Micro-Tidal Disruption Events at Galactic Centers

TL;DR

This paper proposes and tests a micro-TDE mechanism in which a high-speed white dwarf encounters a stellar-mass black hole near a galactic-center massive black hole. The authors combine a dynamical-rate calculation based on Tremaine’s cluster model with Newtonian hydrodynamic simulations to quantify how disruption, accretion, and unbound debris depend on encounter parameters. They identify a disruption criterion and provide empirical fits for the bound and unbound mass fractions, predicting prompt X-ray emission from sBH accretion and delayed optical flares from MBH accretion, plus low-frequency gravitational waves. The work forecasts detectable signatures for current and future observatories (X-ray, optical, GW) and offers a new pathway to probe the stellar distribution and EMRI-like dynamics in galactic centers.

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.

Figures (13)

• Figure 1: Schematic diagram of the orbits of a WD and a sBH, orbiting around a MBH at the galactic center.
• Figure 2: Upper panel shows evolution of the peri-center passage rate with time for the different nuclear cluster with different MBH masses. The $\Gamma$ in lower MBH mass systems ($M\lesssim10^7M_\odot$) will reach a maximum value $\Gamma_{\rm max}$, the stable state, because of the mass segregation, and will decrease because of loss cone mechanism. The lower panel shows the relation between the average rate $\overline{\Gamma}$, the maximum rate $\Gamma_{max}$ and MBH masses. For the larger MBH mass ($M\gtrsim10^7M_\odot$), the rates have a sharp decrease because these systems don't evolve to a stable state in the cosmic time.
• Figure 3: The upper panel shows the differential number of event rate with respect to redshift for two different MBH mass functions. It reaches the maximum value at $z\sim 1$. The lower panel shows the expected event rate within redshift for the two MBH mass functions. The rate reaches $\sim 0.1 / {\rm yr}$ within $z\leq1$ and $\sim 1 / {\rm yr}$ within $z\leq3$.
• Figure 4: Initial condition for the simulations. The sBH of $10M_\odot$ is placed at the center shown as the black dot. The WD of $1M_\odot$ starts with impact parameter $b$ along the $y$-direction and moves with $v_0$ along the $z$-direction.
• Figure 5: Snapshots of gas density at $t=1.26$ s with $v_0=0.1c$ and different impact parameter $b$.