GCN Circular 40416

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

We have analysed 6.8 ks of XRT data for GRB 250509A, from 63 s to 28.3
ks after the   trigger. The data comprise 156 s in Windowed Timing (WT)
mode (the first 6 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.20 (+/-0.09). At T+349 s  the decay
flattens to an alpha of -1.1 (+0.7, -0.4) before breaking again at
T+1377 s to a final decay with index alpha=1.44 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.02 (+0.11, -0.10). The
best-fitting absorption column is  6.6 (+/-0.7) x 10^21 cm^-2, in
excess of the Galactic value of 2.0 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.13 (+/-0.15) and a
best-fitting absorption column of 6.0 (+1.0, -0.9) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.1 x 10^-11 (7.5 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.0 (+1.0, -0.9) x 10^21 cm^-2
Galactic foreground: 2.0 x 10^20 cm^-2
Excess significance: 10.7 sigma
Photon index:	     2.13 (+/-0.15)

If the light curve continues to decay with a power-law decay index of
1.44, the count rate at T+24 hours will be 9.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.9 x
10^-13 (7.1 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/01311764.

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