A Bright First Day for Tidal Disruption Events

Abstract

Stream–stream collision may be an important prepeak energy dissipation mechanism in tidal disruption events (TDEs). We perform local three-dimensional radiation hydrodynamic simulations in a wedge geometry including the gravity to study stream self-crossing with an emphasis on resolving the collision and following the subsequent outflow. We find that the collision can contribute to prepeak optical emissions by converting ≳5% of the stream kinetic energy to radiation, yielding prompt emission of ∼10 ^42−44 erg s ^−1 . The radiative efficiency is sensitive to stream mass fallback rates and strongly depends on the downstream gas optical depth. Even for a sub-Eddington (10%) mass fallback rate, the strong radiation pressure produced in the collision can form a local super-Eddington region near the collision site, where a fast, aspherical outflow is launched. A higher mass fallback rate usually leads to more optically thick outflow and lower net radiative efficiency. For $\dot{M}\gtrsim 0.1{\dot{M}}_{\mathrm{Edd}}$ , the estimated photosphere size of the outflow can expand by 1–2 orders of magnitude, reaching ∼10 ^14 cm. The average gas temperature at this photospheric surface is a few × 10 ^4 K, roughly consistent with inferred prepeak photosphere properties for some optical TDEs. We find that the dynamics is sensitive to the collision angle and radius, but the radiative efficiency or outflow properties show more complex dependency than is often assumed in ballistic models.

Keywords

• Astrophysical black holes
• Tidal disruption
• Accretion
• Astronomical radiation sources