GCN Circular 17608

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

We have analysed 6.3 ks of XRT data for GRB 150318A (Pagani et al. GCN
Circ. 17599
Not found
 
 
), from 130 s to 40.1 ks after the  BAT trigger. The data
comprise 191 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 Evans et
al. (GCN Circ. 17600
Not found
 
 
).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.25 (+0.23, -0.29). At T+187 s  the decay
steepens to an alpha of 5.30 (+/-0.18) before breaking again at T+628 s
to a final decay with index alpha=0.94 (+0.19, -0.21).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.46 (+/-0.06). The
best-fitting absorption column is  3.17 (+0.22, -0.21) x 10^21 cm^-2,
in excess of the Galactic value of 3.3 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 2.9 x 10^-11 (5.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     3.17 (+0.22, -0.21) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^20 cm^-2
Excess significance: 21.9 sigma
Photon index:	     2.46 (+/-0.06)

If the light curve continues to decay with a power-law decay index of
0.94, the count rate at T+24 hours will be 1.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.4 x
10^-14 (8.7 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00635289.

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