GCN Circular 37681

J.P. Osborne (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), V. D'Elia (SSDC & INAF-OAR), M. A. Williams (PSU), S.
Dichiara (PSU), J.A. Kennea (PSU), 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 10 ks of XRT data for GRB 241002A, from 95 s to 38.9
ks after the  BAT trigger. The data comprise 212 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 light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=0.8 (+1.1, -1.8). At T+122 s  the decay
steepens to an alpha of 5.1 (+/-0.7) before breaking again at T+268 s
to a final decay with index alpha=0.57 (+0.12, -0.18).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.1 (+/-0.3). The
best-fitting absorption column is  1.01 (+0.21, -0.19) x 10^22 cm^-2,
in excess of the Galactic value of 1.4 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.1 (+0.5, -0.4) and
a best-fitting absorption column of 8.6 (+4.2, -3.4) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.3 x 10^-11 (8.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.6 (+4.2, -3.4) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: 4.1 sigma
Photon index:	     2.1 (+0.5, -0.4)

If the light curve continues to decay with a power-law decay index of
0.57, the count rate at T+24 hours will be 0.011 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.6 x
10^-13 (9.3 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/01257556.

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