GCN Circular 15926

K.L. Page (U. Leicester), C. Pagani (U. Leicester), A. Maselli 
(INAF-IASFPA), A. Melandri (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU),
J.A. Kennea (PSU), V. Mangano (PSU), M.C. Stroh (PSU), A.P. Beardmore
(U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 140304A (Evans  et al. GCN
Circ. 15915
Not found
 
 
),  from 81 s to 23.1 ks after the  BAT trigger. The data
comprise 42 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Beardmore et al. (GCN Circ. 15925
Not found
 
 
). The late-time light curve (from
T0+5.2 ks) is consistent with a constant source of mean count rate
1.1e+00 ct/sec. A power-law fit gives an index of 0.2 (+/-0.7).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.00 (+0.15, -0.14). The
best-fitting absorption column is  9.8 (+3.4, -3.1) x 10^20 cm^-2, in
excess of the Galactic value of 6.0 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (4.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.8 (+3.4, -3.1) x 10^20 cm^-2
Galactic foreground: 6.0 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     2.00 (+0.15, -0.14)

If the light curve continues to decay with a power-law decay index of
0.2, the count rate at T+24 hours will be 0.024 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.7 x
10^-13 (1.1 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00590206.

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