Quasi-periodic eruptions from the helium envelope of hydrogen-deficient stars stripped by supermassive black holes

¹ School of Astronomy and Space Science, Nanjing University, Nanjing, 210093, PR China
e-mail: fayinwang@nju.edu.cn
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ School of Physics, Huazhong University of Science and Technology, Wuhan 430074, PR China
⁴ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
⁵ Department of Astronomy, School of Physical Sciences, University of Science and Technology of China, Hefei, 230026 Anhui, PR China

Received: 25 October 2021
Accepted: 18 March 2022

Abstract

Quasi-periodic eruptions (QPEs), a new kind of X-ray burst 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 eROSITA showed asymmetric light curves with a fast rise and a slow decline. Current models cannot explain the observational characteristics of QPEs. In this work, we show that QPEs can be generated from the Roche lobe overflows at each periapsis passage of an evolved star orbiting a SMBH. The properties of the companion stars are constrained via analytic estimations. We find that hydrogen-deficient post-AGB stars are promising candidates for exhibiting this phenomenon. We used the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to construct the hydrogen-deficient stars that 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 ∼10⁵ between the SMBH and the donor leads 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. These instruments would provide a new way to measure the Hubble constant and further test the Hubble constant tension.