GCN Circular 17517

M. de Pasquale (INAF-IASFPA), A. Maselli  (INAF-IASFPA), J.A. Kennea
(PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), P.A. Evans (U.
Leicester), A. Amaral-Rogers (U. Leicester), J.P. Osborne (U.
Leicester), A. Melandri (INAF-OAB) and A.Y. Lien report on behalf of
the Swift-XRT team:

We have analysed 147 s of XRT data for GRB 150301A (Lien et al. GCN
Circ. 17510
Not found
 
 
), from 50 s to 43.7 ks after the  BAT trigger. The data are
entirely in Windowed Timing (WT) mode (the first 6 s were taken while
Swift was slewing). 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.53 (+0.18, -0.59), followed by a break at T+112 s to
an alpha of 2.6 (+0.6, -0.7).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.71 (+0.21, -0.20). The
best-fitting absorption column is  5.6 (+/-0.7) x 10^22 cm^-2, in
excess of the Galactic value of 1.3 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 9.1 x 10^-11 (2.0 x 10^-10) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     5.6 (+/-0.7) x 10^22 cm^-2
Galactic foreground: 1.3 x 10^22 cm^-2
Excess significance: 10.5 sigma
Photon index:	     1.71 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
2.6, the count rate at T+24 hours will be 9.7 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.8 x
10^-17 (1.9 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00632995.

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