GCN Circular 22530

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), V. D'Elia
(ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), D.N. Burrows
(PSU), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), A.P. Beardmore (U.
Leicester) and E. Sonbas report on behalf of the Swift-XRT team:

We have analysed 7.1 ks of XRT data for GRB 180324A (Sonbas et al. GCN
Circ. 22524
Not found
 
 
), from 80 s to 13.9 ks after the  BAT trigger. The data are
entirely in Photon Counting (PC) mode. The enhanced XRT position for
this burst was given by Goad et al. (GCN Circ. 22525
Not found
 
 
).

The late-time light curve (from T0+5.7 ks) can be modelled with a
power-law decay with a decay index of alpha=1.07 (+/-0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.36 (+0.13, -0.12). The
best-fitting absorption column is  9.0 (+1.6, -1.4) x 10^21 cm^-2, in
excess of the Galactic value of 3.8 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 6.7 x 10^-11 (8.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.0 (+1.6, -1.4) x 10^21 cm^-2
Galactic foreground: 3.8 x 10^21 cm^-2
Excess significance: 5.9 sigma
Photon index:	     1.36 (+0.13, -0.12)

If the light curve continues to decay with a power-law decay index of
1.07, the count rate at T+24 hours will be 0.032 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.2 x
10^-12 (2.8 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/00817345.

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