GCN Circular 38523

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

We have analysed 4.9 ks of XRT data for GRB 241209A, from 91 s to 40.8
ks after the  BAT trigger. The data comprise 42 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 an initial power-law decay with an
index of alpha=1.16 (+0.17, -0.16), followed by a break at T+485 s to
an alpha of 8.0 (+0.0, -1.2).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.8 (+/-0.3). The
best-fitting absorption column is  3.0 (+1.7, -1.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 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 3.7 x 10^-11 (5.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     3.0 (+1.7, -1.4) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 3.4 sigma
Photon index:	     1.8 (+/-0.3)

If the light curve continues to decay with a power-law decay index of
8.0, the count rate at T+24 hours will be 1.3 x 10^-18 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.0 x
10^-29 (6.8 x 10^-29) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01273071.

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