A Dark Year for Tidal Disruption Events

TL;DR

This paper explores how relativistic precession and hydrodynamics cause delays and suppression of observable flares in tidal disruption events, especially around lower-mass black holes, affecting detection biases.

Contribution

It introduces a new understanding of the 'dark period' in TDEs caused by stream self-intersection suppression and models how black hole mass influences flare observability.

Findings

Small relativistic precession can prevent stream self-intersection.

A 'dark period' can last up to years, delaying observable flares.

Higher-mass black holes tend to produce prompt, observable TDEs.

Abstract

The disruption of a main-sequence star by a supermassive black hole results in the initial production of an extended debris stream that winds repeatedly around the black hole, producing a complex three-dimensional figure that may self-intersect. Both analytical work and simulations have shown that typical encounters generate streams that are extremely thin. In this paper we show that this implies that even small relativistic precessions attributed to black hole spin can induce deflections that prevent the stream from self-intersecting even after many windings. Additionally, hydrodynamical simulations have demonstrated that energy is deposited very slowly via hydrodynamic processes alone, resulting in the liberation of very little gravitational binding energy in the absence of stream-stream collisions. This naturally leads to a "dark period" in which the flare is not observable for some time, persisting for up to a dozen orbital periods of the most bound material, which translates to years for disruptions around black holes with mass $\sim 10^{7} M_{\odot}$. We find that more-massive black holes tend to have more violent stream self-intersections, resulting in short viscous times that lead to prompt accretion onto the black hole. For these tidal disruption events (TDEs), the accretion rate onto the black hole should still closely follow the original fallback rate after a fixed delay time $t_{\rm delay}$. For lower black hole masses ($M_{\rm h} \lesssim 10^{6}$), we find that flares are typically slowed down by about an order of magnitude, and because the accretion rates for TDEs about higher-mass black hole are already sub-Eddington, this results in the majority of TDEs being sub-Eddington at peak. This also implies that current searches for TDEs are biased towards prompt flares, with slowed flares likely having been unidentified. [abridged]