TITLE: GCN CIRCULAR NUMBER: 21897 SUBJECT: LIGO/Virgo G298048: Milliarcsecond imaging of the NGC 4993 central radio source DATE: 17/09/19 07:51:22 GMT FROM: Igor Andreoni at Swinburne U of Tech 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.