A light echo from the warm outflow in the ULIRG F01004-2237 following a
  major flare in its optical continuum emission
  Emission-line variability studies have the potential to provide key
information about the structures of the near-nuclear outflow regions of AGN.
Here we present a VLT/Xshooter spectrum of the nucleus of the ULIRG F01004-2237
that was taken in August 2018, about 8 yr after a major flare in its integrated
optical emission. Compared with our WHT/ISIS spectrum from September 2015, the
broad, red wings of the emission lines most closely associated with the flaring
event, including HeII(4686), NIII(4100,4640) and HeI(5876), have substantially
declined in flux. In contrast, the broad, blue wings that dominate the [OIII],
[NeIII], [NeV] and [OI] forbidden lines have increased in flux by a factor 1.4
- 4.4 (depending on the line). Moreover, the [FeVII](6087) line is detected in
the new spectrum for the first time. We interpret these results in terms of a
light echo from the outflowing warm gas: the direct emission from the flaring
event is continuing to fade, but due to light travel time effects we are only
now observing the impact of the flare on the emission from the extended outflow
region. Unless the outflow is confined to a small range of angles close to our
line of sight, these observations imply that the outflow must be relatively
compact (r<50 pc). In terms of the nature of the flare event, we speculate that
the properties of the F01004-2237 flare may be the result of a tidal disruption
event (TDE) occurring in an object with pre-existing AGN activity.
