GCN Circular 37406

M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), M. A.
Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and
P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.6 ks of XRT data for GRB 240905E, from 96 s to 50.6
ks after the  BAT trigger. The data comprise 146 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The refined XRT position is RA,
Dec = 345.8021, +35.5227 which is equivalent to:

RA (J2000): 23 03 12.51
Dec(J2000): +35 31 21.6

with an uncertainty of 4.1 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=1.65 (+/-0.13), followed by a break at T+260 s to an
alpha of 4.5 (+3.5, -0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.25 (+/-0.10). The
best-fitting absorption column is  1.3 (+0.5, -0.4) x 10^21 cm^-2,
consistent with the Galactic value of 9.3 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 5.3 x 10^-11 (5.7 x
10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.3 (+0.5, -0.4) x 10^21 cm^-2
Galactic foreground: 9.3 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.25 (+/-0.10)

If the light curve continues to decay with a power-law decay index of
4.5, the count rate at T+24 hours will be 3.0 x 10^-11 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x
10^-21 (1.7 x 10^-21) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01252695.

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