[vsnet-grb-info 17796] GRB 160327A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19238
SUBJECT: GRB 160327A: Swift-XRT refined Analysis
DATE:    16/03/27 20:13:31 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

P. D'Avanzo (INAF-OAB), T.G.R. Roegiers (PSU), L.M. McCauley (PSU),
J.A. Kennea (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U.
Leicester), J.P. Osborne (U. Leicester), V. D'Elia (ASDC), A. D'Ai
(INAF-IASFPA) and J.L. Racusin report on behalf of the Swift-XRT team:

We have analysed 6.0 ks of XRT data for GRB 160327A (Racusin et al. GCN
Circ. 19235), from 51 s to 22.1 ks after the  BAT trigger. The data
comprise 7 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. The enhanced
XRT position for this burst was given by Goad et al. (GCN Circ. 19237).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.53 (+0.14, -0.13), followed by a break at T+291 s to
an alpha of 0.59 (+/-0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.97 (+0.15, -0.09). The
best-fitting absorption column is  consistent with the Galactic value
of 1.2 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.90 (+0.16, -0.15) and a best-fitting absorption
column of 4.2 (+3.5, -3.0) 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 (3.7 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.2 (+3.5, -3.0) x 10^20 cm^-2
Galactic foreground: 1.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.90 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
0.59, the count rate at T+24 hours will be 0.023 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.7 x
10^-13 (8.5 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/00680655.

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