[vsnet-grb-info 13725] GRB 130725B: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15040
SUBJECT: GRB 130725B: Swift-XRT refined Analysis
DATE:    13/07/26 15:10:08 GMT
FROM:    Boris Sbarufatti at INAF-OAB/IASFPA  <boris.sbarufatti at brera.inaf.it>

B. Sbarufatti (INAF-OAB/PSU) and C. A. Swenson (PSU) report on behalf
of the Swift-XRT team:

We have analysed 8.2 ks of XRT data for GRB 130725B (Swenson  et al.
GCN Circ. 15029),  from 53 s to 35.0 ks after the  BAT trigger. The
data comprise 235 s in Windowed Timing (WT) mode (the first 8 s were
taken while Swift was slewing) with the remainder in Photon Counting
(PC) mode. Using 6414 s of PC mode data and 9 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 214.24059, -11.12851
which is equivalent to:

RA (J2000): 14h 16m 57.74s
Dec(J2000): -11d 07' 42.6"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.09 (+0.15, -0.18), followed by a break at T+534 s to
an alpha of 0.95 (+/-0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+0.17, -0.16). The
best-fitting absorption column is  8.3 (+4.2, -3.2) x 10^20 cm^-2,
consistent with the Galactic value of 5.2 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 (4.5 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.3 (+4.2, -3.2) x 10^20 cm^-2
Galactic foreground: 5.2 x 10^20 cm^-2
Photon index:	     1.97 (+0.17, -0.16)

If the light curve continues to decay with a power-law decay index of
0.95, the count rate at T+24 hours will be 0.012 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.5 x
10^-13 (5.5 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/00564028.

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