TITLE:   GCN CIRCULAR
NUMBER:  30576
SUBJECT: GRB 210731A: Swift-XRT refined Analysis
DATE:    21/08/01 17:24:27 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T.
Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), D.N. Burrows (PSU) and E. Troja report on behalf of the
Swift-XRT team:

We have analysed 7.9 ks of XRT data for GRB 210731A (Troja et al. GCN
Circ. 30568), from 191 s to 60.1 ks after the  BAT trigger. The data
comprise 62 s in Windowed Timing (WT) mode (the first 8 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 3002 s of PC mode data and 8 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 300.30466, -28.06096
which is equivalent to:

RA (J2000): 20h 01m 13.12s
Dec(J2000): -28d 03' 39.4"

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.6 (+/-0.5), followed by a break at T+424 s to an alpha
of 0.86 (+0.18, -0.19).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.07 (+0.29, -0.21). The
best-fitting absorption column is  1.36 (+0.47, -0.21) x 10^21 cm^-2,
consistent with the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.97 (+0.26,
-0.19) and a best-fitting absorption column of 1.35 (+0.77, -0.20) x
10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.4 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.35 (+0.77, -0.20) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.97 (+0.26, -0.19)

If the light curve continues to decay with a power-law decay index of
0.86, the count rate at T+24 hours will be 0.022 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.3 x
10^-13 (9.3 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/01062336.

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