GCN Circular 37121

A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), J. D. Gropp
(PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 240809A, from 77 s to 45.8
ks after the  BAT trigger. The data comprise 1.3 ks in Windowed Timing
(WT) mode (the first 9 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=1.02 (+0.06, -0.05). At T+257 s  the decay
flattens to an alpha of 0.825 (+/-0.028) before breaking again at
T+2884 s to a final decay with index alpha=1.57 (+/-0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.706 (+/-0.030). The
best-fitting absorption column is  1.94 (+0.13, -0.12) x 10^21 cm^-2,
in excess of the Galactic value of 1.5 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.66 (+/-0.10) and a
best-fitting absorption column of 1.9 (+/-0.4) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.4 x 10^-11 (5.3 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.9 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: 1.8 sigma
Photon index:	     1.66 (+/-0.10)

If the light curve continues to decay with a power-law decay index of
1.57, the count rate at T+24 hours will be 0.050 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.2 x
10^-12 (2.6 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/01247745.

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