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