[vsnet-grb-info 11750] GRB 120327A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13141
SUBJECT: GRB 120327A: Swift-XRT refined Analysis
DATE:    12/03/27 16:17:03 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at star.le.ac.uk>

M.C. Stroh (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), C.
Pagani (U. Leicester), A.P. Beardmore (U. Leicester), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC), A. Maselli  (INAF-IASFPA), J.A. Kennea
(PSU) and B. Sbarufatti report on behalf of the Swift-XRT team:

We have analysed 7.3 ks of XRT data for GRB 120327A (Sbarufatti  et al.
GCN Circ. 13123), from 66 s to 0 s after the  BAT trigger. The data
comprise 267 s in Windowed Timing (WT) mode (the first 7 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 Osborne et
al. (GCN. Circ 13135).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=2.95 (+0.10, -0.11). At T+217 s  the decay
flattens to an alpha of 0.62 (+0.09, -0.13) before breaking again at
T+2540 s to a final decay with index alpha=1.42 (+0.13, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.98 (+/-0.08). The
best-fitting absorption column is  1.5 (+0.5, -0.4) x 10^22 cm^-2, at a
redshift of 2.81, in addition to the Galactic value of 1.4 x 10^21
cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index
of 1.79 (+0.11, -0.10) and a best-fitting absorption column of 1.5
(+0.8, -0.7) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10
keV flux conversion factor deduced from this spectrum  is 4.4 x 10^-11
(5.8 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.4 x 10^21 cm^-2
Intrinsic column:    1.5 (+0.8, -0.7) x 10^22 cm^-2 at z=2.81
Photon index:	     1.79 (+0.11, -0.10)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00518731.

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