TITLE:   GCN CIRCULAR
NUMBER:  28781
SUBJECT: GRB 201020B: 1.3m DFOT continued optical follow-up observations
DATE:    20/10/26 12:23:26 GMT
FROM:    Amit Kumar at ARIES, India  <amitkundu515@gmail.com>

Amit Kumar (ARIES), Rahul Gupta (ARIES), Dimple (ARIES), Ankur Ghosh
(ARIES), Kaushal Sharma (ARIES), Amar Aryan (ARIES), Brajesh Kumar
(ARIES), Kuntal Misra (ARIES), and Shashi B. Pandey (ARIES) report:

We followed-up Fermi GBM triggered GRB 201020B (Fermi team, GCN 28702;
Lipunov et al. GCN 28704; Malacaria and Meegan, GCN 28710; Ursi et al.
GCN 28714; Arimoto et al., GCN 28716; Lipunov et al., GCN 28718; Xu et
al., GCN 28719; Kann et al. GCN 28765) using the 1.3m Devasthal Fast
Optical Telescope (DFOT) at Devasthal observatory of Aryabhatta
Research Institute of observational sciencES (ARIES), India.
The observations were carried out on 2020-10-23 (from 19:14:51 to
19:26:16 UT) and 2020-10-24 (from 18:18:43 to 19:24:14 UT) in multiple
bands. We took a series of images with the exposure time of 120
seconds in multiple Bessel filters, covering Swift UVOT position
(Sbarufatti et al. GCN 28740).
The optical transient reported by V. Lipunov et. al. GCN 28718; D. Xu
et. al. GCN 28719 is clearly visible in the stacked images.

The R-band magnitudes of the GRB afterglow estimated from the
photometry of stacked images are as follows:

Date       Start_UT   T_start-T0 (hrs)  Filter  Exp time (s)
Magnitude   Mag_error
----------------------------------------------------------------------------------------------------------------------------

2020-10-23   19:14:51   73.683            R      15*120         21.34       0.05
2020-10-24   19:24:14   97.839            R      30*120         21.83
     0.05
----------------------------------------------------------------------------------------------------------------------------

The estimated magnitudes have not been corrected for the Galactic
extinction in the direction of the GRB. Photometric calibration is
performed using the following standard stars from the USNO-B1.0
catalog:

1670-0037558
1670-0037526
1670-0037460
1670-0037451
1670-0037450
1670-0037447
Our estimated magnitudes in the R-band, along with the measurements
reported by Belkin et al., GCN 28725; Vinko et al. GCN 28750; Belkin
et al. GCN 28768; Vinko et al. GCN 28769; Dimple et al. GCN 28772;
Nicastro et al. GCN 28773, and Kumar et al. GCN 28776, suggest that
the afterglow of GRB 201020B is fading with a power-law index of 1.66
+/- 0.12.
The magnitudes reported by Vinko et al. GCN 28750, Vinko et al. GCN
28769, and Nicastro et al. GCN 28773 are convetred to R-band
magnitudes using the tranformation equations provided by Lupton et al.
(2005).
This power-law index is well consistent with those reported by Kumar
et al. GCN 28753 and Belkin et al. GCN 28768.

This circular may be cited.