TITLE:   GCN CIRCULAR
NUMBER:  25851
SUBJECT: GRB 190926A: Swift-XRT refined Analysis
DATE:    19/09/26 19:45:30 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB) and A.
Melandri report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 190926A (Melandri et al. GCN
Circ. 25848), from 140 s to 23.8 ks after the  BAT trigger. The data
comprise 685 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN Circ. 25850).

The late-time light curve (from T0+4.1 ks) can be modelled with an
initial power-law decay with an index of alpha=0.661 (+0.053, -0.030),
followed by a break at T+677 s to an alpha of 3.34 (+0.30, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.804 (+/-0.023). The
best-fitting absorption column is  3.00 (+/-0.11) x 10^21 cm^-2, in
excess of the Galactic value of 7.5 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.1 x 10^-11 (5.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     3.00 (+/-0.11) x 10^21 cm^-2
Galactic foreground: 7.5 x 10^20 cm^-2
Excess significance: 33.3 sigma
Photon index:	     1.804 (+/-0.023)

If the light curve continues to decay with a power-law decay index of
3.34, the count rate at T+24 hours will be 3.2 x 10^-6 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-16 (1.7 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00926515.

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