{
  "bibcode": "2022GCN.31868....1K",
  "body": "Amit Kumar, Rahul Gupta, Amit K. Ror, Nikita Rawat, Dimple, Ankur Ghosh,\nAmar Aryan, Brajesh Kumar, Shashi B. Pandey and Kuntal Misra (ARIES)\nreport:\n\nWe observed the field of the Swift-BAT (Caputo et al., GCN 31848) and\nFermi-GBM (GBM Team, GCN 31847) detected GRB 220408A with the 1.3m\nDevasthal Fast Optical Telescope (DFOT) located at Devasthal observatory of\nAryabhatta Research Institute of Observational Sciences (ARIES), India. We\nstarted the observations on 2022-04-08 at 16:27:57 UT, i.e., ~10.698 hours\nafter the GBM trigger. We have observed 300s*23 frames in each Bessel R and\nI filters. We stacked the images after the pre-processing and alignment. We\ndid not detect the optical afterglow at the Swift-XRT position (Beardmore\net al., GCN 31852) in our stacked images; see also Caputo et al., GCN\n31848; Zheng & Filippenko, GCN 31850; Jiang et al., GCN 31853; Oates &\nCaputo, GCN 31855; Kumar et al., GCN 31857; Hu et al., GCN 31859;\nStrausbaugh et al., GCN 31862; Zhu et al., GCN 31864; and Gregory et al.,\nGCN 31867.\n\n\nWe obtain the following 3-sigma upper limits in the stacked images.\n\nDate Start_UT T_start-T0 (hour) Filter Exp time(s) Limiting magnitude\n====================================================\n2022-04-08 16:27:57 ~10.698 R 300s*23 >22.5\n2022-04-08 18:29:56 ~12.731 I  300s*23 >22.1\n\n\nThe magnitudes are not corrected for the Galactic extinction in the burst\ndirection. Photometric calibration is performed using the standard stars\nfrom the USNO-B1.0 catalogue.\n\nDuring our observations, the moon phase and its distance were ~45% and\n~55-66 degrees, respectively, which affected our deeper limits\nsignificantly; see Kumar et al. 2021, JAA, arXiv:2111.13018 (\nhttps://ui.adsabs.harvard.edu/abs/2000A%26A...363..705M/abstract) for the\nsky brightness of the Devasthal site w.r.t. the moon phase and distance.\n\nAssuming a typical temporal flux decay index of the early afterglows\n(alpha~0.7), the extrapolated brightness at the epoch of our observation is\nconsistent with the quoted non-detection. Deeper observations using the\nlarger facilities will be useful to establish the fast decaying nature of\nthe burst and to investigate for this burst to happen in any\ndistant/background galaxy rather than Whirlpool Galaxy (M51a).\n\nThis message may be cited.",
  "circularId": 31868,
  "createdOn": 1649592420000,
  "email": "amitkundu515@gmail.com",
  "subject": "GRB 220408A: 1.3m DFOT Optical upper limit",
  "submitter": "Amit Kumar at ARIES, India  <amitkundu515@gmail.com>",
  "eventId": "GRB 220408A"
}