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GCN Circular 17048

Subject
GRB 141109A: Swift-XRT refined Analysis
Date
2014-11-09T23:04:08Z (10 years ago)
From
Paolo D'Avanzo at INAF-OAB <paolo.davanzo@brera.inaf.it>
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),  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).

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.
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