GCN Circular 38508

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR),
S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 241209D, from 260 s to 37.7
ks after the  BAT trigger. The data comprise 22 s in Windowed Timing
(WT) mode 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=4.3 (+3.4, -2.5), followed by a break at T+626 s to an
alpha of 0.66 (+/-0.13).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.89 (+0.29, -0.26). The
best-fitting absorption column is  5.9 (+6.9, -3.6) x 10^20 cm^-2,
consistent with the Galactic value of 2.3 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 3.5 x 10^-11 (4.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.9 (+6.9, -3.6) x 10^20 cm^-2
Galactic foreground: 2.3 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.89 (+0.29, -0.26)

If the light curve continues to decay with a power-law decay index of
0.66, the count rate at T+24 hours will be 0.014 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.1 x
10^-13 (5.8 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/01273106.

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