Period Evolution of Repeating Transients in Galactic Nuclei

TL;DR

This paper investigates the physical mechanisms behind period changes in repeating transients near galactic nuclei, emphasizing star-disc interactions and their role in orbital decay and transient phenomena.

Contribution

It introduces a model where hydrodynamical drag from star-disc interactions explains observed period decay in sources like ASASSN-14ko, highlighting the importance of tidal heating and mass stripping.

Findings

Orbital decay in ASASSN-14ko is consistent with hydrodynamical drag from star-disc interactions.

Star's envelope may be inflated due to tidal heating, affecting transient behavior.

Recurrence time of Quasi-Periodic Eruptions is predicted to decay at a measurable rate.

Abstract

Wide-field survery have recently detected recurring optical and X-ray sources near galactic nuclei, with period spanning hours to years. These phenomena could result from repeated partial tidal disruptions of stars by supermassive black holes (SMBHs) or by interaction between star and SMBH-accretion discs. We study the physical processes that produce period changes in such sources, highlighting the key role of the interaction between the orbiting star and the accretion disc. We focus on ASASSN-14ko - a repeatedly flaring optical source with a mean period $P_0 = 115 \, \rm d$ and a detected period decay $\dot{P} = -2.6\times 10^{-3}$ (Payne et al. 2022). We argue that the system's $\dot{P}$ is most compatible with true orbital decay produced by hydrodynamical drag as a star passes through the accretion disc on an inclined orbit, twice per orbit. The star is likely a sun-like star whose envelope is somewhat inflated, possibly due to tidal heating. Star-disc interaction inevitably leads to drag-induced stripping of mass from the star, which may be the dominant component in powering the observed flares. We discuss ASASSN-14ko's possible formation history and observational tests of our interpretation of the measured $\dot P$. Our results imply that partial tidal disruption events manifesting as repeating nuclear transients cannot be modeled without accounting for the cumulative impact of tidal heating over many orbits. We discuss the implications of our results for other repeating transients, and predict that the recurrence time of Quasi-Periodic Eruptions is expected to decay at a rate of order $|\dot{P}| \approx 10^{-6}-10^{-5}$.