{
  "circularId": 40005,
  "body": "N. Pankov (HSE, IKI), R. Ya. Inasaridze (AbAO), A. Pozanenko (IKI) report\non behalf of IKI GRB-FuN:\n\nWe observed the entire localisation region of BAT-GUANO/Swift of GRB250331D\n(Ronchini et al., GCN 39979), initially detected by GBM/Fermi (Fermi GBM\nteam, GCN 39961) with AS-32 telescope of AbAO starting on 2025-03-31 (UT)\n18:56:37. We report the results of optical transient searches in five\nXRT/Swift localisations reported in\nhttps://www.swift.ac.uk/ToO_GRBs/00021825/ and (Melandri et al., GCN\n39998). Details of the preliminary photometry of the stacked image and the\nsources detected so far are given below.\n\nDate UT start t-T0 Filter Exp. UL(3sigma) Telescope\n(mid, d) (s)\n2025-03-31 18:56:37 0.56447 R 61x60 20.8 AS-32\n\nXRT #1\n------\nRA = 48.26026d\nDec = 62.30756d\nErr90 = 5.1\"\n\nInside the XRT error circle the single source is detected:\nRA = 48.25957d +/- 0.05\"\nDec = 62.30756 +/- 0.02\"\nR(AbAO) = 13.24 +/- 0.05\nUSNO-B1.0 R2 = 13.39\n\n\nXRT #2\n------\nRA = 48.65718d\nDec = 62.28326d\nErr90 = 3.5\"\n\nInside the XRT error circle the single source is detected:\nRA = 48.66142d +/- 0.14\"\nDec = 62.28324d +/- 0.07\"\nR(AbAO) = 18.72 +/- 0.07\nUSNO-B1.0 R2 = 18.97\n\n\nXRT #3\n------\nRA = 48.86152d\nDec = 62.34017d\nErr90 = 7.0\"\n\nInside the XRT error circle the single source is detected:\nRA = 48.85897d +/- 0.12\"\nDec = 48.85897d +/- 0.06\"\nR(AbAO) = 18.64 +/- 0.07\nUSNO-B1.0 R2 = 18.93\n\n\nXRT #4\n------\nRA = 48.77053d\nDec = 62.37337d\nErr90 = 5.9\"\n\nInside the XRT error circle the single source is detected:\nRA = 48.76831d +/- 0.3\"\nDec = 62.37292d +/- 0.3\"\nR(AbAO) = 19.68 +/- 0.26\nUSNO-B1.0 R2 = 19.94\n\nXRT #5\n------\nRA = 48.3703896d\nDec = 62.10607d\nErr90 = 6.9\"\n\nInside the XRT error circle the single source is detected:\nRA = 48.37039d +/- 0.3\"\nDec = 62.10664d +/- 0.3\"\nR(AbAO) ~ 21.0\nPS1 r = 21.17 +/- 0.15, i = 20.21 +/- 0.06\n\nWe did not identify any variable sources in any of the XRT localisations (\nhttps://www.swift.ac.uk/ToO_GRBs/00021825/).\n",
  "eventId": "GRB 250331D",
  "createdOn": 1743516927756,
  "bibcode": "2025GCN.40005....1P",
  "submitter": "Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>",
  "subject": "GRB250331D: AbAO optical observations",
  "submittedHow": "email"
}