TITLE: GCN CIRCULAR NUMBER: 29759 SUBJECT: GRB 210207B: Prompt and very late time observations with HCT. DATE: 21/04/04 16:50:36 GMT FROM: Firoza Sutaria at Indian Inst. of Astrophysics F. Sutaria (IIA, Bangalore, India) and A. Ray (TIFR /HBCSE, Mumbai, India) report on the observations of the optical counterpart of GRB 210207B (Lien et al., GCN #29420; Strausbaugh, R, GCN #29441 and references therein) taken with the Himalyan Chandra telescope (HCT) on two epochs. The first observation was on MJD 59254.91163194 -- i.e 48.0103 hr after the Swift/BAT trigger. The field of the GRB was observed in the Bessel R filter. We stacked 12 frames to achieve a total exposure time of 570 s. The photometry was carried out using standard IRAF utilities for faint objects in relatively uncrowded fields. The instrumental magnitudes were calibrated against photometric standards in the field of PG0918+029, taken on the same and on the previous night, resulting in an average atmospheric extinction coefficient of 0.087(\pm 0.0208) /mag in this filter. At the position of the GRB (18:02:32.40 +53:40:56.0) we find a faint optical source with apparent magnitude R=22.0 \pm 0.1, uncorrected for galactic extinction. We observed the same field once again on 17th Mar. 2021 (MJD 59290.91730324), in the same filter. A stacked exposure of 10 frames leading to a total exposure time of 600s did not reveal any source at the location of the GRB, down to a 3-sigma limiting magnitude of R= 23.66. We thus conclude that there was no contribution from the host galaxy in the previous detection. Our results are also tabulated below. ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- MJD Filter Detected? Magnitude (uncorrected for galactic extinction) ------------------------------------------------------------------------------- 59254.9116 R Yes 22.0 \pm 0.1 59290.9173 R No. < 23.66 (3-sigma) ------------------------------------------------------------------------------- We note that the galactic extinction for the CTIO R filter in this direction is estimated at A_R = 0.087 mag [Schlafly E. F. & Finkbeiner D., 2011, ApJ, 737, 103], based on a SDSS dust map. However, the IRAS and COBE/DIRBE survey [D.J. Schlegel, D.P. Finkbeiner, & M. Davis, 1998, ApJ, 500, 525] sets it at A_R=0.109 mag. This GCN may be cited. We thank the staff of IAO, Hanle and CREST, Hosakote, that made these observations possible. The facilities at IAO and CREST are operated by the Indian Institute of Astrophysics, Bangalore.