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/).