GCN Circular 41658

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea
(PSU), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
MAXI-detected burst GRB 250830B. The observations now extend from
T0+11.1 ks to T0+223.0 ks and have a total exposure time of 6.0 ks. .
The source previously reported, "Source 1", is fading with 2.5 sigma
significance and thus is believed to be the GRB afterglow. Using 2349 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 = 105.74037, -64.85289 which is equivalent
to:

RA (J2000): 07h 02m 57.69s
Dec(J2000): -64d 51' 10.4"

with an uncertainty of 3.0 arcsec (radius, 90% confidence). This
position is 2.3 arcmin from the EP position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.43 (+0.15, -0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+0.8, -0.5). The
best-fitting absorption column is  1.5 (+6.0, -0.4) x 10^21 cm^-2,
consistent with the Galactic value of 1.1 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 5.4 x 10^-11 (6.1 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.5 (+6.0, -0.4) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.4 (+0.8, -0.5)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021858.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021858.

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