GCN Circular 45934

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

We have analysed 4.2 ks of XRT data for GRB 261011A, from 301 s to 28.0
ks after the   trigger. The data comprise 1.2 ks in Windowed Timing
(WT) mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. Using 1938 s of PC mode data
and 4 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 = 21.89722, -21.56052 which is equivalent to:

RA (J2000): 01h 27m 35.33s
Dec(J2000): -21d 33' 37.9"

with an uncertainty of 2.9 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=4.1 (+/-0.6). At T+362 s  the decay
flattens to an alpha of 1.68 (+0.11, -0.10) before breaking again at
T+792 s to a final decay with index alpha=1.26 (+0.06, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.80 (+/-0.05). The
best-fitting absorption column is  7.7 (+1.7, -1.6) x 10^20 cm^-2, in
excess of the Galactic value of 0.0 x 10^00 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (4.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     7.7 (+1.7, -1.6) x 10^20 cm^-2
Galactic foreground: 1.5 x 10^20 cm^-2
Excess significance: 6.4 sigma
Photon index:	     1.80 (+/-0.05)

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01516664.

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