TITLE:   GCN CIRCULAR
NUMBER:  10992
SUBJECT: GRB 100725B: refined XRT analysis 
DATE:    10/07/26 11:41:00 GMT
FROM:    Boris Sbarufatti at INAF-OAB/IASFPA  <boris.sbarufatti@brera.inaf.it>

V. Mangano (INAF IASF-PA), B. Sbarufatti (INAF OAB/INAF IASF-PA)
and W. H. Baumgartner (GSFC/UMBC) report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 100725B (Baumgartner et al.
GCN Circ. 10983), from 68 s to 58.8 ks after the BAT trigger. The data
comprise 288 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 light curve shows flaring activity for the initial 300 s (i.e. the
WT observation), then can be modeled with a series of power-law decays
and a small Gaussian flare.
The initial decay index is alpha=4.66 (+0.4 -0.2). The Gaussian flare peaks
at T+780 (+/-37) s with a sigma of 145 (+/- 30) s.
At T+770 s the decay change in a possible slow rise with an alpha of
0.30 (+/- 0.27) before breaking again at T+8660 s to a final
decay with index alpha=1.3 (+/- 0.2).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index    of 1.75 (+/-0.06). The
best-fitting absorption column is  4.7 (+0.4, -0.3) x 10^21 cm^-2,
in excess of the Galactic value of 5.4 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum (extracted from T+359 s to T+6761 s, 2.9 ks
exposure) has a photon index of 2.5 (+/-0.2) and a best-fitting absorption
column of 5.1 (+0.7, -0.6) x 10^21 cm^-2. The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum is
4.1 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
1.3, the count rate at T+48 hours will be 4.3 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of
1.8 x 10-13 (4.7 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/00429969.

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