TITLE:   GCN CIRCULAR
NUMBER:  40268
SUBJECT: GRB 250427A: Swift-XRT refined Analysis
DATE:    25/04/27 17:49:43 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

V. D'Elia (ASI-SSDC), E. Ambrosi (INAF/IASFPA), K.L. Page (U
Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows
(PSU), M. A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U.
Leicester), A. D'Ai (INAF-IASFPA) and P.A. Evans (U. Leicester) reports
on behalf of the Swift-XRT team:

We have analysed 1.7 ks of XRT data for the Einstein Probe/WXT-detected
burst GRB 250427A, from 22.9 ks to 28.9 ks after the  Einstein
Probe/WXT trigger. The data are entirely in Photon Counting (PC) mode. 

We found an uncatalogued X-ray source within the estimated 3-sigma
EP-WXT error region. We find an XRT position: RA, Dec = 277.27280,
+7.5632 which is equivalent
to:

RA (J2000): 18h 29m 05.47s
Dec (J2000): +07° 33′ 47.6″

with an uncertainty of 3.9 arcsec (radius, 90% confidence).
This position is 6.9 arcsec from the EP-WXT position, and consistent
with the X-ray counterpart found in EP/FXT data (Wang et al. GCN 40257)
and with the optical counterpart reported by Perez-Garcia et al. (GCN
40259), Becerra et al. (GCN 40261), Ghosh et al. (GCN 40263), Brivio et
al. (GCN 40264).

The light curve is consistent with a constant source with a hint of
fading in the last segment. However, given the corresponding optical
counterpart and the measured redshift, we believe this to be the X-ray
afterglow. Further observations are planned.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.78 (+0.39, -0.28). The
best-fitting absorption column is  consistent with the Galactic value
of 2.9 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 4.3 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+/-1.3) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.78 (+0.39, -0.28)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019750.

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