GCN Circular 43050
Subject
GRB 251208B: Swift-XRT afterglow detection
Event
Date
2025-12-09T13:53:09Z (2 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.A. Kennea (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester),
J.P. Osborne (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB),
A. D'Ai (INAF-IASFPA), S. Lanava (PSU) and P.A. Evans (U. Leicester)
reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the GUANO (DeLaunay
et al. GCN 43033) and Fermi/LAT-detected (Bissaldi et al. GCN 43034)
burst GRB 251208B. We searched for X-ray sources in 4.8 ks of Photon
Counting (PC) mode data. The total exposure at the position of the
afterglow (see below) is 4.8 ks, obtained between T0+33.1 ks and
T0+45.4 ks.
Three uncatalogued X-ray sources are detected within the estimated
3-sigma Fermi/LAT error region (414 arcsec), of which one ("Source 1")
is 2.9 sigma above the RASS limit and it is fading with 2.2 sigma
significance and thus is believed to be the GRB afterglow. Using 4576 s
of PC mode data and 5 UVOT images, we find an enhanced XRT position
(using the XRT-UVOT alignment and matching UVOT field sources to the
USNO-B1 catalogue): RA, Dec = 228.48108, +29.04575 which is equivalent
to:
RA (J2000): 15h 13m 55.46s
Dec(J2000): +29d 02' 44.7"
with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 87 arcsec from the Fermi/LAT position. The light curve is
consistent with a constant source of mean count rate 5.0e-02 ct/sec. A
power-law fit gives an index of 1.2 (+/-1.2).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+0.4, -0.3). The
best-fitting absorption column is 1.4 (+1.1, -0.8) x 10^21 cm^-2, in
excess of the Galactic value of 2.6 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 3.1 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.4 (+1.1, -0.8) x 10^21 cm^-2
Galactic foreground: 2.6 x 10^20 cm^-2
Excess significance: 2.3 sigma
Photon index: 2.2 (+0.4, -0.3)
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/00021893.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021893.
This circular is an official product of the Swift-XRT team.