GCN Circular 37045

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), A.
Melandri (INAF-OAR), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto),
D.N. Burrows (PSU) and P.A. Evans report on behalf of the Swift-XRT
team:

We have analysed 1.7 ks of XRT data for GRB 240805A, from 89 s to 5.1
ks after the  BAT trigger. The data comprise 75 s in Windowed Timing
(WT) mode (the first 6 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. Using 1639 s of PC mode data
and 4 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 = 190.91475, -53.59526 which is equivalent to:

RA (J2000): 12h 43m 39.54s
Dec(J2000): -53d 35' 42.9"

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=7.82 (+0.18, -2.12), followed by a break at T+97.3 s to
an alpha of 1.292 (+0.027, -0.026).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.79 (+/-0.08). The
best-fitting absorption column is  5.1 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 2.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.52 (+0.23, -0.22)
and a best-fitting absorption column of 3.8 (+1.5, -1.2) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 5.3 x 10^-11 (6.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.8 (+1.5, -1.2) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^21 cm^-2
Excess significance: 2.0 sigma
Photon index:	     1.52 (+0.23, -0.22)

If the light curve continues to decay with a power-law decay index of
1.292, the count rate at T+24 hours will be 0.020 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.4 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/01246907.

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