Discovery of late-time X-ray flare and anomalous emission line
  enhancement after the nuclear optical outburst in a narrow-line Seyfert 1
  Galaxy
  CSS J102913+404220 is a peculiar narrow line Seyfert 1 galaxy with an
energetic nuclear optical outburst. We present a detailed analysis of its
multi-wavelength photometric and spectroscopic observations covering a period
of decade since outburst. We detect mid-infrared (MIR) flares delayed by about
two months relative to the optical outburst, with an extremely high peak
luminosity of log(L_4.6um)>44 erg/s. The MIR peak luminosity is at least an
order of magnitude higher than any known supernovae explosions, suggesting the
optical outburst might be due to a stellar tidal disruption event (TDE). We
find late-time X-ray brightening by a factor of >30 with respect to what is
observed about 100 days after the optical outburst peak, followed by a flux
fading by a factor of ~4 within two weeks, making it one of Active Galactic
Nuclei (AGNs) with extreme variability. Despite the dramatic X-ray variability,
there are no coincident strong flux variations in optical, UV and MIR bands.
This unusual variability behavior has been seen in other highly accreting AGNs
and could be attributed to absorption variability. In this scenario, the
decrease in the covering factor of absorber with accretion rate could cause the
X-ray brightening, possibly induced by the TDE. Most strikingly, while the
UV/optical continuum remains little changes with time, an evident enhancement
in the flux of H_alpha broad emission line is observed, about a decade after
the nuclear optical outburst, which is an anomalous behavior never seen in any
other AGNs. Such an H_alpha anomaly could be explained by the replenishment of
gas clouds and excitation within Broad Line Region (BLR) that originates,
perhaps from the interaction of outflowing stellar debris with BLR. The results
highlight the importance of late-time evolution of TDE that could affect the
accreting properties of AGN, as suggested by recent simulations.
