Micro-Tidal Disruption Events at Galactic Centers

TL;DR

This paper investigates micro-tidal disruption events caused by white dwarfs interacting with stellar-mass black holes in galactic centers, predicting observable X-ray and optical signals and matching some observed transients.

Contribution

It introduces a new model for micro-TDEs involving white dwarf and black hole encounters, supported by hydrodynamical simulations and analytical criteria.

Findings

Estimated event rate of a few per year within z≤3.

Disruption outcomes depend on impact parameter and collision speed.

Some observed X-ray transients are consistent with the micro-TDE model.

Abstract

This work explores a scenario for micro-tidal disruption events (TDEs) triggered by close encounters between high-speed white dwarfs (WDs) and stellar-mass black holes (sBHs) in galactic centers. In this model, a WD orbiting the central massive black hole (MBH) is scattered by an sBH during the sBH's early extreme mass-ratio inspiral phase. We conservatively estimate these events occur a few times per year within $z\leq 3$. Significant disruption of the WD occurs when the impact parameter is comparable to the WD's radius. We derive a mathematical criterion and confirm numerically by hydrodynamical simulations. With the increase of the impact parameter and the collision speed, the WD material captured by the sBH decreases while the material remain self-gravitating increases. A part of the WD material becomes unbound from the sBH-WD system, and its mass ranges from nearly zero to $\ge 50\%$, reaching the peak value when the impact parameter is comparable to the WD's radius. We expect the subsequent capture of WD material by the sBH to produce a prompt X-ray burst (a micro-TDE), and the accretion of unbound debris onto the MBH can power a fainter, delayed optical flare. The properties of certain transient X-ray bursts observed by Einstein Probe are consistent with this micro-TDE picture.