Using the Hills Mechanism to Generate Repeating Partial Tidal Disruption Events and ASASSN-14ko

Cufari, M.; Coughlin, Eric R.; Nixon, Chris

doi:10.3847/2041-8213/ac6021

“…However, it is not clear that these would constitute TDEs that yield detectable emission, as it seems likely that such stars would be progressively stripped of their mass over many pericenter passages, leading to underluminous events spread out over long (humanly inaccessible) timescales (as suggested by MacLeod et al 2012); this is especially true at the high SMBH mass where the fallback time of the material becomes  years. It is also not possible to substantially reduce the orbital period of such starts through traditional tidal dissipation owing to the extreme mass ratio (Cufari et al 2022). Therefore, we expect the rate suppression derived here to be substantial, even with the empty loss-cone regime included, though we leave a detailed investigation of the importance of the latter regime to future work.…”

Section: The Definition Of R T and "Observable" Tdesmentioning

confidence: 86%

On the Impact of Relativistic Gravity on the Rate of Tidal Disruption Events

Coughlin

¹

,

Nixon

²

2022

ApJ

Self Cite

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The tidal disruption of stars by supermassive black holes (SMBHs) probes relativistic gravity. In the coming decade, the number of observed tidal disruption events (TDEs) will grow by several orders of magnitude, allowing statistical inferences of the properties of the SMBH and stellar populations. Here we analyze the probability distribution functions of the pericenter distances of stars that encounter an SMBH in the Schwarzschild geometry, where the results are completely analytic, and the Kerr metric. From this analysis we calculate the number of observable TDEs, defined to be those that come within the tidal radius r t but outside the direct capture radius (which is, in general, larger than the horizon radius). We find that relativistic effects result in a steep decline in the number of stars that have pericenter distances r p ≲ 10 r g, where r g = GM/c 2, and that for maximally spinning SMBHs the distribution function of r p at such distances scales as f r p ∝ r p 4 / 3 , or in terms of β ≡ r t/r p scales as f β ∝ β −10/3. We find that spin has little effect on the TDE fraction until the very-high-mass end, where instead of being identically zero the rate is small (≲1% of the expected rate in the absence of relativistic effects). Effectively independent of spin, if the progenitors of TDEs reflect the predominantly low-mass stellar population and thus have masses ≲1M ⊙, we expect a substantial reduction in the rate of TDEs above 107 M ⊙.

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“…I thank the authors of Cufari et al (2022) for sending me a copy of their paper in advance of publication. I am particularly grateful to the anonymous referee for a very thoughtful and perceptive report.…”

Section: Acknowledgmentsmentioning

confidence: 99%

Quasi-periodic eruptions from galaxy nuclei

King

¹

2022

Monthly Notices of the Royal Astronomical Society

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I consider quasi-periodic eruptions (QPEs) from galaxy nuclei. All the known cases fit naturally into a picture of accretion from white dwarfs (WDs) in highly eccentric orbits about the central black holes which decay through gravitational wave emission. I argue that ASASSN–14ko and ESO 243-39 HLX–1 are QPE sources at earlier stages of this evolution, with correspondingly longer periods, more extreme eccentricities, and significantly more massive WD donors. The derived parameters for ASASSN–14ko correctly give the measured period derivative. I show explicitly that mass transfer in QPE systems is always highly stable, despite recent claims to the contrary in the literature. This stability may explain the alternating long-short eruptions seen in some QPE sources. As the WD orbit decays, the eruptions occupy larger fractions of the orbit and become brighter, making searches for quasi-periodicities in bright low-mass galaxy nuclei potentially fruitful.

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“…These arguments suggest that the star that initially fueled the outburst from AT2018fyk was partially disrupted (see also Coughlin & Nixon 2022b). Typically tidally disrupted stars are on approximately parabolic orbits (e.g., Merritt 2013), which begs the question of how a partial TDE could yield a rebrighten-ing because, as noted by Cufari et al (2022a), tidal dissipation within the partially disrupted star yields a minimum orbital period of a few ×10 3 years for a 10 7.7 M SMBH (see their Equation 1). One can bind the partially disrupted star more tightly if the star was initially part of a binary system that was destroyed through Hills capture (Hills 1988).…”

Section: Explaining the Rebrightening: A Repeating Partial Tidal Disr...mentioning

confidence: 99%

“…Setting M = 10 7.7 M and taking solar-like values gives T acc 0.8 yr and L acc 6.7×10 44 erg s −1 (Figure 3, panel b). Partial TDEs typically rise, peak, and decay as ∝ t −9/4 Miles et al 2020;Nixon et al 2021), but for a star on a bound orbit, the fallback rate plummets as the star returns to pericenter (Hayasaki et al 2013b;Darbha et al 2018;Cufari et al 2022a). The reason for this sharp decline in the fallback rate is that the stellar core has a Hill sphere -an approximately spherical region within which the star's gravitational field dominates over that of the SMBHnear to which the stream density is much smaller than that of the bulk of the stream (Figure 3, panel e).…”

Section: Explaining the Rebrightening: A Repeating Partial Tidal Disr...mentioning

confidence: 99%

Live to die another day: the rebrightening of AT2018fyk as a repeating partial tidal disruption event

Wevers¹,

Coughlin²,

Pasham³

et al. 2022

Preprint

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Stars that interact with supermassive black holes (SMBHs) can either be completely or partially destroyed by tides. In a partial tidal disruption event (TDE) the high-density core of the star remains intact, and the low-density, outer envelope of the star is stripped and feeds a luminous accretion episode. The TDE AT2018fyk, with an inferred black hole mass of 10 7.7±0.4 M , experienced an extreme dimming event at X-ray (factor of >6000) and UV (factor ∼15) wavelengths ∼500-600 days after discovery. Here we report on the re-emergence of these emission components roughly 1200 days after discovery. We find that the source properties are similar to those of the pre-dimming accretion state, suggesting that the accretion flow was rejuvenated to a similar state. We propose that a repeated partial TDE, where the partially disrupted star is on a ∼ 1200 day orbit about the SMBH and is periodically stripped of mass during each pericenter passage, powers its unique lightcurve. This scenario provides a plausible explanation for AT2018fyk's overall properties, including the rapid dimming event and the rebrightening at late times. We also provide testable predictions for the behavior of the accretion flow in the future: if the second encounter was also a partial disruption then we predict another strong dimming event around day 1800 (August 2023), and a subsequent rebrightening around day 2400 (March 2025). This source provides strong evidence of the partial disruption of a star by a SMBH.

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Using the Hills Mechanism to Generate Repeating Partial Tidal Disruption Events and ASASSN-14ko

Cited by 34 publications

References 41 publications

On the Impact of Relativistic Gravity on the Rate of Tidal Disruption Events

On the Impact of Relativistic Gravity on the Rate of Tidal Disruption Events

Quasi-periodic eruptions from galaxy nuclei

Live to die another day: the rebrightening of AT2018fyk as a repeating partial tidal disruption event

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