GCN Circular 17048

P. D'Avanzo (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf
of the Swift-XRT team:

We have analysed 5.7 ks of XRT data for GRB 141109A (D'Avanzo  et al.
GCN Circ. 17037
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 ),  from 119 s to 46.5 ks after the  BAT trigger. The
data comprise 409 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 enhanced XRT position for this burst was given by
Osborne et al. (GCN Circ. 17043
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=0.80 (+0.25, -0.14). At T+212 s  the decay
steepens to an alpha of 3.25 (+0.51, -0.20) before breaking again at
T+363 s to a final decay with index alpha=1.254 (+0.021, -0.020). Some
flaring activity is observed between T+300 s and T+500 s.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.673 (+0.028, -0.027). The
best-fitting absorption column is  1.73 (+0.11, -0.10) x 10^21 cm^-2,
in excess of the Galactic value of 3.3 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.99 (+0.14, -0.13)
and a best-fitting absorption column of 1.3 (+/-0.4) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.4 x 10^-11 (4.4 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^20 cm^-2
Excess significance: 4.5 sigma
Photon index:	     1.99 (+0.14, -0.13)

If the light curve continues to decay with a power-law decay index of
1.254, the count rate at T+24 hours will be 0.033 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.4 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00618024.

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