Quasi-periodic Eruptions from Helium Envelope of Hydrogen-deficient Stars Stripped by Supermassive Black Holes

TL;DR

This paper proposes that quasi-periodic eruptions (QPEs) observed in galactic nuclei originate from hydrogen-deficient stars orbiting supermassive black holes, and predicts associated gravitational wave signals detectable by future space-based observatories.

Contribution

It introduces a novel model linking QPEs to Roche lobe overflow from hydrogen-deficient post-AGB stars and predicts gravitational wave signals from extreme mass-ratio inspirals.

Findings

QPEs can be explained by Roche lobe overflow of evolved stars.

Hydrogen-deficient post-AGB stars fit the observed QPE properties.

QPEs are potential sources of millihertz gravitational waves for LISA and Tianqin.

Abstract

Quasi-periodic eruptions (QPEs), which are a new kind of X-ray bursts with a recurrence time of several hours, have been detected from supermassive black holes (SMBHs) in galactic nuclei. Recently, the two QPEs discovered by the \textit{eROSITA} show asymmetric light curves with a fast rise and a slow decline. Current models cannot explain the observational characteristics of QPEs. Here we show that QPEs can be generated from the Roche lobe overflows at each periapsis passage of an evolved star orbiting an SMBH. The properties of the companion stars are constrained via analytic estimations. We find that hydrogen-deficient post-AGB stars are promising candidates. Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code is used to construct the hydrogen-deficient stars which can fulfill the requirements, as obtained through analytical estimates, to produce the properties of QPEs, including the fast-rise and slow-decay light curves, periods, energetics, and rates. Furthermore, the extreme mass ratio $\sim 10^5$ between the SMBH and the donor will lead to a phenomenon called extreme mass-ratio inspiral (EMRI), producing millihertz gravitational waves. These QPEs would be detected as EMRI sources with electromagnetic counterparts for space-based GW detectors, such as Laser Interferometer Space Antenna (LISA) and Tianqin. They would provide a new way to measure the Hubble constant and further test the Hubble constant tension.