GCN Circular 25851

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
Not found
 
 
), 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
Not found
 
 
).

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.