Unstable Mass Transfer from a Main-sequence Star to a Supermassive Black Hole and Quasiperiodic Eruptions

Abstract

We discuss the formation and evolution of systems composed of a low-mass (M ⋆ ≲ 4 M ⊙) main-sequence star orbiting a 105–107 M ⊙ supermassive black hole with an orbital period of order ∼hours and a mild eccentricity (e ≈ 0.1–0.2), episodically shedding mass at each pericenter passage. We argue that the resulting mass transfer is likely unstable, with Roche lobe overflow initially driven by gravitational-wave emission, but then being accelerated by the star’s expansion in response to its mass loss, undergoing a runaway process. We show that such systems are naturally produced by two-body gravitational encounters within the inner parsec of a galaxy, followed by gravitational-wave circularization and inspiral from initially highly eccentric orbits. We argue that such systems can produce recurring flares similar to the recently identified class of X-ray transients known as quasiperiodic eruptions, observed at the centers of a few distant galaxies.

EMRI + TDE = QPE: Periodic X-Ray Flares from Star–Disk Collisions in Galactic Nuclei

“…Furthermore, while it is tempting to attribute the QPEgenerating body orbiting the SMBH to be the surviving stellar core of a partial TDE (King 2020;Sheng et al 2021;Xian et al 2021;Zhao et al 2022), the core will generally inherit the highly eccentric orbit of the original star if not become even more eccentric due to the kick received during the disruption process (e.g., Manukian et al 2013;Gafton et al 2015;Cufari et al 2023; see discussion in Metzger et al 2022), inconsistent with the mildly eccentric orbit needed to explain the long-short QPE behavior (Xian et al 2021). As in previous related models (Krolik & Linial 2022;Linial & Sari 2023;Lu & Quataert 2023;Metzger et al 2022), we find it more natural to consider the QPE-generating star as one that has separately migrated…”

“…In Appendix B we show that the effect of multiple interactions between the star and disk of radial density profile Σ ∝ r 3/2 (Equation ( 2)) is to damp an initially mild eccentricity e  0.1. As a consequence, the mild eccentricity e  0.1 responsible for the observed long/short alternating recurrence times must reflect the residual eccentricity of the EMRI, before it started interacting with the gas disk (the residual eccentricity of a stellar EMRI is indeed typically ∼10%; Linial & Sari 2023). This sets an additional constraint on the lifetime of the system as a QPE of the alternating long-short variety, as this eccentricity is expected to dampen over a timescale τ e ≈ eτ decay  centuries.…”

“…Both the rate of stellar EMRIs (Equation ( 40)) and TDEs (e.g., Stone & Metzger 2016) are predicted to rise toward lower SMBH mass, but only relatively gradually µ -M • 1 4 (e.g., Linial & Sari 2023). The stellar EMRI production rate is typically dominated by the Hill's mechanism, occurring when tidally split binaries leave behind a bound star on a highly eccentric orbit of semimajor axis a Hills (Linial & Sari 2023). Since the overall number of stars residing within the SMBH's radius of influence is smaller in low-mass systems, the average number of stars orbiting the SMBH at similar radii ∼a Hills may be less than unity, implying that captured stars evolve primarily by gravitational wave emission rather than stochastic angular momentum diffusion, enhancing the stellar EMRI rate and consequentially, the QPE rate, in low-M • systems.…”

“…Several theoretical models have been proposed to explain the QPE phenomenon, including (i) accretion disk instabilities (Raj & Nixon 2021;Kaur et al 2023;Pan et al 2022;Sniegowska et al 2023); (ii) gravitational lensing of an SMBH binary (Ingram et al 2021); (iii) mass transfer onto an SMBH from one or more orbiting bodies (Zalamea et al 2010;King 2020;Chen et al 2023;King 2022King , 2023Krolik & Linial 2022;Linial & Sari 2023;Lu & Quataert 2023;Metzger et al 2022;Wang et al 2022;Zhao et al 2022);and (iv) collisions between an orbiting secondary body and the SMBH accretion disk (Suková et al 2021;Xian et al 2021;Franchini et al 2023). We focus on the latter scenario here.…”

Application of the Disk Instability Model to All Quasiperiodic Eruptions

“…The primary challenges of this phenomenon are how to construct a physical scenario to produce such short periodic eruptions (several to a dozen of hours). A number of models have been proposed, which can be roughly divided into two categories: while the first one suggests that the periodic outbursts in QPEs originate from the periodic orbital motion of a star captured by a black hole (King 2020(King , 2022Ingram et al 2021;Suková et al 2021;Xian et al 2021;Chen et al 2023;Krolik & Linial 2022;Lu & Quataert 2022;Linial & Sari 2023;Metzger et al 2022;Wang et al 2022b;Franchini et al 2023;Linial & Metzger 2023;Tagawa & Haiman 2023), another one ascribes the periodic behavior to the instability of inner accretion disk dominated by radiation pressure (Sniegowska et al 2020;Pan et al 2021Pan et al , 2022Raj & Nixon 2021;Kaur et al 2023;Śniegowska 2023). Notably, only the model of Pan et al (2022) is able to fit both the light curves and the phase-resolved X-ray spectra simultaneously during outbursts in GSN 069.…”