Mass Function of Binary Massive Black Holes in Active Galactic Nuclei

If the activity of active galactic nuclei (AGNs) is predominantly induced by major galaxy mergers, then a significant fraction of AGNs should harbor binary massive black holes in their centers. We study the mass function of binary massive black holes in nearby AGNs based on the observed AGN black-hole mass function and theory of evolution of binary massive black holes interacting with a massive circumbinary disk in the framework of coevolution of massive black holes and their host galaxies. The circumbinary disk is assumed to be steady, axisymmetric, geometrically thin, self-regulated, self-gravitating but non-fragmenting with a fraction of Eddington accretion rate, which is typically one tenth of Eddington value. The timescale of orbital decay is {then} estimated as ~10^8yr for equal mass black-hole, being independent of the black hole mass, semi-major axis, and viscosity parameter but dependent on the black-hole mass ratio, Eddington ratio, and mass-to-energy conversion efficiency. This makes it possible for any binary massive black holes to merge within a Hubble time by the binary-disk interaction. We find that (1.8+-0.6%) for the equal mass ratio and (1.6+-0.4%) for the one-tenth mass ratio of the total number of nearby AGNs have close binary massive black holes with orbital period less than ten years in their centers, detectable with on-going highly sensitive X-ray monitors such as Monitor of All-sky X-ray Image and/or Swift/Burst Alert Telescope. Assuming that all binary massive black holes have the equal mass ratio, about 20% of AGNs with black hole masses of 10^{6.5-7}M_sun has the close binaries and thus provides the best chance to detect them.Comment: 22 pages, 11 figures, accepted for publication in PASJ. The draft was significantly revised. The major differences from the previous version are as follows: (1)The circumbinary disk is assumed to be a steady, axisymmetric, geometrically thin, self-gravitating, self-regulated but non-fragmenting. (2)The stellar scattering process is taken account of in the merging process of binary black hole

Binary Black Hole Accretion Flows in Merged Galactic Nuclei

We study the accretion flows from the circumbinary disks onto the supermassive binary black holes in a subparsec scale of the galactic center, using a smoothed particles hydrodynamics (SPH) code. Simulation models are presented in four cases of a circular binary with equal and unequal masses, and of an eccentric binary with equal and unequal masses. We find that the circumblack-hole disks are formed around each black holes regardless of simulation parameters. There are two-step mechanisms to cause an accretion flow from the circumbinary disk onto supermassive binary black holes: First, the tidally induced elongation of the circumbinary disk triggers mass inflow towards two closest points on the circumbinary disk from the black holes. Then, the gas is increasingly accumulated on these two points owing to the gravitational attraction of black holes. Second, when the gas can pass across the maximum loci of the effective binary potential, it starts to overflow via their two points and freely infalls to each black hole. In circular binaries, the gas continues to be supplied from the circumbinary disk (i.e. the gap between the circumbinary disk and the binary black hole is always closed.) In eccentric binaries, the mass supply undergoes the periodic on/off transitions during one orbital period because of the variation of periodic potential. The gap starts to close after the apastron and to open again after the next periastron passage. Due to this gap closing/opening cycles, the mass-capture rates are eventually strongly phase dependent. This could provide observable diagnosis for the presence of supermassive binary black holes in merged galactic nuclei.Comment: 16 pages, 27 figures, 2 tables, accepted for publication in PASJ. "High Resolution Version is Available at "http://www2.yukawa.kyoto-u.ac.jp/~kimitake/bbhs.html" Three observational references are added. Grammatical errors and typos are correcte

Impact of scale-height derivative on general relativistic slim disks in tidal disruption events

We construct a numerical model of steady-state, general relativistic (GR) super-Eddington accretion flows in an optically thick, advection-dominated regime, motivated by tidal disruption events wherein super-Eddington accretion assumes a pivotal role. Our model takes into account the loss of angular momentum due to radiation and the scale-height derivative in the basic equations of the GR slim disk. For comparison purposes, we also provide a new analytical solution for a radiation-pressure-dominant GR slim disk, which neglects the angular momentum loss due to radiation and the scale-height derivative. We find that the radiation pressure enhances by incorporating the scale height derivative into the basic equations. As a result, the surface density near the disk's inner edge decreases, whereas the disk temperature and scale height increase, brightening the disk spectrum in the soft X-ray waveband. Notably, an extremely high mass accretion rate significantly enhances the effect of the scale-height derivative, affecting the entire disk. In contrast, the inclusion of the radiation-driven angular momentum loss only slightly influences the disk surface density and temperature compared with the case of the scale-height derivative inclusion. The X-ray luminosity increases significantly due to scale height derivative for $\dot{M}/\dot{M}_{\rm Edd} \gtrsim 2$. In addition, the increment is higher for the non-spinning black hole than the spinning black hole case, resulting in a one-order of magnitude difference for $\dot{M}/\dot{M}_{\rm Edd}\gtrsim100$. We conclude that incorporating the scale-height derivative into a GR slim disk model is crucial as it impacts the disk structure and its resultant spectrum, particularly on a soft-X-ray waveband.Comment: 21 pages, 15 figures, 1 table, accepted for publication in Physical Review

Disk-wind-driven Expanding Radio-emitting Shell in Tidal Disruption Events

We study the evolution of a non-relativistically expanding thin shell in radio-emitting tidal disruption events (TDEs) based on a one-dimensional spherically symmetric model considering the effect of both a time-dependent mass loss rate of the disk wind and the ambient mass distribution. The analytical solutions are derived in two extreme limits: one is the approximate solution near the origin in the form of the Taylor series, and the other is the asymptotic solution in which the ambient matter is dominant far away from the origin. Our numerical solutions are confirmed to agree with the respective analytical solutions. We find that no simple power-law of time solution exists in early to middle times because the mass loss rate varies over time, affecting the shell dynamics. We also discuss the application of our model to the observed radio-emitting TDE, AT2019dsg.Comment: 28 pages, 6 figures, Accepted for publication in Ap