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Abstract Periodic collisions between a star on an inclined orbit around a supermassive black hole and its accretion disk offer a promising explanation for X-ray “quasiperiodic eruptions” (QPEs). Each passage through the disk midplane shocks and compresses gas ahead of the star, which subsequently re-expands above the disk as a quasi-spherical cloud. We present spherically symmetric Monte Carlo radiation transport simulations that follow the production of photons behind the radiation-mediated shock, Comptonization by hot electrons, and the eventual escape of the radiation through the expanding debris. Such 1D calculations are approximately justified for thin disks (scale-heighth ≲ few × R_⋆), through which the star of radiusR_⋆passes more quickly than the shocked gas can flow around the star. For collision speedsv_coll ≳ 0.15cand disk surface densities Σ ∼ 10³g cm⁻²characteristic of those encountered by stellar orbits consistent with QPE recurrence times, the predicted transient light curves exhibit peak luminosities ≳10⁴²erg s⁻¹and Comptonized quasi-thermal (Wien-like) spectra that peak at energieshν ∼ 100 eV, which is broadly consistent with QPE properties. For these conditions, gas and radiation are out of equilibrium, rendering the emission temperature harder than the blackbody value due to inefficient photon production behind the radiation-mediated shock. The predicted eruptions execute counterclockwise loops in hardness–luminosity space, qualitatively similar to QPE observations. Reproducing the observed eruption properties (duration, luminosity, temperature) requires a large radiusR_⋆ ≳ 10R_⊙, which may point to inflation of the star’s atmosphere from repeated collisions. 

Abstract A modest fraction of the stars in galactic nuclei fed toward the central supermassive black hole (SMBH) approach on low-eccentricity orbits driven by gravitational-wave radiation (extreme mass ratio inspiral (EMRI)). In the likely event that a gaseous accretion disk is created in the nucleus during this slow inspiral (e.g., via an independent tidal disruption event (TDE)), star–disk collisions generate regular short-lived flares consistent with the observed quasiperiodic eruption (QPE) sources. We present a model for the coupled star-disk evolution, which self-consistently accounts for mass and thermal energy injected into the disk from stellar collisions and associated mass ablation. For weak collision/ablation heating, the disk is thermally unstable and undergoes limit-cycle oscillations, which modulate its properties and lead to accretion-powered outbursts on timescales of years to decades, with a time-averaged accretion rate ∼0.1Ṁ Edd. Stronger collision/ablation heating acts to stabilize the disk, enabling roughly steady accretion at the EMRI-stripping rate. In either case, the stellar destruction time through ablation, and hence the maximum QPE lifetime, is ∼10²–10³yr, far longer than fallback accretion after a TDE. The quiescent accretion disks in QPE sources may at the present epoch be self-sustaining and fed primarily by EMRI ablation. Indeed, the observed range of secular variability broadly matches those predicted for collision-fed disks. Changes in the QPE recurrence pattern following such outbursts, similar to that observed in GSN 069, could arise from temporary misalignment between the EMRI-fed disk and the SMBH equatorial plane as the former regrows its mass after a state transition. 

Abstract “Quasiperiodic eruptions” (QPE) are recurrent nuclear transients with periods of several hours to almost a day, which thus far have been detected exclusively in the X-ray band. We have shown that many of the key properties of QPE flares (period, luminosity, duration, emission temperature, alternating long-short recurrence time behavior, and source rates) are naturally reproduced by a scenario involving twice-per-orbit collisions between a solar-type star on a mildly eccentric orbit, likely brought into the nucleus as an extreme mass-ratio inspiral (EMRI), and the gaseous accretion disk of a supermassive black hole (SMBH). The flare is generated by the hot shocked debris expanding outwards from either side of the disk midplane, akin to dual miniature supernovae. Here, we consider the conditions necessary for disk–star collisions to generate lower-temperature flares that peak in the ultraviolet (UV) instead of the X-ray band. We identify a region of parameter space at low SMBH massM_•∼ 10^5.5M_⊙and QPE periodsP≳ 10 hr for which the predicted flares are sufficiently luminousL_UV∼ 10⁴¹erg s⁻¹to outshine the quiescent disk emission at these wavelengths. The prospects to discover such “UV QPEs” with future satellite missions such as ULTRASAT and Ultraviolet Explorer depend on the prevalence of very low-mass SMBHs and the occurrence rate of stellar EMRIs onto them. For gaseous disks produced by the tidal disruption of stars, we predict that X-ray QPEs will eventually shut off, only to later reappear as UV QPEs as the accretion rate continues to drop. 

