GCN Circular 40361
Subject
GRB 250506A: Swift XRT observations and possible counterpart detection
Date
2025-05-06T10:03:08Z (16 days ago)
From
P.A. Evans at U. Leicester <pae9@leicester.ac.uk>
Via
email
A.P. Beadmore, K.L. Page and P.A. Evans (U. Leicester) report on behalf
of the Swift-XRT team,
Swift-XRT has observed the location of the SVOM and Fermi-detected GRB 250506A
(GCN Circs. 40355, 40358), gathering 1.7 ks of data between 1.5 ks and 13.7ks
after the Fermi trigger. The observations are centred on the SVOM/ECLAIRs
position.
We find a single, uncatalogued X-ray source with an enhanced XRT position of RA,
Dec = 219.64184, 29.21742 which is equivalent to:
RA (J2000) = 14h 38m 34.04s
Dec (J2000) = +29° 13′ 02.7
with an uncertainty of 4.6 arcsec (radius, 90% confidence). This source shows
evidence for fading at the 2.2-sigma level.
As noted by Liu et al. (GCN Circ. 50360), this is spatially consistent with an
optical source which Gaia classifies as a "quasar with 100.00% probability”
(Gaia DR3 1281175103580872064). The source also appears in the ALLWISEAGN
catalogue (Secrest et al., 2015, ApJS, 221, 12).
The XRT observations show a mean count-rate of 2.9 (+/- 0.007) ct sec and the
spectrum can be modelled with an absorbed power-law with photon index 1.8 (+0.8,
-0.5). The intrinsic absorption is <1.8e21 cm^-2, in addition to the Galactic
column of 1.44e20 cm^-2. Using these paramters the mean 0.3-10 keV flux is 8.2
(+1.8, -1.7)e-13 erg cm^-2 s^-1.
No other X-ray sources are detected, to an upper limit of ~6e-3 ct/sec which
corresponds to 2.6e-13 erg cm^-2 s^-1 (0.3-10 keV) assuming a standard power-law
spectrum (photon index 1.7, absorption column 3e20 cm^-2).
Given the lack of other sources and how early the XRT observations started,
combined with the apparent fading of the XRT afterglow, we consider it likely
that this source is the counterpart to the Fermi and SVOM triggers; however,
follow up observations are encouraged to determine the nature of this event.
Automated analysis of the observations is available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00014/.
This circular is an official product of the Swift-XRT team.