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TITLE: GCN CIRCULAR
NUMBER: 28916
SUBJECT: GRB 201116A: Swift-XRT afterglow detection
DATE: 20/11/17 08:48:42 GMT
FROM: Phil Evans at U of Leicester
A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P.
D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU) and
P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 201116A (Axelsson et al. GCN Circ. 28907)
in a series of observations tiled on the sky. The total exposure time
is 4.8 ks, distributed over 3 tiles; the maximum exposure at a single
sky location was 3.1 ks. The data were collected between T0+46.5 ks and
T0+62.3 ks, and are entirely in Photon Counting (PC) mode.
Four uncatalogued X-ray sources are detected, of which one ("Source 3")
is above the RASS 3-sigma upper limit at this position, and shows
signes of fading, and is therefore likely the GRB afterglow. Using 2405
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 = 149.32493, +0.27586 which is equivalent
to:
RA (J2000): 09h 57m 17.98s
Dec(J2000): +00d 16' 33.1"
with an uncertainty of 2.5 arcsec (radius, 90% confidence). This
position is 2.4 arcmin from the Fermi/LAT position. As already
reported by de Ugarte Postigo et al (GCN Circ. 28915), there is an
optical afterglow counterpart consistent with this X-ray object. The
light curve is consistent with a constant source of mean count rate
5.2e-02 ct/sec, however fading also cannot be ruled out. A power-law
fit gives an index of 1.6 (+1.4, -2.0), thus the large uncertainties
hinder a confident statement about fading.
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+0.6, -0.5). The
best-fitting absorption column is 2.7 (+2.5, -1.9) x 10^21 cm^-2, in
excess of the Galactic value of 2.8 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 3.2 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 2.7 (+2.5, -1.9) x 10^21 cm^-2
Galactic foreground: 2.8 x 10^20 cm^-2
Excess significance: 2.1 sigma
Photon index: 1.9 (+0.6, -0.5)
The results of the XRT-team automatic analysis of the afterglow are at:
https://www.swift.ac.uk/xrt_products/00021040.
The results of the full analysis of the tiled XRT observations are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00095.
This circular is an official product of the Swift-XRT team.
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