GCN Circular 23266

A. Melandri (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti
(INAF-OAB/PSU), D.N. Burrows (PSU), Z. Liu (NAOC / U. Leicester), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC) and T.N. Ukwatta report on behalf of the
Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 180924A (Ukwatta et al. GCN
Circ. 23261
Not found
 
 
), from 82 s to 52.3 ks after the  BAT trigger. The data
comprise 141 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 23262
Not found
 
 
).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.23 (+0.11, -0.09), followed by a break at T+485 s to
an alpha of 0.70 (+0.06, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.96 (+0.14, -0.13). The
best-fitting absorption column is  1.0 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.0 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.88 (+0.20, -0.19)
and a best-fitting absorption column of 6.9 (+5.2, -4.5) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.9 (+5.2, -4.5) x 10^20 cm^-2
Galactic foreground: 2.0 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     1.88 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
0.70, the count rate at T+24 hours will be 0.017 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.8 x
10^-13 (6.6 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/00863186.

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