[vsnet-grb-info 20078] GRB 171124A: Swift-XRT afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22174
SUBJECT: GRB 171124A: Swift-XRT afterglow detection
DATE:    17/12/01 11:21:08 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu
(PSU), S. J. LaPorte (PSU), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester) and P.A. Evans (U. Leicester) report on behalf of the
Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
Fermi/LAT-detected burst GRB 171124A (Longo et al. GCN Circ. 22156).
The observations now extend from T0+21.8 ks to T0+579.1 ks. 

Of the sources reported by Osborne et al. (GCN Circ. 22165), "Source 2"
is fading with 3-sigma significance, and is therefore likely the GRB
afterglow. Using 4982 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 = 335.47598, +35.33213
which is equivalent to:

RA (J2000): 22h 21m 54.24s
Dec(J2000): +35d 19' 55.7"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 7.1 arcmin from the Fermi/LAT position.  The source is
fading with alpha >0.7.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.8 (+0.8, -0.7). The
best-fitting absorption column is  7.9 (+4.7, -3.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.3 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 2.9 x 10^-11 (1.0 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.9 (+4.7, -3.5) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: 3.1 sigma
Photon index:	     2.8 (+0.8, -0.7)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020784.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020784.

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