GCN Circular 36538

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

We have analysed 6.5 ks of XRT data for GRB 240523A, from 132 s to 27.6
ks after the  BAT trigger. The data comprise 8 s in Windowed Timing
(WT) mode (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=2.8 (+0.6, -0.4). At T+400 s  the decay
flattens to an alpha of -0.4 (+0.3, -0.5) before breaking again at
T+2137 s to a final decay with index alpha=1.41 (+0.26, -0.24).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.06 (+0.24, -0.22). The
best-fitting absorption column is  1.1 (+0.6, -0.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.6 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.3 x 10^-11 (4.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.1 (+0.6, -0.5) x 10^21 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: 2.9 sigma
Photon index:	     2.06 (+0.24, -0.22)

If the light curve continues to decay with a power-law decay index of
1.41, the count rate at T+24 hours will be 2.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.5 x
10^-14 (8.2 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01230423.

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