TITLE:   GCN CIRCULAR
NUMBER:  41479
SUBJECT: GRB 250821A: Swift-XRT refined Analysis
DATE:    25/08/21 22:32:18 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

M.A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai
(INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri (INAF-OAR), J.A.
Kennea (PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.4 ks of XRT data for GRB 250821A, from 150 s to 29.3
ks after the   trigger. The data are entirely in Photon Counting (PC)
mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.32 (+/-0.20).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.59 (+0.42, -0.28). The
best-fitting absorption column is  3 (+13, -2) x 10^20 cm^-2,
consistent with the Galactic value of 1.6 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 4.2 x 10^-11 (4.4 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3 (+13, -2) x 10^20 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.59 (+0.42, -0.28)

If the light curve continues to decay with a power-law decay index of
1.32, the count rate at T+24 hours will be 1.0 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x
10^-15 (4.6 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/01344131.

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