[vsnet-grb-info 26641] GRB 201020B: Swift-XRT afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28740
SUBJECT: GRB 201020B: Swift-XRT afterglow detection
DATE:    20/10/22 09:04:39 GMT
FROM:    Paolo D'Avanzo at INAF - OAB  <paolo.davanzo at inaf.it>

B. Sbarufatti (PSU), D.N. Burrows (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), 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
Fermi/LAT-detected burst GRB 201020B (Arimoto et al. GCN Circ. 28716),
collecting 1.7 ks of Photon Counting (PC) mode data between T0+72.1 ks
and T0+74.3 ks. 

An uncatalogued X-ray source is detected and is above the RASS limit,
and is therefore likely the GRB afterglow. Using 1015 s of PC mode data
and 2 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 = 75.46680, +77.06830 which is equivalent to:

RA (J2000): 05h 01m 52.03s
Dec(J2000): +77d 04' 05.9"

with an uncertainty of 2.0 arcsec (radius, 90% confidence). This
position is 9.6 arcmin from the Fermi/LAT position and consistent with
the optical counterpart detected by MASTER (Lipunov et al. GCN Circ.
28718).  The light curve is consistent with a constant source of mean
count rate 1.5e-01 ct/sec. With the currently available dataset we
cannot constrain the temporal evolution.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+0.4, -0.3). The
best-fitting absorption column is  4.0 (+1.7, -1.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.4 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 3.3 x 10^-11 (5.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.0 (+1.7, -1.4) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^21 cm^-2
Excess significance: 3.0 sigma
Photon index:	     2.2 (+0.4, -0.3)

The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00021035/Source1.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021035.

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