GCN Circular 42551
Subject
GRB 251103A: Swift-XRT afterglow detection
Event
Date
2025-11-03T15:18:01Z (2 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.P. Osborne (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB),
A. D'Ai (INAF-IASFPA), S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea
(PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A.
Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of GRB 251103A (Hu et
al., GCN Circ. 42534). We searched for X-ray sources in 1.6 ks of
Photon Counting (PC) mode data. The total exposure at the position of
the afterglow (see below) is 1.6 ks, obtained between T0+4.8 ks and
T0+10.2 ks.
An uncatalogued X-ray source is detected within the estimated 3-sigma
SVOM/ECLAIRs error region (210 arcsec) and is above the LSXPS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow. Using 1768 s of PC mode data and 2 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 = 146.83866, +16.12272
which is equivalent to:
RA (J2000): 09h 47m 21.28s
Dec(J2000): +16d 07' 21.8"
with an uncertainty of 2.6 arcsec (radius, 90% confidence).
The source position is consistent with the one of the optical afterglow
reported by Schneider et al. (GCN Circ. 42535, 42549), Turpin et al.
(GCN Circ. 42538), Gritsevich et al. (GCN Circ. 42539), Izzo et al.
(GCN Circ. 42540), O'Neill et al. (GCN Circ. 42543) and Ghosh et al.
(GCN Circ. 42545).
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.2 (+0.4, -0.3).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.82 (+0.16, -0.15). The
best-fitting absorption column is 9.9 (+4.7, -4.2) x 10^20 cm^-2, in
excess of the Galactic value of 4.0 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 3.8 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 9.9 (+4.7, -4.2) x 10^20 cm^-2
Galactic foreground: 4.0 x 10^20 cm^-2
Excess significance: 2.3 sigma
Photon index: 1.82 (+0.16, -0.15)
If the light curve continues to decay with a power-law decay index of
1.2, the count rate at T+24 hours will be 0.035 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-12 (1.5 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/03000169.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/SVOM/SVOM_FIELD00047.
This circular is an official product of the Swift-XRT team.