X-Ray Quasi-periodic Eruptions Driven by Star–Disk Collisions: Application to GSN069 and Probing the Spin of Massive Black Holes

Abstract

X-ray quasi-periodic eruptions (QPEs) were discovered recently in active galaxies with an unknown driving mechanism. Under the assumption that QPEs are caused by star–disk collisions, we adopt full relativistic method and show that both the orbital parameters of the star and also the mass and spinning of the massive black hole (MBH) can be revealed by using the time of arrival (TOA) of the QPEs. By applying the model to the observed QPEs of GSN069, we find that the star is in a near-circular orbit ( e • = 0.05 − 0.02 + 0.02 ) with a semimajor axis of ∼ 365 − 49 + 54 r g around an MBH with M • = 3.0 − 0.6 + 0.9 × 10 5 M ⊙ . The alternative short and long recurring time of the QPEs of GSN069 can be well explained by the small eccentricity and the orbital precession of the star. We find that the QPEs of GSN069 are possibly driven by a striped stellar core colliding with accretion disk after a partial tidal disruption event around the MBH. For GSN069-like galaxies, if continuous X-ray monitoring of QPE events can be accumulated with uncertainties of TOA ≲100–150 s, the spin of the MBH can be constrained by fitting to QPEs. Our results show that the timing of QPEs can provide a unique probe for measuring the spinning of the MBH and tests of the no-hair theorem.

Host galaxy properties of quasi-periodically erupting X-ray sources

“…If the association between TDEs and QPEs (based on their host galaxy properties) is real, this may suggest that QPEs are related to special and exotic TDE scenarios, for example as the result of a partial disruption and the subsequent capture of the stellar remnant, which interacts with a pre-existing AGN accretion flow (e.g. Xian et al 2021). Differences in selection methods between X-ray and UV/optical surveys of TDEs could also contribute to this effect; for example, TDE follow-up observations are more likely to target quiescent galaxies than AGNs.…”

“…In light of these properties, the first potential explanations that were put forward focussed on instabilities of the inner accretion flow (Sniegowska et al 2020;Raj & Nixon 2021). Alternatively, extreme mass ratio inspirals (EMRIs) may also explain the observed QPE properties (King 2020;Suková et al 2021;Xian et al 2021;Zhao et al 2021;Metzger et al 2022).…”

Milli-Hertz Gravitational Wave Background Produced by Quasi-Periodic Eruptions

“…We first consider a model in which QPEs are produced by stellar-mass objects moving on relatively circular orbits around MBHs (Metzger et al 2021;Xian et al 2021). Such an orbit has three parameters, the mass of the MBH M , the mass of the stellar-mass object m, and the orbital period P .…”

“…The orbital period P is determined by the time interval between successive eruptions. Note that in some models, the small object collides with the accretion disk of the MBH twice per orbital period, and hence P is twice the time interval between eruptions (Xian et al 2021). We neglect this factor of two because it does not qualitatively affect the amplitude and detectability of the GWs.…”

“…Further theoretical studies suggest that WD-MBH binaries may be too short-lived to explain the detection rate of QPEs because the WDs would expand in a runaway fashion as soon as the mass transfer starts (Metzger et al 2021). One way of alleviating the problem invokes less eccentric orbits for those objects (not necessarily WDs) around MBHs, so that mass transfer can be avoided (Metzger et al 2021;Xian et al 2021;Ingram et al 2021). In these models, QPEs also emit GWs because they are essentially still EMRIs.…”