TITLE:   GCN CIRCULAR
NUMBER:  26961
SUBJECT: GRB 200131A: Swift-XRT refined Analysis
DATE:    20/02/01 08:55:16 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne
(U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) ,
M. Capalbi (INAF-IASFPA), D.N. Burrows (PSU), J. D. Gropp (PSU) and B.
Sbarufatti report on behalf of the Swift-XRT team:

We have analysed 6.1 ks of XRT data for GRB 200131A (Sbarufatti et al.
GCN Circ. 26953), from 61 s to 17.4 ks after the  BAT trigger. The data
comprise 249 s in Windowed Timing (WT) mode (the first 5 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 Goad et al.
(GCN Circ. 26956). We cannot determine at the present time whether the
source is fading.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.83 (+0.04, -0.03). The
best-fitting absorption column is  consistent with the Galactic value
of 1.8 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.81 (+0.15, -0.14) and a best-fitting absorption
column of 2.3 (+0.6, -0.4) x 10^21 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 4.0 x 10^-11 (5.2 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.3 (+0.6, -0.4) x 10^21 cm^-2
Galactic foreground: 1.8 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.81 (+0.15, -0.14)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00953689.

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