[vsnet-grb-info 18410] GRB 160824A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19858
SUBJECT: GRB 160824A: Swift-XRT refined Analysis
DATE:    16/08/25 12:29:35 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), T.G.R. Roegiers
(PSU), L.M. McCauley (PSU), J.A. Kennea (PSU) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 160824A (Evans et al. GCN
Circ. 19855), from 81 s to 29.6 ks after the  BAT trigger. The data
comprise 105 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 Goad et al.
(GCN Circ. 19857).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.33 (+0.17, -0.18). At T+258 s  the decay
flattens to an alpha of 0.44 (+0.05, -0.06) before breaking again at
T+7762 s to a final decay with index alpha=1.31 (+/-0.14).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.07 (+/-0.10). The
best-fitting absorption column is  1.18 (+0.11, -0.10) x 10^22 cm^-2,
in excess of the Galactic value of 6.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.96 (+/-0.13) and a
best-fitting absorption column of 1.05 (+0.14, -0.13) x 10^22 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 5.0 x 10^-11 (9.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.05 (+0.14, -0.13) x 10^22 cm^-2
Galactic foreground: 6.3 x 10^21 cm^-2
Excess significance: 5.3 sigma
Photon index:	     1.96 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
1.31, the count rate at T+24 hours will be 0.018 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.8 x
10^-13 (1.6 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/00709765.

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