[vsnet-grb-info 18664] GRB 161023A: Swift-XRT afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20112
SUBJECT: GRB 161023A: Swift-XRT afterglow detection
DATE:    16/10/24 17:39:57 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows
(PSU), T.G.R. Roegiers (PSU), K.L. Page (U. Leicester), B. Mingo (U.
Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of
the Swift-XRT team:

Swift-XRT has performed follow-up observations of the INTEGRAL-detected
burst GRB 161023A (Gorbovskoy et al. GCN Circ. 20102), collecting 1.1
ks of Photon Counting (PC) mode data between T0+34.8 ks and T0+53.2 ks.


A fading, uncatalogued X-ray source is detected consistent with the
MASTER position (Gorbovskoy et al., GCN Circ. 20102) and is therefore
believed to be the afterglow. Using 3643 s of PC mode data and 4 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 =
311.02161, -47.66348 which is equivalent to:

RA (J2000): 20h 44m 05.19s
Dec(J2000): -47d 39' 48.5"

with an uncertainty of 1.5 arcsec (radius, 90% confidence). This
position is 0.2 arcsec from the MASTER position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.3 (+0.9, -1.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.09 (+0.30, -0.16). The
best-fitting absorption column is  consistent with the Galactic value
of 3.2 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.1 x 10^-11 (3.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.2 (+/-5.6) x 10^20 cm^-2
Galactic foreground: 3.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.09 (+0.30, -0.16)

If the light curve continues to decay with a power-law decay index of
2.3, the count rate at T+24 hours will be 9.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x
10^-13 (3.1 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/00020709.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020709.

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