GCN Circular 40644

A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB),
M.A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester)
and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.0 ks of XRT data for GRB 250605A, from 90 s to 50.2
ks after the   trigger. The data comprise 149 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=4.36 (+0.19, -0.15). At T+164 s  the decay
flattens to an alpha of 3.19 (+0.29, -0.32) before breaking again at
T+326 s to a final decay with index alpha=0.79 (+/-0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.12 (+0.06, -0.05). The
best-fitting absorption column is  2.59 (+0.20, -0.19) x 10^21 cm^-2,
in excess of the Galactic value of 4.3 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.10 (+0.13, -0.12)
and a best-fitting absorption column of 2.9 (+/-0.5) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (5.5 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 4.3 x 10^20 cm^-2
Excess significance: 8.7 sigma
Photon index:	     2.10 (+0.13, -0.12)

If the light curve continues to decay with a power-law decay index of
0.79, the count rate at T+24 hours will be 0.085 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-12 (4.7 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01321323.

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