GCN Circular 41885

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Ferro
(INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Lanava
(PSU), S. Dichiara (PSU), J.A. Kennea (PSU), A.P. Beardmore (U.
Leicester) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of GRB
250919A/EP250919a. We searched for X-ray sources in  1.7 ks of Photon
Counting (PC) mode data. The total exposure at the position of the
afterglow (see below) is 1.7 ks, obtained between T0+21.4 ks and
T0+23.1 ks, taking T0 as the Fermi trigger time.

An uncatalogued X-ray source is detected within the estimated 3-sigma
Einstein Probe/WXT error region (35 arcsec), also consistent with the
Fermi-LAT error region, and is above the RASS 3-sigma upper limit at
this position, and is therefore likely the GRB afterglow. Using 1732 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 = 298.55415, -48.83775 which is equivalent
to:

RA (J2000): 19h 54m 13.00s
Dec(J2000): -48d 50' 15.9"

with an uncertainty of 2.5 arcsec (radius, 90% confidence). The light
curve is consistent with a constant source of mean count rate 2.3e+00
ct/sec. A power-law fit gives an index of 1.0 (+1.9, -1.0). However,
given the position of this X-ray saource is consistent with the optical
counterpart with a measured redshift of 1.145 (GCN 41883
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 ), as well as
being above the RASS 3-sigma upper limit, we believe this to be 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 1.52 (+0.13, -0.11). The
best-fitting absorption column is  6.7 (+4.1, -1.6) x 10^20 cm^-2,
consistent with the Galactic value of 5.1 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 4.5 x 10^-11 (4.8 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.7 (+4.1, -1.6) x 10^20 cm^-2
Galactic foreground: 5.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.52 (+0.13, -0.11)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/03000101.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/EP/EP_FIELD00075.

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