X-ray Variability and Photosphere Evolution during Accretion Disk Formation in Tidal Disruption Events

TL;DR

This study models the formation and evolution of accretion disks in tidal disruption events, revealing how shocks, photosphere changes, and relativistic effects influence multi-band emission and variability.

Contribution

It introduces detailed 3D radiation hydrodynamic simulations of TDE disk formation, highlighting the timing of disk circularization and the origins of optical and X-ray emissions.

Findings

Disk forms about 24 days after initial collision.

Optical-UV peaks when the disk forms and shocks subside.

Soft X-ray emission is highly angle-dependent.

Abstract

The early time emission in tidal disruption events (TDEs) originates from both accretion and shocks, which produce photons that eventually emerge from an inhomogeneous photosphere. In this work, we model the disk formation following the debris stream self-intersection in a TDE. We track the multi-band emission using three-dimensional, frequency-integrated and multi-group radiation hydrodynamic simulations. We find a more circularized disk forms about 24 days following the initial stream-stream collision, after the mass fallback rate peaks and once the debris stream density decreases. Despite the absence of a circularized disk at early times, various shocks and the asymmetric photosphere are sufficient to drive a wide range of optical-to-X-ray ratios and soft-X-ray variability. We find that with strong apsidal precession, the first light is from the stream-stream collision. It launches an optically-thick outflow, but only produces modest prompt emission. The subsequent optical and ultraviolet (UV) light curve rise is mainly powered by shocks in the turbulent accretion flow close to the black hole. The optical-UV luminosity peaks roughly when the disk forms and shock-driven outflows subside. The disk is optically and geometrically thick, extending well beyond the circularization radius. Radiation pressure clears the polar region and leaves optically-thin channels. We obtain the broad-band spectral energy distribution (SED) directly from multi-group simulations with 16-20 frequency groups. The SED has a black body component that peaks in the extreme UV. The soft X-ray component either resembles a thermal tail, or can be described by a shallower power law associated with bulk Compton scattering. The blackbody parameters are broadly consistent with observed optical TDEs and vary weakly with viewing angle. In contrast, soft X-ray emission is highly angle-dependent.