GCN Circular 42891
Subject
GRB 251129A: Swift-XRT afterglow detection
Event
Date
2025-11-29T12:29:18Z (10 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
K.L. Page (U. Leicester), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti
(INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU),
A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester) and P.A.
Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the
SVOM/ECLAIRs-detected burst GRB 251129A. We searched for X-ray sources
in 732 s of Photon Counting (PC) mode data. The total exposure at the
position of the afterglow (see below) is 2.3 ks, obtained between
T0+16.5 ks and T0+28.5 ks.
An uncatalogued X-ray source is detected within the estimated 3-sigma
SVOM/ECLAIRs error region (49 arcsec) and is above the RASS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow. Using 2336 s of PC mode data and 4 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 = 224.33074, +79.29312
which is equivalent to:
RA (J2000): 14h 57m 19.38s
Dec(J2000): +79d 17' 35.2"
with an uncertainty of 2.1 arcsec (radius, 90% confidence). This
position is 26 arcsec from the SVOM/ECLAIRs position. The position is
consistent with the optical afterglow (GCNs 42880, 42882, 42883, 42885,
42887) and the EP X-ray transient (GCN 42884).
The light curve can be modelled can be modelled with a power-law decay
with a decay index of alpha=6.23 (+0.32, -0.29).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.71 (+0.22, -0.20). The
best-fitting absorption column is 7.3 (+3.7, -3.3) x 10^20 cm^-2, in
excess of the Galactic value of 2.9 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 2.6 x 10^-11 (3.7 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 7.3 (+3.7, -3.3) x 10^20 cm^-2
Galactic foreground: 2.9 x 10^20 cm^-2
Excess significance: 2.2 sigma
Photon index: 2.71 (+0.22, -0.20)
If the light curve continues to decay with a power-law decay index of
6.2, the count rate at T+24 hours will be 1.1 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-15 (4.2 x 10^-15) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/00021892.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021892.
This circular is an official product of the Swift-XRT team.