Why are Quasiperiodic Eruptions only Found in Low-Mass Galaxies?

TL;DR

This paper analyzes the limited occurrence of quasiperiodic eruptions in low-mass galaxies, suggesting they result from highly eccentric stellar orbits near black holes, likely involving white dwarfs and gravitational radiation effects.

Contribution

It constrains models of QPEs by linking their occurrence to low black hole masses and eccentric stellar orbits, proposing a tidal disruption near-miss mechanism involving white dwarfs.

Findings

QPEs are only observed in low-mass galaxies with black holes less than ~10^6 solar masses.

Eccentric stellar orbits near black holes are favored, with eccentricity close to 1.

A tidal disruption near-miss scenario involving gravitational radiation is supported.

Abstract

I consider the current sample of galaxy nuclei producing quasiperiodic eruptions (QPEs). If the quasiperiod results from the orbital motion of a star around the central black hole, the dearth of associated black hole masses $\gtrsim 10^6\msun$ places tight constraints on models. It disfavours those assuming wide orbits and small eccentricities, because there is ample volume within pericentre to allow significantly more massive holes in QPE systems than are currently observed. If instead the orbiting star is assumed to pass close to the black hole, the same lack of large black hole masses strongly suggests that the stellar orbits must be significantly eccentric, with $1 - e \lesssim {\rm few}\times 10^{-2}$. This favours a tidal disruption near-miss picture where QPEs result from repeated accretion from an orbiting star (in practice a white dwarf) losing orbital angular momentum to gravitational radiation, even though this is not assumed in deriving the eccentricity constraint. Given the tight constraints resulting from the current small observed sample, attempts to find QPE systems in more massive galaxies are clearly important.