AT2019 avd: A tidal disruption event with a two-phase evolution

TL;DR

This paper analyzes the tidal disruption event AT2019avd, revealing a two-phase evolution involving stream circularization and delayed accretion, with implications for SMBH properties and disk precession.

Contribution

It introduces a two-phase model for TDE evolution, combining self-crossing and delayed accretion models, and interprets X-ray variability as disk precession due to SMBH spin.

Findings

AT2019avd shows a double-peaked light curve with ~400 and 600 days durations.

The event is consistent with partial disruption of a 0.9 M_sun star by a 7 million solar mass SMBH.

X-ray variability suggests disk precession caused by SMBH spin with a period of 10-25 days.

Abstract

Tidal disruption events (TDEs) can uncover the quiescent supermassive black holes (SMBHs) at the center of galaxies. After the disruption of a star by a 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 ~400 and 600 days, respectively, and the separation between them is ~700 days. We fit its spectral energy distribution (SED) 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 the delayed accretion model to fit the first and the second peaks, respectively. The fitting result implies that AT 2019avd can be interpreted by the partial disruption of a 0.9 M_sun star by a 7 * 10^6 M_sun SMBH, but this result is sensitive to the stellar model. Furthermore, we find the large-amplitude (by factors up to $\sim 5$) 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 the precession period of 10 - 25 days.