Repeating Nuclear Transients from Repeating Partial Tidal Disruption Events

Abstract

Extragalactic nuclear transients that exhibit repeating outbursts can be modeled as the repeated dynamical interaction between bound stars and supermassive black holes (SMBHs). A subset of these transients, with recurrence timescales of months-to-years, have been explained as accretion flares from the repeated tidal stripping of a star by an SMBH, in a repeating partial tidal disruption event (rpTDE). We outline the scope of the rpTDE model and discuss hydrodynamical simulations and analytical predictions for the stability of stars undergoing repeated mass loss, and the long-term evolution of these flares as a function of stellar type and orbital parameters. Our findings demonstrate that high-mass and centrally concentrated stars undergo negligible changes in structure in response to small amounts ($\sim 1-10\% M_\star$) of mass loss, and can survive many mass-stripping encounters with an SMBH. Contrarily, low-mass and less evolved stars are unstable to mass loss, and would be destroyed within a few orbits. We discuss the implications of these results for constraining the stellar type and orbital parameters of observed sources, such as ASASSN-14ko, for which $\gtrsim 20$ flares have been observed, and AT2020vdq, which exhibits a second flare that is brighter than its primary outburst.

Figures (15)

• Figure 1: Fallback rates for the first fifteen pericenter passages of a $3M_\odot$ TAMS star on a $\beta=1$ orbit around a $10^6$ SMBH. The peak of the fallback rates remains relatively constant across all fifteen encounters, whereas the peak timescale $t_{\rm peak}$ progressively shifts towards earlier times.
• Figure 2: The amount of mass stripped $\Delta M$ for the first fifteen pericenter passages of the $3M_\odot$ TAMS star on a $\beta=1$ orbit.
• Figure 3: The angular velocity imparted to the core of the $3M_\odot$ TAMS star on a $\beta=1$ orbit, normalized by its break-up angular velocity $\sqrt{G M_{\rm c}/R_{\rm c}^3}$, for the first 15 pericenter passages.
• Figure 4: Fallback rates for the first two encounters of a $1 M_{\odot}$ ZAMS star on a $\beta=1$ orbit. The amount of mass stripped increases from $N=1$ to $N=2$, leading to a brighter second peak.
• Figure 5: Fallback rates for the $1M_\odot$ ZAMS star on a $\beta=1.5$ orbit around a $10^6M_\odot$ SMBH. The star loses a significant fraction of its mass on its first pericenter passage ($\beta_{\rm c}$ for complete disruption being $\sim 1.8$), but yields a partial disruption, as indicated by the $\propto t^{-9/4}$ scaling of the fallback rate at late times. The star gets completely destroyed on its second encounter, (as indicated by the $\propto t^{-5/3}$ scaling) and yields a dimmer second peak.