GCN Circular 37479

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

We have analysed 6.9 ks of XRT data for GRB 240912A, from 99 s to 45.7
ks after the  BAT trigger. The data comprise 939 s in Windowed Timing
(WT) mode (the first 5 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+4.8 ks) can be modelled with a
power-law decay with a decay index of alpha=0.82 (+/-0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.925 (+/-0.020). The
best-fitting absorption column is  2.61 (+/-0.29) x 10^21 cm^-2, at a
redshift of 1.234, in addition to the Galactic value of 5.6 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.86 (+/-0.09) and a best-fitting absorption column of 9 (+11, -9) x
10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.6 x 10^-11 (4.1 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 5.6 x 10^20 cm^-2
Intrinsic column:    9 (+11, -9) x 10^20 cm^-2 at z=1.234
Photon index:	     1.86 (+/-0.09)

If the light curve continues to decay with a power-law decay index of
0.82, the count rate at T+24 hours will be 0.12 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x
10^-12 (4.9 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/01253910.

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