[vsnet-grb-info 9396] GRB 100621A: Swift XRT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10877
SUBJECT: GRB 100621A: Swift XRT refined analysis
DATE:    10/06/21 14:10:13 GMT
FROM:    Giulia Stratta at ASDC  <giulia.stratta at gmail.com>

G. Stratta (ASDC), V. D'Elia (ASDC) and T. N. Ukwatta (GSFC/GWU) report on
behalf of the Swift-XRT team:

We have analysed 3.6 ks of XRT data for GRB 100621A (Ukwatta et al. GCN
Circ. 10870), from 79 s to 7.0 ks after the BAT trigger. The data comprise
322 s in Windowed Timing (WT) mode with the remainder in Photon Counting
(PC) mode. The enhanced XRT position for this burst was given by Evans et
al. (GCN. Circ 10872).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.41 (+0.11, -0.18). At T+153 s the decay
steepens to an alpha of 4.34 (+0.20, -0.12) before breaking again at T+389 s
to a final decay with index alpha=0.55 (+0.13, -0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.73 (+0.07, -0.06). The
best-fitting absorption column is 6.11 (+0.27, -0.26) x 10^21 cm^-2, in
excess of the Galactic value of 2.9 x 10^20 cm^-2 (Kalberla et al. 2005).
The PC mode spectrum has a photon index of 2.15 (+0.17, -0.16) and a
best-fitting absorption column of 6.6 (+1.0, -0.9) x 10^21 cm^-2. The counts
to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum is 5.0 x 10^-11 (1.0 x 10^-10) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 0.55,
the count rate at T+24 hours will be 0.23 count s^-1, corresponding to an
observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-11 (2.4 x 10^-11) erg
cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00425151.

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