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

   https://doi.org/10.3847/1538-4357/acf65b  

   Linial, Itai ; Metzger, Brian D. ( October 2023 , The Astrophysical Journal)

   Roughly half of the quasiperiodic eruption (QPE) sources in galactic nuclei exhibit a remarkably regular alternating “long-short” pattern of recurrence times between consecutive flares. We show that a main-sequence star (brought into the nucleus as an extreme mass-ratio inspiral; EMRI) that passes twice per orbit through the accretion disk of the supermassive black hole (SMBH) on a mildly eccentric inclined orbit, each time shocking and ejecting optically thick gas clouds above and below the midplane, naturally reproduces observed properties of QPE flares. Inefficient photon production in the ejecta renders the QPE emission much harder than the blackbody temperature, enabling the flares to stick out from the softer quiescent disk spectrum. Destruction of the star via mass ablation limits the QPE lifetime to decades, precluding a long-lived AGN as the gaseous disk. By contrast, a tidal disruption event (TDE) naturally provides a transient gaseous disk on the requisite radial scale, with a rate exceeding the EMRI inward migration rate, suggesting that many TDEs should host a QPE. This picture is consistent with the X-ray TDE observed several years prior to the QPE appearance from GSN 069. Remarkably, a second TDE-like flare was observed from this event, starting immediately after detectable QPE activity ceased; this event could plausibly result from the (partial or complete) destruction of the QPE-generating star triggered by runaway mass loss, though other explanations cannot be excluded. Our model can also be applied to black hole–disk collisions, such as those invoked in the context of the candidate SMBH binary OJ 287.

    

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2. Interacting Stellar EMRIs as Sources of Quasi-periodic Eruptions in Galactic Nuclei

   https://doi.org/10.3847/1538-4357/ac3ee1  

   Metzger, Brian D. ; Stone, Nicholas C. ; Gilbaum, Shmuel ( February 2022 , The Astrophysical Journal)

   A star that approaches a supermassive black hole (SMBH) on a circular extreme mass ratio inspiral (EMRI) can undergo Roche lobe overflow (RLOF), resulting in a phase of long-lived mass transfer onto the SMBH. If the interval separating consecutive EMRIs is less than the mass-transfer timescale driven by gravitational wave emission (typically ∼1–10 Myr), the semimajor axes of the two stars will approach each another on scales of ≲ hundreds to thousands of gravitational radii. Close flybys tidally strip gas from one or both RLOFing stars, briefly enhancing the mass-transfer rate onto the SMBH and giving rise to a flare of transient X-ray emission. If both stars reside in a common orbital plane, these close interactions will repeat on a timescale as short as hours, generating a periodic series of flares with properties (amplitudes, timescales, sources lifetimes) remarkably similar to the “quasi-periodic eruptions” (QPEs) recently observed from galactic nuclei hosting low-mass SMBHs. A cessation of QPE activity is predicted on a timescale of months to years, due to nodal precession of the EMRI orbits out of alignment by the SMBH spin. Channels for generating the requisite coplanar EMRIs include the tidal separation of binaries (Hills mechanism) or Type I inward migration through a gaseous AGN disk. Alternative stellar dynamical scenarios for QPEs, that invoke single stellar EMRIs on an eccentric orbit undergoing a runaway sequence of RLOF events, are strongly disfavored by formation rate constraints.

    

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3. Cooling Envelope Model for Tidal Disruption Events

   https://doi.org/10.3847/2041-8213/ac90ba  

   Metzger, Brian D. ( September 2022 , The Astrophysical Journal Letters)

