GCN Circular 42611

A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), J.A. Kennea (PSU), D.N.
Burrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester), J.P.
Osborne (U. Leicester), A. Melandri (INAF-OAR) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of GRB 251106A. We
searched for X-ray sources in  2.0 ks of Photon Counting (PC) mode
data. The total exposure at the position of the afterglow (see below)
is 2.0 ks, obtained between T0+4.0 ks and T0+9.9 ks.

Three uncatalogued X-ray sources are detected, of which one ("Source
1") is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 2020 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 = 234.46260, +63.36779 which is equivalent to:

RA (J2000): 15h 37m 51.02s
Dec(J2000): +63d 22' 04.0"

with an uncertainty of 2.6 arcsec (radius, 90% confidence). This
position is consistent with the optical afterglow candidate identified
by Angulo et al. (GCN 42607
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.87 (+0.33, -0.29).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.96 (+0.23, -0.22). The
best-fitting absorption column is  1.1 (+0.6, -0.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 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 3.2 x 10^-11 (3.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.1 (+0.6, -0.5) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 2.8 sigma
Photon index:	     1.96 (+0.23, -0.22)

If the light curve continues to decay with a power-law decay index of
0.87, the count rate at T+24 hours will be 0.038 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-12 (1.5 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/03000182.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/SVOM/SVOM_FIELD00048.

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