A Tidal Disruption Event Candidate Discovered in the Active Galactic Nucleus SDSS J022700.77-042020.6

Abstract

We report the discovery of a tidal disruption event (TDE) candidate occurring in the active galactic nucleus (AGN) SDSS J022700.77-042020.6. A sudden increase in flux from J0227-0420 during the second half of 2009 appears in the long-term optical, UV, and near-IR (NIR) light curves. A plateau phase, following an initial decline, is seen in the near-UV (NUV) and optical light curves. We find possible evidence that the plateau in the NUV band may lag behind the optical ones by similar to 70-80 days with also a much shorter duration, that is, similar to 7-15 days against similar to 40-50 days. The long-term NUV/optical (after the plateau phase), NIR, and mid-infrared (MIR) light curves can be well characterized with a form of L(t) proportional to t(-beta), consistent with the expectation of a TDE. The plateaus can be explained if stellar streams collide with the preexisting disk at different radii. Though the overall fallback rate decreases, the material in the outer disk drifts inward and increases the local accretion rate in the inner region, producing the NUV/optical plateaus. The possible lag is then attributed to viscosity delay. The index beta of the NIR J, H, and K-s bands (similar to 1.4-3.3) is steeper than that of the UV/optical (similar to 0.7-1.3) and MIR bands (similar to 0.9-1.8), which may suggest that a certain fraction of the dust in the inner region of the torus may be sublimated during the TDE phase. Our results indicate that, due to collisions between stellar debris and the preexisting disk, the light curves of TDEs occurring in AGNs may show distinctive features, which may shed new light on the accretion process.