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CSS J102913+404220 is a peculiar narrow line Seyfert 1 galaxy with anenergetic nuclear optical outburst. We present a detailed analysis of itsmulti-wavelength photometric and spectroscopic observations covering a periodof decade since outburst. We detect mid-infrared (MIR) flares delayed by abouttwo months relative to the optical outburst, with an extremely high peakluminosity of log(L_4.6um)>44 erg/s. The MIR peak luminosity is at least anorder of magnitude higher than any known supernovae explosions, suggesting theoptical outburst might be due to a stellar tidal disruption event (TDE). Wefind late-time X-ray brightening by a factor of >30 with respect to what isobserved about 100 days after the optical outburst peak, followed by a fluxfading by a factor of ~4 within two weeks, making it one of Active GalacticNuclei (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 AGNsand could be attributed to absorption variability. In this scenario, thedecrease in the covering factor of absorber with accretion rate could cause theX-ray brightening, possibly induced by the TDE. Most strikingly, while theUV/optical continuum remains little changes with time, an evident enhancementin the flux of H_alpha broad emission line is observed, about a decade afterthe nuclear optical outburst, which is an anomalous behavior never seen in anyother AGNs. Such an H_alpha anomaly could be explained by the replenishment ofgas clouds and excitation within Broad Line Region (BLR) that originates,perhaps from the interaction of outflowing stellar debris with BLR. The resultshighlight the importance of late-time evolution of TDE that could affect theaccreting properties of AGN, as suggested by recent simulations.

Discovery of late-time X-ray flare and anomalous emission line enhancement after the nuclear optical outburst in a narrow-line Seyfert 1 Galaxy