GCN Circular 21897
Subject
LIGO/Virgo G298048: Milliarcsecond imaging of the NGC 4993 central radio source
Date
2017-09-19T07:51:22Z (7 years ago)
From
Igor Andreoni at Swinburne U of Tech <igor.andreoni@gmail.com>
A. Deller (Swinburne/OzGrav), M. Bailes (Swinburne/OzGrav), I. Andreoni
(Swinburne/OzGrav/AAO), K. Bannister (CSIRO), J. Cooke (Swinburne/OzGrav),
D. Dobie (University of Sydney), D. Kaplan (UWM), C. Lynch (University of
Sydney), T. Murphy (University of Sydney), on behalf of a joint effort
between OzGrav and the VAST collaboration.
We have further analysed the Very Long Baseline Array (VLBA) observations of
the field containing NGC 4993 and SSS17a/DLT17ck (Deller et al., LVC GCN
21588; Deller et al., LVC GCN 21850), producing images centered on the
sub-mJy radio source at the centre of NGC 4993 (Alexander et al., LVC GCN
21545; Alexander et al., LVC GCN 21548; Bannister et al., LVC GCN 21559;
Alexander et al., LVC GCN 21589; Corsi et al., LVC GCN 21614).
The source is detected with a flux density of 0.22 mJy (9 sigma
significance, flux calibration scale uncertainty estimated at 20%). The
data is consistent with either an unresolved or marginally resolved source,
while comparison with the flux densities estimated by the ATCA and VLA
observations indicate that most (or possibly all) of the source flux is
contained within this milliarcsecond-scale component. Taking the synthesized
beam size of 2.5 x 1.0 mas as an upper limit to the source size, the
inferred lower limit for the brightness temperature is 1.6 x 10^6 K,
consistent with an AGN interpretation. The position obtained is
13:09:47.69398 -23:23:02.3195 (J2000), with estimated uncertainties
(dominated by systematics) of <=1 mas in each coordinate.