrated with the Pan-STARRS catalog. Further observations are needed. We thank the staff at Roque de Los Muchachos Observatory for their excellent support.