AT 2019avd: A Tidal Disruption Event with a Two-phase Evolution

Tidal disruption events (TDEs) can uncover the quiescent supermassive black holes (SMBHs) at the center of galaxies and also offer a promising method to study them. After the disruption of a star by an SMBH, the highly elliptical orbit of the debris stream will be gradually circularized due to the self-crossing, and then the circularized debris will form an accretion disk. The recent TDE candidate AT 2019avd has double peaks in its optical light curve, and the X-ray emerges near the second peak. The durations of the peaks are similar to 400 and 600 days, respectively, and the separation between them is similar to 700 days. We fit its spectral energy distribution and analyze its light curves in the optical/UV, mid-infrared, and X-ray bands. We find that this source can be interpreted as a two-phase scenario in which the first phase is dominated by the stream circularization, and the second phase is the delayed accretion. We use the succession of the self-crossing model and delayed accretion model to fit the first and second peaks, respectively. The fitting result implies that AT 2019avd can be interpreted by the partial disruption of a 0.9 M (circle dot) star by a 7 x 10(6) M (circle dot) SMBH, but this result is sensitive to the stellar model. Furthermore, we find that the large-amplitude (by factors up to similar to 5) X-ray variability in AT 2019avd can be interpreted as the rigid-body precession of the misaligned disk due to the Lense-Thirring effect of a spinning SMBH, with a precession period of 10-25 days.