GCN Circular 11794

A. Melandri, B. Sbarufatti, R. Margutti and P. D'Avanzo (INAF-OAB)
report on behalf of the Swift-XRT team:

We have analysed 14 ks of XRT data for GRB 110315A (Melandri  et al.
GCN Circ. 11790
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 ), from 94 s to 34.8 ks after the  BAT trigger. The data
comprise 97 s in Windowed Timing (WT) mode (the first 8 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 Beardmore
et al. (GCN. Circ 11792).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.48 (+0.64, -0.53), followed by a break at T+150 s to
an alpha of 1.03 (+0.03, -0.03). Flaring activity is detected around
T+500 s.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.14 (+0.26, -0.24). The
best-fitting absorption column is  2.5 (+0.8, -0.7) x 10^21 cm^-2, in
excess of the Galactic value of 1.8 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.19 (+0.15, -0.12)
and a best-fitting absorption column of 2.7 (+/-0.4) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.9 x 10^-11 (6.7 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.7 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 1.8 x 10^21 cm^-2
Excess significance: 10.0 sigma
Photon index:	     2.19 (+0.15, -0.12)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00449399.

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