GCN Circular 16065

P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), D.N. Burrows (PSU), J.A.
Kennea (PSU), V. Mangano (PSU), P.A. Evans (U. Leicester), B.P.
Gompertz (U. Leicester), J.P. Osborne (U. Leicester), A. Maselli 
(INAF-IASFPA) and B.-B. Zhang report on behalf of the Swift-XRT team:

We have analysed 8.0 ks of XRT data for GRB 140331A (-B. Zhang	et al.
GCN Circ. 16049
Not found
 
 
),  from 374 s to 42.6 ks after the  BAT trigger. The
data comprise 118 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
Beardmore et al. (GCN Circ. 16052
Not found
 
 
).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.57 (+0.22, -0.19), followed by a break at T+1389 s to
an alpha of 0.69 (+/-0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.97 (+0.13, -0.12). The
best-fitting absorption column is  6.3 (+2.9, -2.0) x 10^20 cm^-2,
consistent with the Galactic value of 4.4 x 10^20 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.90 (+0.16,
-0.11) and a best-fitting absorption column of 4.8 (+3.7, -0.4) x 10^20
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.4 x 10^-11 (3.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.8 (+3.7, -0.4) x 10^20 cm^-2
Galactic foreground: 4.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.90 (+0.16, -0.11)

If the light curve continues to decay with a power-law decay index of
0.69, the count rate at T+24 hours will be 0.011 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x
10^-13 (4.2 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/00594081.

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