TITLE:   GCN CIRCULAR
NUMBER:  31868
SUBJECT: GRB 220408A: 1.3m DFOT Optical upper limit
DATE:    22/04/10 12:07:00 GMT
FROM:    Amit Kumar at ARIES, India  <amitkundu515@gmail.com>

 Amit Kumar, Rahul Gupta, Amit K. Ror, Nikita Rawat, Dimple, Ankur Ghosh,
Amar Aryan, Brajesh Kumar, Shashi B. Pandey and Kuntal Misra (ARIES)
report:

We observed the field of the Swift-BAT (Caputo et al., GCN 31848) and
Fermi-GBM (GBM Team, GCN 31847) detected GRB 220408A with the 1.3m
Devasthal Fast Optical Telescope (DFOT) located at Devasthal observatory of
Aryabhatta Research Institute of Observational Sciences (ARIES), India. We
started the observations on 2022-04-08 at 16:27:57 UT, i.e., ~10.698 hours
after the GBM trigger. We have observed 300s*23 frames in each Bessel R and
I filters. We stacked the images after the pre-processing and alignment. We
did not detect the optical afterglow at the Swift-XRT position (Beardmore
et al., GCN 31852) in our stacked images; see also Caputo et al., GCN
31848; Zheng & Filippenko, GCN 31850; Jiang et al., GCN 31853; Oates &
Caputo, GCN 31855; Kumar et al., GCN 31857; Hu et al., GCN 31859;
Strausbaugh et al., GCN 31862; Zhu et al., GCN 31864; and Gregory et al.,
GCN 31867.


We obtain the following 3-sigma upper limits in the stacked images.

Date Start_UT T_start-T0 (hour) Filter Exp time(s) Limiting magnitude
====================================================
2022-04-08 16:27:57 ~10.698 R 300s*23 >22.5
2022-04-08 18:29:56 ~12.731 I  300s*23 >22.1


The magnitudes are not corrected for the Galactic extinction in the burst
direction. Photometric calibration is performed using the standard stars
from the USNO-B1.0 catalogue.

During our observations, the moon phase and its distance were ~45% and
~55-66 degrees, respectively, which affected our deeper limits
significantly; see Kumar et al. 2021, JAA, arXiv:2111.13018 (
https://ui.adsabs.harvard.edu/abs/2000A%26A...363..705M/abstract) for the
sky brightness of the Devasthal site w.r.t. the moon phase and distance.

Assuming a typical temporal flux decay index of the early afterglows
(alpha~0.7), the extrapolated brightness at the epoch of our observation is
consistent with the quoted non-detection. Deeper observations using the
larger facilities will be useful to establish the fast decaying nature of
the burst and to investigate for this burst to happen in any
distant/background galaxy rather than Whirlpool Galaxy (M51a).

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