GCN Circular 42084
Subject
GRB 251002A: Swift-XRT afterglow detection
Event
Date
2025-10-03T10:06:49Z (5 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), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), E. Ambrosi (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 GRB 251002A. We
searched for X-ray sources in 1.5 ks of Photon Counting (PC) mode
data. The total exposure at the position of the afterglow (see below)
is 1.5 ks, obtained between T0+3.6 ks and T0+9.2 ks.
Three uncatalogued X-ray sources are detected within the estimated
3-sigma SVOM/ECLAIRs error region (392 arcsec), of which one ("Source
1") is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 1503 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 = 13.90813, -5.56678 which is equivalent to:
RA (J2000): 00h 55m 37.95s
Dec(J2000): -05d 34' 00.4"
with an uncertainty of 2.6 arcsec (radius, 90% confidence). This
position is consistent with the optical candidate (Palmerio et al. GCN
42061) and the EP-FXT X-ray candidate (Sun et al. GCN 24075).
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.27 (+0.28, -0.25).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.62 (+0.20, -0.18). The
best-fitting absorption column is 8.3 (+6.5, -2.8) x 10^20 cm^-2,
consistent with the Galactic value of 5.5 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 4.2 x 10^-11 (4.7 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 8.3 (+6.5, -2.8) x 10^20 cm^-2
Galactic foreground: 5.5 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.62 (+0.20, -0.18)
If the light curve continues to decay with a power-law decay index of
1.27, the count rate at T+24 hours will be 0.014 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.9 x
10^-13 (6.5 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/03000125.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/SVOM/SVOM_FIELD00039.
This circular is an official product of the Swift-XRT team.