[vsnet-grb-info 17601] GRB 160220B: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19036
SUBJECT: GRB 160220B: Swift-XRT refined Analysis
DATE:    16/02/20 20:21:46 GMT
FROM:    Sarah Gibson at U.of Leicester  <slg44 at leicester.ac.uk>

J. R. Cummings (NASA/UMBC) reports on behalf of the Swift-XRT team:

We have analysed 6.0 ks of XRT data for GRB 160220B (Cummings et al.
GCN Circ. 19029), from 3.3 ks to 16.9 ks after the  BAT trigger. The
data are entirely in Photon Counting (PC) mode. Using 4686 s of PC mode
data and 8 UVOT images, we find an enhanced XRT position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue): RA, Dec = 259.86518, -18.12401 which is equivalent to:

RA (J2000): 17h 19m 27.64s
Dec(J2000): -18d 07' 26.5"

with an uncertainty of 1.5 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.37 (+0.18, -0.17).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.17 (+0.25, -0.23). The
best-fitting absorption column is  4.3 (+1.2, -1.1) x 10^21 cm^-2, in
excess of the Galactic value of 2.6 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (6.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.3 (+1.2, -1.1) x 10^21 cm^-2
Galactic foreground: 2.6 x 10^21 cm^-2
Excess significance: 2.5 sigma
Photon index:	     2.17 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of
1.37, the count rate at T+24 hours will be 2.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.7 x
10^-14 (1.7 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/00674734.

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