GCN Circular 41407
Subject
GRB 250818B: Swift-XRT afterglow detection
Event
Date
2025-08-18T05:50:58Z (7 days ago)
Edited On
2025-08-20T15:14:34Z (4 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
M. Ferro (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M.A.
Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA) and P.A. Evans
(U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of GRB 250818B. We
searched for X-ray sources in 604 s of Photon Counting (PC) mode data.
The total exposure at the position of the afterglow (see below) is 604
s, obtained between T0+1.7 ks and T0+2.3 ks.
An uncatalogued X-ray source is detected and is above the RASS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow. The position of this source is RA, Dec=46.0552, -3.1251
which is equivalent to:
RA (J2000): 03h 04m 13.24s
Dec(J2000): -03d 07' 30.24"
with an uncertainty of 3.6 arcsec (radius, 90% confidence). The light
curve is consistent with a constant source of mean count rate 7.1e-01
ct/sec, though only covers a short interval of time. The position is
also consistent with the fading optical counterpart candidate
GOTO25fzq/AT 2025ukm reported by GOTO (GCN Circ. 41406).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.67 (+0.26, -0.20). The
best-fitting absorption column is 8.6 (+8.1, -2.1) x 10^20 cm^-2,
consistent with the Galactic value of 6.5 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 4.0 x 10^-11 (4.5 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 8.6 (+8.1, -2.1) x 10^20 cm^-2
Galactic foreground: 6.5 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.67 (+0.26, -0.20)
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/03000030.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00031.
This circular is an official product of the Swift-XRT team.