Abstract 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. 

Abstract Like most galaxies, the Milky Way harbors a supermassive black hole (SMBH) at its center, surrounded by a nuclear star cluster. In this dense star cluster, direct collisions can occur between stars before they evolve off the main sequence. Using a statistical approach, we characterize the outcomes of these stellar collisions within the inner parsec of the Galactic center (GC). Close to the SMBH, where the velocity dispersion is larger than the escape speed from a Sun-like star, collisions lead to mass loss. We find that the stellar population within 0.01 pc is halved within about a billion years because of destructive collisions. Additionally, we predict a diffuse population of peculiar low-mass stars in the GC. These stars have been divested of their outer layers in the inner 0.01 pc before migrating to larger distances from the SMBH. Between 0.01 and 0.1 pc from the SMBH, collisions can result in mergers. Our results suggest that repeated collisions between lower-mass stars can produce massive (≳10M_⊙) stars, and that there may be ∼100 of them residing in this region. We provide predictions on the number of so-called G objects, dust- and gas-enshrouded stellar objects, that may result from main-sequence stellar collisions. Lastly, we comment on uncertainties in our model and possible connections between stellar collisions and the missing red giants in the GC. 

Abstract Quasiperiodic eruptions (QPEs) represent a novel class of extragalactic X-ray transients that are known to repeat at roughly regular intervals of a few hours to days. Their underlying physical mechanism is a topic of heated debate, with most models proposing that they originate either from instabilities within the inner accretion flow or from orbiting objects. At present, our knowledge of how QPEs evolve over an extended timescale of multiple years is limited, except for the unique QPE source GSN 069. In this study, we present results from strategically designed Swift observing programs spanning the past 3 yr, aimed at tracking eruptions from eRO-QPE1. Our main results are as follows: (1) the recurrence time of eruptions can vary from flare to flare and is in the range of 0.6–1.2 days; (2) there is no detectable secular trend in evolution of the recurrence times; (3) consistent with prior studies, their eruption profiles can have complex shapes; and (4) the peak flux of the eruptions has been declining over the past 3 yr, with the eruptions barely detected in the most recent Swift data set taken in 2023 June. This trend of weakening eruptions has been reported recently in GSN 069. However, because the background luminosity of eRO-QPE1 is below our detection limit, we cannot verify whether the weakening is correlated with the background luminosity (as is claimed to be the case for GSN 069). We discuss these findings within the context of various proposed QPE models. 

Abstract Quasiperiodic erupters are a remarkable class of objects exhibiting very-large-amplitude quasiperiodic X-ray flares. Although numerous dynamical models have been proposed to explain them, relatively little attention has been given to using the properties of their radiation to constrain their dynamics. Here we show that the observed luminosity, spectrum, repetition period, duty cycle, and fluctuations in the latter two quantities point toward a model in which a main-sequence star on a moderately eccentric orbit around a supermassive black hole periodically transfers mass to the Roche lobe of the black hole; orbital dynamics lead to mildly relativistic shocks near the black hole; and thermal X-rays at the observed temperature are emitted by the gas as it flows away from the shock. Strong X-ray irradiation of the star by the flare itself augments the mass transfer, creates fluctuations in flare timing, and stirs turbulence in the stellar atmosphere that amplifies the magnetic field to a level at which magnetic stresses can accelerate infall of the transferred mass toward the black hole.