GCN Circular 42356

P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi 
(INAF-IASFPA) , S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), 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 251016A. We
searched for X-ray sources in  3.5 ks of Photon Counting (PC) mode
data. The total exposure at the position of the afterglow (see below)
is 5.6 ks, obtained between T0+6.6 ks and T0+107.3 ks.

Thirty-two uncatalogued X-ray sources are detected, of which one
("Source 1") is fading with >3-sigma significance, and is therefore
likely the GRB afterglow. Using 1190 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 =
266.30279, -39.56008 which is equivalent to:

RA (J2000): 17h 45m 12.67s
Dec(J2000): -39d 33' 36.3"

with an uncertainty of 3.2 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=4.53 (+0.08, -3.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+1.0, -0.6). The
best-fitting absorption column is  4.8 (+8.8, -0.9) x 10^21 cm^-2,
consistent with the Galactic value of 3.9 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 6.4 x 10^-11 (8.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.8 (+8.8, -0.9) x 10^21 cm^-2
Galactic foreground: 3.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.4 (+1.0, -0.6)

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/03000141.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/SVOM/SVOM_FIELD00042.

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