Tidal Disruption Event AT2020ocn: Early Time X-Ray Flares Caused by a Possible Disk Alignment Process

Abstract

Abstract A tidal disruption event (TDE) may occur when a star is torn apart by the tidal force of a black hole (BH). Eventually, an accretion disk is thought to form out of stellar debris falling back toward the BH. If the star’s orbital angular momentum vector prior to disruption is not aligned with the BH spin angular momentum vector, the disk will be tilted with respect to the BH equatorial plane. The disk will eventually be drawn into the BH equatorial plane due to a combination of the Bardeen–Petterson effect and internal torques. Here, we analyze the X-ray and UV observations of the TDE AT2020ocn obtained by Swift, XMM-Newton, and Neutron star Interior Composition ExploreR. The X-ray light curve shows strong flares during the first ≈100 days, while, over the same period, the UV emission decays gradually. We find that the X-ray flares can be explained by a model that also explains the spectral evolution. This model includes a slim disk viewed under a variable inclination plus an inverse-Comptonization component processing the slim disk emission. A scenario where the ongoing Lense–Thirring precession during the disk alignment process is responsible for the observed inclination variations is consistent with the data. In later observations, we find that the X-ray spectrum of AT2020ocn becomes harder, while the mass accretion rate remains at super-Eddington levels, suggesting the formation of a corona in line with accretion onto other compact objects. We constrain the BH mass to be ( 7 − 3 + 13 ) × 10 5 M ⊙ at the 1σ (68%) confidence level.