Kimitake Hayasaki

Tidal disruption events are exotic astrophysical phenomena where matter from a star or the interstellar medium is captured by a supermassive black hole. The process liberates enormous energy, within a few months to a year timescale, enough to detect dormant black holes in near as well as the farthest galaxies. We revisit the long-term spectral variabilities associated with the jetted tidal disruption event Swift J1644+57 by exploring the archival X-ray data obtained with the Swift X-ray Telescope and the XMM-Newton observatory. Our analysis reveals that the spectral indices decrease nonmonotonically as Swift J1644+57 evolves with time. We also find that the soft (0.3–1.5 keV) and hard (1.5–10 keV) X-ray photon counts are highly correlated with a maximum correlation coefficient of 0.95 and peak at zero lag. Moreover, the soft and hard band variabilities obtained from XMM-Newton observations are highly correlated with a Pearson cross-correlation coefficient of 0.96. This indicates that the soft and hard X-ray photons are emitted from the same site, which is most likely a Compton cloud, i.e., the corona. Assuming the hard X-ray photons originate from the corona, we find that the coronal parameter undergoes rapid expansion during the early phases when accompanied by a relativistic jet launching and subsequently evolves toward a state of saturation with minor fluctuations in the latter stages. The temporal variation in the coronal radius parameter ( R _cor ) is consistent with a simple theoretical conjecture. We also discuss the application of our analytical outcomes to other jetted and nonjetted tidal disruption events.

We study the thermal and spectral properties of irradiated circumbinary disks (CBDs) around binary black holes (BBHs), using analytic, hydrogen-based opacity models that capture dependencies on temperature, density, and ionization. We solve the vertical hydrostatic equilibrium and energy balance, assuming gas pressure only, using Rosseland-mean opacities from free–free and bound–free absorption plus electron scattering, with ionization fractions given by the Saha equation. Four opacity models are considered, including a reference model with no physical opacity, constructed by Y. Lee et al., and three physically motivated alternatives. The midplane temperature profiles show significant variation across models, while the surface temperature remains largely unchanged in regions dominated by viscous heating. Opacity effects become pronounced in the outer disk, where irradiation reprocessing shapes the IR-optical continuum. Bound-free opacity introduces flattening and a mid-frequency peak in the spectral energy distribution. We compute spectra of a triple-disk system including the CBD and two accreting minidisks. The high-frequency peak arises from the hot minidisks, while the low-frequency excess originates from irradiated outer CBD layers. Comparing model spectra with the detection limits of Subaru, JWST, and Swift, we find that BBH systems within ∼10 Mpc can exhibit a detectable IR excess. Our results highlight the need for physically consistent opacity modeling to interpret electromagnetic (EM) signatures of BBHs approaching coalescence and support integration of metallicity-dependent opacity tables. Our opacity-informed framework for irradiated CBDs provides an EM template for identifying stellar- to intermediate-mass BBHs in a mass range sparsely sampled by LISA, thereby bridging the gravitational-wave–EM gap with testable IR/optical signatures.

If supermassive black hole binaries (SMBHBs) are driven together by gas disks in galactic nuclei, then a surrounding nuclear star cluster or in situ star formation should deliver stars to the disk plane. Migration through the circumbinary disk will quickly bring stars to the edge of a low-density cavity cleared by the binary, where the stellar orbit becomes trapped and locked with the binary decay. Here we explore the scenario where the trapped stellar orbit decays with the binary until the binary tidally strips the star in a runaway process. For Sun-like stars, this occurs preferentially for 10 ⁴ –10 ⁶ M _⊙ SMBHBs, as the SMBHB enters the LISA band. We estimate that the runaway stripping process will generate Eddington-level X-ray flares repeating on hours-to-days timescales and lasting for decades. The flaring timescales and energetics of these circumbinary disk tidal disruption events (CBD-TDEs) match well with the recently discovered quasiperiodic eruptions. However, the inferred rates of the two phenomena are in tension, unless low-mass SMBHB mergers are more common than expected. For less-dense stars, stripping begins earlier in the SMBHB inspiral, has longer repetition times, lasts longer, is dimmer, and can occur for more-massive SMBHBs. Whether CBD-TDEs are a known or a yet-undiscovered class of repeating nuclear transients, they could provide a new probe of the elusive SMBH mergers in low mass/dwarf galaxies, which lie in the sweet-spot of the LISA sensitivity.

In this Letter, we simulate the collision between outflows from the tidal disruption of a 1 M _⊙ main-sequence star around a 10⁶ M _⊙ black hole and an initially spherically symmetric circumnuclear cloud. We launch super-Eddington outflows self-consistently by simulating the disruption of stars on both bound and unbound initial orbits using general relativistic smoothed particle hydrodynamics. We find that shocks formed as early as ∼10 days after the initial stellar disruption produce prompt radio emission. The shock radius (≈10¹⁷ cm), velocity (∼0.15c), and total energy (∼10⁵¹ erg) in our simulations match those inferred from radio observations of tidal disruption events (TDEs). We ray-trace to produce synthetic radio images and spectra to compare with the observations. While the TDE outflow is quasi-spherical, the synchrotron emitting region is aspherical but with reflection symmetry above and below the initial orbital plane. Our synthetic spectra show a continuous decay in peak frequency, matching prompt radio TDE observations. Our model supports the hypothesis that synchrotron radio flares from TDEs result from the collision between outflows and the circumnuclear material.

We study the effect of irradiation from two accretion disks (minidisks) around respective black holes of stellar-to-intermediate masses in a circular binary on the spectrum of a circumbinary disk (CBD) surrounding them. We assume the CBD to be a standard disk and adopt the orbit-averaged irradiation flux because the viscous timescale is much longer than the orbital period. We then solve the energy equation both analytically and numerically to compute the CBD temperature distribution and the corresponding disk spectrum. We find that the analytically calculated spectra are in good agreement with the numerical ones. The CBD spectrum is almost independent of the binary mass ratio. We also find that the combined spectra of two minidisks and the CBD have double peaks, one peak in the soft X-ray band and the other in the infrared (IR) band. The former peak comes from the two minidisks, while the latter peak from the CBD. The observed flux density increases with frequency as ν ^1/3 toward the soft X-ray peak, while it decreases with frequency away from the IR peak as ν ^−5/3. The latter feature is testable with near-IR observations with Subaru and JWST.