   We present a toy model for the thermal optical/UV/X-ray emission from tidal disruption events (TDEs). Motivated by recent hydrodynamical simulations, we assume that the debris streams promptly and rapidly circularize (on the orbital period of the most tightly bound debris), generating a hot quasi-spherical pressure-supported envelope of radiusR_v∼ 10¹⁴cm (photosphere radius ∼10¹⁵cm) surrounding the supermassive black hole (SMBH). As the envelope cools radiatively, it undergoes Kelvin–Helmholtz contractionR_v∝t⁻¹, its temperature risingT_eff∝t^1/2while its total luminosity remains roughly constant; the optical luminosity decays as$\nu L_{\nu }\propto R_v^2{T}_{\mathrm{eff}}\propto t^{-3/2}$. Despite this similarity to the mass fallback rate${\dot{M}}_{\mathrm{fb}}\propto t^{-5/3}$, envelope heating from fallback accretion is subdominant compared to the envelope cooling luminosity except near optical peak (where they are comparable). Envelope contraction can be delayed by energy injection from accretion from the inner envelope onto the SMBH in a regulated manner, leading to a late-time flattening of the optical/X-ray light curves, similar to those observed in some TDEs. Eventually, as the envelope contracts to near the circularization radius, the SMBH accretion rate rises to its maximum, in tandem with the decreasing optical luminosity. This cooling-induced (rather than circularization-induced) delay of up to several hundred days may account for the delayed onset of thermal X-rays, late-time radio flares, and high-energy neutrino generation, observed in some TDEs. We compare the model predictions to recent TDE light-curve correlation studies, finding both agreement and points of tension.

    

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4. Live to Die Another Day: The Rebrightening of AT 2018fyk as a Repeating Partial Tidal Disruption Event

   https://doi.org/10.3847/2041-8213/ac9f36  

   Wevers, T. ; Coughlin, E. R. ; Pasham, D. R. ; Guolo, M. ; Sun, Y. ; Wen, S. ; Jonker, P. G. ; Zabludoff, A. ; Malyali, A. ; Arcodia, R. ; et al ( January 2023 , The Astrophysical Journal Letters)

   Stars that interact with supermassive black holes (SMBHs) can be either 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 AT 2018fyk, with an inferred black hole mass of 10^7.7±0.4M_⊙, experienced an extreme dimming event at X-ray (factor of >6000) and UV (factor of ∼15) wavelengths ∼500–600 days after discovery. Here we report on the reemergence of these emission components roughly 1200 days after discovery. We find that the source properties are similar to those of the predimming 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 an ∼1200 day orbit about the SMBH and periodically stripped of mass during each pericenter passage, powers its unique light curve. This scenario provides a plausible explanation for AT 2018fyk’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 (2023 August) and a subsequent rebrightening around day 2400 (2025 March). This source provides strong evidence of the partial disruption of a star by an SMBH.

    

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5. Orbital Decay in an Accreting and Eclipsing 13.7 Minute Orbital Period Binary with a Luminous Donor

   https://doi.org/10.3847/2041-8213/ace7cf  

   Burdge, Kevin B. ; El-Badry, Kareem ; Rappaport, Saul ; Sunny Wong, Tin Long ; Bauer, Evan B. ; Bildsten, Lars ; Caiazzo, Ilaria ; Chakrabarty, Deepto ; Chickles, Emma ; Graham, Matthew J. ; et al ( August 2023 , The Astrophysical Journal Letters)

   We report the discovery of ZTF J0127+5258, a compact mass-transferring binary with an orbital period of 13.7 minutes. The system contains a white dwarf accretor, which likely originated as a post–common envelope carbon–oxygen (CO) white dwarf, and a warm donor (T_eff,donor= 16,400 ± 1000 K). The donor probably formed during a common envelope phase between the CO white dwarf and an evolving giant that left behind a helium star or white dwarf in a close orbit with the CO white dwarf. We measure gravitational wave–driven orbital inspiral with ∼51σsignificance, which yields a joint constraint on the component masses and mass transfer rate. While the accretion disk in the system is dominated by ionized helium emission, the donor exhibits a mixture of hydrogen and helium absorption lines. Phase-resolved spectroscopy yields a donor radial velocity semiamplitude of 771 ± 27 km s⁻¹, and high-speed photometry reveals that the system is eclipsing. We detect a Chandra X-ray counterpart withL_X∼ 3 × 10³¹erg s⁻¹. Depending on the mass transfer rate, the system will likely either evolve into a stably mass-transferring helium cataclysmic variable, merge to become an R CrB star, or explode as a Type Ia supernova in the next million years. We predict that the Laser Space Interferometer Antenna (LISA) will detect the source with a signal-to-noise ratio of 24 ± 6 after 4 yr of observations. The system is the first LISA-loud mass-transferring binary with an intrinsically luminous donor, a class of sources that provide the opportunity to leverage the synergy between optical and infrared time domain surveys, X-ray facilities, and gravitational-wave observatories to probe general relativity, accretion physics, and binary evolution.

    

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