TITLE:   GCN CIRCULAR
NUMBER:  31041
SUBJECT: GRB 211023A: continued 1.3m DFOT Optical Observations and possible jet break
DATE:    21/11/05 06:00:44 GMT
FROM:    Rahul Gupta at ARIES, India  <rahulbhu.c157@gmail.com>

Rahul Gupta, S. B. Pandey, Amit Ror, Amit Kumar, Dimple, Ankur Ghosh, Amar
Aryan, Bhavya, and Kuntal Misra (ARIES) report:

We further observed the bright GRB 211023A (Fermi GBM Team 2021, GCN
30958; Lesage et al. 2021, GCN 30965; N. Di Lalla et al. 2021, GCN
30961; Ursi et al. 2021, GCN 30969; and Ridnaia et al. 2021, GCN
31022) with 1.3m Devasthal Fast Optical Telescope (DFOT) located at
Devasthal observatory of Aryabhatta Research Institute of
Observational Sciences (ARIES), India, starting on 2021-10-29 at
19:19:13 UT, i.e., ~ 6.26 days after the Fermi trigger. We obtained 24
and 18 frames having an exposure time of 300 sec in the R and I
filters, respectively. We detected the optical afterglow of GRB
211023A (Lipunov et al. 2021, GCN 30970; Zhirkov et al. 2021, GCN
30977; Kann et al. 2021, GCN 30982; Hu et al. 2021, GCN 30990; Belkin
et al. 2021, GCN 31004, 31018, 31020; Vinko et al. 2021, GCN 31008;
and Kumar et al. 2021, GCN 31023) in our stacked images of both the
filters. The preliminary photometric estimate of the afterglow in the
R filter is following :


Date Start UT   T-T0 (mid, days) Filter  Exp time (sec)   Magnitude

=========================================================2021-10-29
19:19:13   ~6.28      R           300*24          22.58 +/- 0.33



Based on the preliminary photometry reported by using various
ground-based optical telescopes (Kann et al. 2021, GCN 30982; Hu et
al. 2021, GCN 30990; and Belkin et al. 2021, GCN 31004, 31018, 31020)
and our first epoch observations using 1.3m DFOT (Kumar et al. 2021,
GCN 31023), we measure the optical flux decay power-law index of the
afterglow light curve (Galactic extinction corrected) of GRB 211023A
equal to 1.49 +/- 0.04, consistent with those reported earlier (Belkin
et al. 2021, GCN 31004, 31020; and Vinko et al. 2021, GCN 31008).
However, we noticed that our second epoch observations are fainter
than the extrapolation of the early temporal decay index of ~1.5,
suggesting a possible break in the light curve. However, due to the
unavailability of near-simultaneous X-ray afterglow observations, we
could not confirm if this optical steeping is due to the jet break.
Further deeper and multiwavelength observations with larger telescopes
will be helpful to establish the possible jet break.

   The reported magnitude value is not corrected for the Galactic
extinction in the direction of the burst. Photometric calibration is
performed using the standard stars from the USNO-B1.0 catalog.

This circular may be cited.