TITLE: GCN CIRCULAR NUMBER: 40005 SUBJECT: GRB250331D: AbAO optical observations DATE: 25/04/01 14:15:27 GMT FROM: Alexei Pozanenko at IKI, Moscow grb.alex@gmail.com

N. Pankov (HSE, IKI), R. Ya. Inasaridze (AbAO), A. Pozanenko (IKI) report on behalf of IKI GRB-FuN:

We observed the entire localisation region of BAT-GUANO/Swift of GRB250331D (Ronchini et al., GCN 39979), initially detected by GBM/Fermi (Fermi GBM team, GCN 39961) with AS-32 telescope of AbAO starting on 2025-03-31 (UT) 18:56:37. We report the results of optical transient searches in five XRT/Swift localisations reported in https://www.swift.ac.uk/ToO_GRBs/00021825/ and (Melandri et al., GCN 39998). Details of the preliminary photometry of the stacked image and the sources detected so far are given below.

Date UT start t-T0 Filter Exp. UL(3sigma) Telescope (mid, d) (s) 2025-03-31 18:56:37 0.56447 R 61x60 20.8 AS-32

XRT #1 ------ RA = 48.26026d Dec = 62.30756d Err90 = 5.1"

Inside the XRT error circle the single source is detected: RA = 48.25957d +/- 0.05" Dec = 62.30756 +/- 0.02" R(AbAO) = 13.24 +/- 0.05 USNO-B1.0 R2 = 13.39

XRT #2 ------ RA = 48.65718d Dec = 62.28326d Err90 = 3.5"

Inside the XRT error circle the single source is detected: RA = 48.66142d +/- 0.14" Dec = 62.28324d +/- 0.07" R(AbAO) = 18.72 +/- 0.07 USNO-B1.0 R2 = 18.97

XRT #3 ------ RA = 48.86152d Dec = 62.34017d Err90 = 7.0"

Inside the XRT error circle the single source is detected: RA = 48.85897d +/- 0.12" Dec = 48.85897d +/- 0.06" R(AbAO) = 18.64 +/- 0.07 USNO-B1.0 R2 = 18.93

XRT #4 ------ RA = 48.77053d Dec = 62.37337d Err90 = 5.9"

Inside the XRT error circle the single source is detected: RA = 48.76831d +/- 0.3" Dec = 62.37292d +/- 0.3" R(AbAO) = 19.68 +/- 0.26 USNO-B1.0 R2 = 19.94

XRT #5 ------ RA = 48.3703896d Dec = 62.10607d Err90 = 6.9"

Inside the XRT error circle the single source is detected: RA = 48.37039d +/- 0.3" Dec = 62.10664d +/- 0.3" R(AbAO) ~ 21.0 PS1 r = 21.17 +/- 0.15, i = 20.21 +/- 0.06

We did not identify any variable sources in any of the XRT localisations ( https://www.swift.ac.uk/ToO_GRBs/00021825/).

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