[vsnet-grb-info 23927] GRB 191016A: Swift-XRT afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26026
SUBJECT: GRB 191016A: Swift-XRT afterglow detection
DATE:    19/10/18 00:12:22 GMT
FROM:    Boris Sbarufatti at PSU  <bxs60 at psu.edu>

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M. Perri
(ASDC), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), B. Sbarufatti (PSU),
D.N. Burrows (PSU), J. D. Gropp (PSU) and P.A. Evans (U. Leicester)
report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Swift/BAT-detected burst GRB 191016A (WeiKang Zheng et al. GCN Circ.
26012), collecting 4.2 ks of Photon Counting (PC) mode data between
T0+45.0 ks and T0+56.9 ks. 

An uncatalogued X-ray source is detected consistent with being within
296 arcsec of the Swift/BAT position and is above the RASS limit, and
is therefore likely the GRB afterglow. Using 3470 s of PC mode data and
3 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 = 30.26933, +24.50988 which is equivalent to:

RA (J2000): 02h 01m 04.64s
Dec(J2000): +24d 30' 35.6"

with an uncertainty of 1.9 arcsec (radius, 90% confidence). This
position is 93 arcsec from the Swift/BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.8 (+1.5, -0.5).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+0.5, -0.4). The
best-fitting absorption column is  1.4 (+1.7, -0.4) x 10^21 cm^-2,
consistent with the Galactic value of 1.0 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 3.4 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+1.7, -0.4) x 10^21 cm^-2
Galactic foreground: 1.0 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.9 (+0.5, -0.4)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00929744.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00929744.

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