# Self-intersection of the Fallback Stream in Tidal Disruption Events

## TL;DR

This paper presents a semi-analytical model explaining how stream self-intersection in tidal disruption events leads to observable optical and infrared features, unifying diverse behaviors and predicting new spectral characteristics.

## Contribution

It introduces a new model for stream self-intersection in TDEs, linking it to observed optical/infrared properties and explaining the diversity in X-ray emissions.

## Key findings

- CIO causes optical emission reprocessing and explains observed line widths.
- Infrared spectrum of CIO-reprocessed emission follows a shallow power law.
- Optical TDE rate is lower than total rate due to filtering by the reprocessing layer.

## Abstract

We propose a semi-analytical model for the self-intersection of the fallback stream in tidal disruption events (TDEs). When the initial periapsis is less than about 15 gravitational radii, a large fraction of the shocked gas is unbound in the form of a collision-induced outflow (CIO). This is because large apsidal precession causes the stream to self-intersect near the local escape speed at radius much below the apocenter. The rest of the fallback gas is left in more tightly bound orbits and quickly joins the accretion flow. We propose that the CIO is responsible for reprocessing the hard emission from the accretion flow into the optical band. This picture naturally explains the large photospheric radius (or low blackbody temperature) and typical widths of the H and/or He emission lines seen in optical TDEs. We predict the CIO-reprocessed spectrum in the infrared to be L_\nu ~ \nu^{~0.5}, shallower than a blackbody. The partial sky coverage of the CIO also provides a unification of the diverse X-ray behaviors of optical TDEs. According to this picture, optical surveys filter out a large fraction of TDEs with low-mass blackholes due to lack of a reprocessing layer, and the volumetric rate of optical TDEs is nearly flat wrt. the blackhole mass for M < 10^7 solar masses. This filtering causes the optical TDE rate to be lower than the total rate by a factor of ~10 or more. When the CIO is decelerated by the ambient medium, radio emission at the level of that in ASASSN-14li may be produced, but the timescales and peak luminosities can be highly diverse. Finally, our method paves the way for global simulations of the disk formation process by injecting gas at the intersection point according to the prescribed velocity and density profiles.