[vsnet-grb-info 8276] GRB 090814A: Swift-XRT Team Refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9801
SUBJECT: GRB 090814A: Swift-XRT Team Refined Analysis
DATE:    09/08/14 13:33:54 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

K.L. Page (U. Leicester) & T. N. Ukwatta (GSFC/GWU) report on behalf of 
the Swift-XRT team: 

We have analysed the first four orbits of data collected for GRB 090814A, 
from 165 s to 25.7 ks after the BAT trigger. These data comprise 258 s in 
Windowed Timing (WT) mode and 7.8 ks in Photon Counting mode. The enhanced 
XRT position for this burst was given by Beardmore et al. in GCN Circ. 
9796.

The light curve can be modelled with a series of power-law decays, 
starting with alpha1 = 2.60 +/- 0.08 until ~T+830 s, at which point the 
decay flattens slightly to alpha2 = 1.91 +/- 0.20. The light curve 
flattens again at ~T+5.4 ks, to a slope of alpha3 = 0.51 +/- 0.48. With 
the limited data at this time, alpha3 has a relatively large uncertainty.

The spectrum extracted from the WT data can be fitted with an absorbed 
power-law, with Gamma = 2.78 +0.12/-0.11, absorbed by the Galactic column 
of NH = 4.76 x 10^20 cm^-2 (Kalberla et al. 2005), together with an 
intrinsic column (assuming z = 0.696; Jakobsson et al., GCN Circ. 9797) of 
(2.0 +/- 0.4) x 10^21 cm^-2. Note, however, that this redshift 
determination is uncertain. The total (including Galactic) column at z = 0 
would be (1.3 +/- 0.2) x 10^21 cm^-2. Alternatively, the spectrum is 
better fitted (at >3 sigma) by a broken power-law with Gamma1 = 0.47 
+0.72/-1.59 below a break energy of 0.62 +0.09/-0.08 keV, followed by 
Gamma2 = 2.62 +/- 0.09; this fit only requires the Galactic column. The 
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced 
from the broken power-law fit is 2.5 x 10^-11 (3.0 x 10^-11) erg cm^-2 
count^-1.

If the light curve continues to decay with a power-law decay index of 
0.51, the count rate at T+24 hours will be 0.0041 count s^-1, 
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x 10^-13 
(1.2 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/00359951.

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