TITLE:   GCN CIRCULAR
NUMBER:  40336
SUBJECT: GRB 250502A: Swift-XRT afterglow detection
DATE:    25/05/03 19:00:30 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), A.
Melandri (INAF-OAR), T. Sbarrato (INAF-OAB) 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 250502A (GCN 40313). We searched for
X-ray sources in  4.6 ks of Photon Counting (PC) mode data. The total
exposure at the position of the afterglow (see below) is 4.6 ks,
obtained between T0+72.1 ks and T0+105.5 ks.

An uncatalogued X-ray source is detected consistent with the
SVOM/COLIBRI error region (GCN 40315) and is believed to be the
afterglow. Using 2113 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 = 206.52396, -10.75879
which is equivalent to:

RA (J2000): 13h 46m 05.75s
Dec(J2000): -10d 45' 31.6"

with an uncertainty of 4.5 arcsec (radius, 90% confidence). This
position is 2.3 arcsec from the SVOM/COLIBRI position.	The source has
a mean count rate of 1.3e-02 ct/sec; we cannot determine at the present
time whether it is fading. However, given the coincidence in position
with the optical counterpart (GCNs 40315, 40319, 40320, 40322, 40330,
40331, 40333, 40334) at a redshift of 2.163 (GCN 40328), we confirm
this as the X-ray afterglow.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.4 (+1.6, -0.7). The
best-fitting absorption column is  consistent with the Galactic value
of 5.0 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 3.1 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5 (+/-31) x 10^20 cm^-2
Galactic foreground: 5.0 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.4 (+1.6, -0.7)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021827.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021827.

This circular is an official product of the Swift-XRT team.