Tidal disruption event AT2020ocn: early-time X-ray flares caused by a possible disc alignment process

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 disc is thought to form out of stellar debris falling back towards the BH. If the star's orbital angular momentum vector prior to disruption is not aligned with the BH spin angular momentum vector, the disc will be tilted with respect to the BH equatorial plane. The disc will eventually be drawn into the BH equatorial plane due to a combination of the Bardeen-Petterson effect and internal torques. Here, we analyse the X-ray and UV observations of the TDE AT2020ocn obtained by Swift, XMM-Newton, and NICER. The X-ray light curve shows strong flares during the first $\approx100$ 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 disc viewed under a variable inclination plus an inverse-Comptonisation component processing the slim disc emission. A scenario where the ongoing Lense-Thirring precession during the disc 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 $(7^{+13}_{-3})\times10^{5}$ M$_\odot$ at the 1$\sigma$ (68%) confidence level.