Archival VLBA observations of the Cygnus A Nuclear Radio Transient (Cyg A-2) Strengthen the Tidal Disruption Event Interpretation

TL;DR

Archival VLBA data analysis of Cygnus A supports the interpretation that the transient Cyg A-2 is caused by a tidal disruption event, with revised timing and energetic estimates based on radio observations from 2002 to 2013.

Contribution

This study extends previous analyses by providing new VLBA data that refine the timing and nature of the transient, strengthening the TDE interpretation for Cyg A-2.

Findings

Cyg A-2's rise time is revised to 4-6 years.

The transient is consistent with a TDE causing a mildly relativistic outflow.

Cyg A-2 has not been in a high luminosity state over the last 30 years.

Abstract

We have analyzed archival VLBA data for Cygnus A between 2002 and 2013, to search for radio emission from the transient discovered in 2015 by \citet{per18} approximately 0.4\arcsec~ from the nucleus of Cygnus A (Cyg A-2). \citet{per18} use VLA and VLBA archival data (between 1989 and 1997) to show that the transient rises in flux density by a factor of at least five in less than approximately 20 years. With the additional data presented here, we revise the rise time to between approximately four years and six years, based on a new detection of the source at 15.4 GHz from October 2011. Our results strengthen the interpretation of Cyg A-2 as the result of a Tidal Disruption Event (TDE), as we can identify the location of the compact object responsible for the TDE and can estimate the angular expansion speed of the resulting radio emitting structures, equivalent to an apparent expansion speed of $<0.9c$. While our results are consistent with recent X-ray analyses, we can rule out a previously suggested date of early 2013 for the timing of the TDE. We favour a timing between early 2009 and late 2011. Applying the model of \citet{nak11}, we suggest a TDE causing a mildly relativistic outflow with a (density-dependent) total energy $>10^{49}$ erg. Due to the improved temporal coverage of our archival measurements, we find that it is unlikely that Cyg A-2 has previously been in a high luminosity radio state over the last 30 years.