[vsnet-grb-info 26247] GRB 200901B: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28347
SUBJECT: GRB 200901B: Swift-XRT refined Analysis
DATE:    20/09/02 07:26:03 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

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

We have analysed 4.6 ks of XRT data for GRB 200901B (Sbarufatti et al.
GCN Circ. 28337), from 11.3 ks to 35.0 ks after the  BAT trigger. The
data are entirely in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Osborne et al. (GCN Circ. 28339).

The light curve can be modelled with a series of power-law decays. The
light curve initially rises, with an index alpha=-0.6 (+0.7, -0.9). At
T+239 s it breaks to an alpha of 0.54 (+0.08, -0.07) before breaking
again at T+950 s to a final decay with index alpha=2.61 (+1.29, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.15 (+/-0.05). The
best-fitting absorption column is  1.66 (+0.27, -0.26) x 10^21 cm^-2,
in excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.8 x 10^-11 (6.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.66 (+0.27, -0.26) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 2.9 sigma
Photon index:	     1.15 (+/-0.05)

If the light curve continues to decay with a power-law decay index of
2.61, the count rate at T+24 hours will be 8.7 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.1 x
10^-15 (5.5 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00994165.

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