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.
Comment: Accepted for publication in A&A, 16 pages, 16 figures and 4 Tables