GCN Circular 18127

D.N. Burrows (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U.
Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia
(ASDC), T.G.R. Roegiers (PSU), L.M. McCauley (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 8.5 ks of XRT data for GRB 150811A (Evans et al. GCN
Circ. 18119
Not found
 
 
), from 128 s to 27.8 ks after the  BAT trigger. The data
comprise 156 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 enhanced XRT position for this burst was given by Goad et al.
(GCN Circ. 18124
Not found
 
 
).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.52 (+/-0.19), followed by a break at T+366 s to an
alpha of 1.17 (+0.06, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.11 (+0.21, -0.13). The
best-fitting absorption column is  1.60 (+0.65, -0.07) x 10^21 cm^-2,
consistent with the Galactic value of 1.5 x 10^21 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 2.03 (+0.13,
-0.15) and a best-fitting absorption column of 2.1 (+1.0, -0.6) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.5 x 10^-11 (4.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.1 (+1.0, -0.6) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.03 (+0.13, -0.15)

If the light curve continues to decay with a power-law decay index of
1.17, the count rate at T+24 hours will be 3.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (1.6 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00651882.

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