GCN Circular 36956

J. D. Gropp (PSU), S. Dichiara (PSU), A.P. Beardmore (U. Leicester),
J.P. Osborne (U. Leicester), C. Salvaggio (INAF-OAB), M. Ferro
(INAF-OAB), R. Brivio (INAF-OAB), J.A. Kennea (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 240727A, from 109 s to 51.8
ks after the  BAT trigger. The data comprise 377 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.89 (+0.26, -0.58), followed by a break at T+177 s to
an alpha of 1.80 (+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 1.98 (+/-0.06). The
best-fitting absorption column is  1.46 (+0.19, -0.18) x 10^21 cm^-2,
in excess of the Galactic value of 3.6 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.25 (+0.14, -0.12)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.3 x 10^-11 (5.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.6 (+3.6, -0.0) x 10^20 cm^-2
Galactic foreground: 3.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.25 (+0.14, -0.12)

If the light curve continues to decay with a power-law decay index of
1.80, the count rate at T+24 hours will be 4.8 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.5 x
10^-14 (2.6 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/01245074.

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