GCN Circular 37059

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai
(INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), A.
Tohuvavohu (U. Toronto), D.N. Burrows (PSU), J. D. Gropp (PSU) and P.A.
Evans report on behalf of the Swift-XRT team:

We have analysed 8.2 ks of XRT data for GRB 240805B, from 71 s to 30.3
ks after the  BAT trigger. The data comprise 4.5 ks in Windowed Timing
(WT) mode (the first 6 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+5.6 ks) can be modelled with a
power-law decay with a decay index of alpha=1.55 (+/-0.26).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.77 (+/-0.06). The
best-fitting absorption column is  1.19 (+0.21, -0.20) x 10^21 cm^-2,
in excess of the Galactic value of 7.6 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.9 (+0.4, -0.3) and
a best-fitting absorption column of 1.5 (+1.4, -0.7) 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 (4.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.5 (+1.4, -0.7) x 10^21 cm^-2
Galactic foreground: 7.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.9 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of
1.55, the count rate at T+24 hours will be 3.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.6 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/01246989.

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