GCN Circular 39939

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Ferro
(INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), J.A. Kennea
(PSU), D.N. Burrows (PSU), M. A. Williams (PSU) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

We have analysed 1.1 ks of XRT data for the SVOM/ECLAIRs-detected burst
GRB 250329B, from 7.4 ks to 8.5 ks after the  SVOM/ECLAIRs trigger. The
data are entirely in Photon Counting (PC) mode. An uncatalogued X-ray
source is found which is substantially brighter than an upper limit
derived from the Rosat All-Sky Survey and is thus likely the afterglow.

Using 1140 s of PC mode data and 1 UVOT image, we find an enhanced XRT
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 207.66921, +52.82343 which is
equivalent to:

RA (J2000): 13h 50m 40.61s
Dec(J2000): +52d 49' 24.3"

with an uncertainty of 3.3 arcsec (radius, 90% confidence). This
position is 3.7 arcmin from the SVOM/ECLAIRs position. 

We cannot at this time determine whether the source is fading.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 3.2 (+0.9, -0.8). The
best-fitting absorption column is  7.1 (+4.0, -3.1) x 10^21 cm^-2, in
excess of the Galactic value of 1.3 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 (1.4 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.1 (+4.0, -3.1) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^20 cm^-2
Excess significance: 3.8 sigma
Photon index:	     3.2 (+0.9, -0.8)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019667.

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