GCN Circular 42205

B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), 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), M. Ferro (INAF-OAB)
and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 5.1 ks of XRT data for GRB 251011B, from 85 s to 61.7
ks after the   trigger. The data comprise 112 s in Windowed Timing (WT)
mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.12 (+/-0.08), followed by a break at T+579 s to an
alpha of 0.57 (+0.07, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.15 (+/-0.12). The
best-fitting absorption column is  1.8 (+0.4, -0.3) 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 3.1 x 10^-11 (4.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.8 (+0.4, -0.3) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 8.4 sigma
Photon index:	     2.15 (+/-0.12)

If the light curve continues to decay with a power-law decay index of
0.57, the count rate at T+24 hours will be 0.013 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.0 x
10^-13 (5.7 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01403191.

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