GCN Circular 29759

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.