Takamitsu Tanaka

Exact time-dependent solutions for the thin accretion disc equation: boundary conditions at finite radius

We discuss Greens function solutions of the equation for a geometrically thin, axisymmetric Keplerian accretion disc with a viscosity prescription ν ∝ R n . The mathematical problem was solved by Lynden-Bell & Pringle for the special cases with boundary conditions of zero-viscous torque and zero mass flow at the disc centre. While it has been widely established that the observational appearance of astrophysical discs depends on the physical size of the central object(s), exact time-dependent solutions with boundary conditions imposed at finite radius have not been published for a general value of the power-law index n. We derive exact Greens function solutions that satisfy either a zero-torque or a zero-flux condition at a non-zero inner boundary R in > 0, for an arbitrary initial surface density profile. Whereas the viscously dissipated power diverges at the disc centre for the previously known solutions with R in = 0, the new solutions with R in > 0 have finite expressions for the disc luminosity that agree, in the limit t → ∞, with standard expressions for steady-state disc luminosities. The new solutions are applicable to the evolution of the innermost regions of thin accretion discs.

X-ray emission from high-redshift miniquasars: self-regulating the population of massive black holes through global warming

Observations of high-redshift quasars at z � 6 imply that supermassive black holes (SMBHs) with masses M � 10 9 M⊙ were in place less than 1 Gyr after the Big Bang. If these SMBHs assembled from “seed” BHs left behind by the first stars, then they must have accreted gas at close to the Eddington limit during a large fraction ( � 50%) of the time. A generic problem with this scenario, however, is that the mass density in M � 10 6 M⊙ SMBHs at z � 6 already exceeds the locally observed SMBH mass density by several orders of magnitude; in order to avoid this overproduction, BH seed formation and growth must become significantly less efficientin less massive protogalaxies through some form of feedback, while proceeding unabated in the most massive galaxies that formed first. Using Monte-Carlo realizations of the merger and growth history of BHs, we show that X-rays from the earliest accreting BHs can provide such a feedback mechanism, on a global scale. Our calculations paint a self-consistent picture of black-hole-made climate change, in which the first miniquasars—among them the ancestors of the z � 6 quasar SMBHs—globally warm the intergalactic medium and suppress the formation and growth of subsequent generations of BHs. We present two specific models with global miniquasar feedback that provide excellent agreement with recent estimates of the z = 6 SMBH mass function. For each of these models, we estimate the rate of BH mergers at z > 6 that could be detected by the proposed gravitational-wave observatory eLISA/NGO.

Hot Accretion with Conduction: Spontaneous Thermal Outflows

Motivated by the low collisionality of gas accreted onto black holes in Sgr Aand other nearby galactic nuclei, we study a family of two-dimensional advective accretion solutions with thermal conduction. While we only impose global inflow,the accretionflow spontaneously develops bipolar outflows.The role of conduction is key in providing the extra degree of freedom (latitudinal energy transport) necessary to launch these rotating thermal outflows. The signoftheBernoulliconstantdoesnotdiscriminate betweeninflowingandoutflowingregions.Ourparametersurvey covers mass outflow rates from � 0% to 13% of the net inflow rate, outflow velocities from � 0% to 11% of the local Keplerian velocity, and outflow opening angles from � 0 � to 60 � . As the magnitude of conduction is increased, out- flows canadopt aconical geometry,pureinflow solutionsemerge, andthelimit oftwo-dimensional nonrotatingBondi- like solutions is eventually reached. These results confirm that radiatively inefficient, hot accretion flows have a hydrodynamic propensity to generate bipolar thermal outflows. Subject headingg accretion, accretion disks — black hole physics — conduction — hydrodynamics Online material: color figures

The formation of massive black holes in z 30 dark matter haloes with large baryonic streaming velocities

The origins of the � 10 9 M J quasar supermassive black holes (BHs) at redshifts z > 6 remain a theoretical puzzle. One possibility is that they grew from � 10 5 M J BHs formed in the ‘direct collapse’ of pristine, atomic-cooling (temperatures & 8000K; PAC) gas that did not fragment to form ordinary stars due to a lack of molecular hydrogen and metals. We propose that baryonic streaming—the relic relative motion of gas with respect to dark matter from cosmological recombination—provides a natural mechanism for establishing the conditions necessary for direct collapse. This effect delays the formation of the first stars by inhibiting the infall of gas into dark matter haloes; streaming velocities more than twice the root-mean-square value could forestall star formation until halo virial temperatures & 8000K. The resulting PAC gas can proceed to form massive BHs by any of the mechanisms proposed in the literature to induce direct collapse in the absence of a ultraviolet background. This scenario produces haloes containing PAC gas at a characteristic redshift z � 30. It can explain the abundance of the most luminous quasars at z � 6, regardless of whether direct collapse occurs in nearly all or less than 1 per cent of PAC haloes.

Recurring flares from supermassive black hole binaries: implications for tidal disruption candidates and OJ 287

I discuss the possibility that accreting supermassive black hole (SMBH) binaries with sub-parsec separations produce periodically recurring luminous outbursts that interrupt periods of relative quiescence. This hypothesis is motivated by two characteristics found generically in simulations of binaries embedded in prograde accretion discs: (i) the formation of a central, low-density cavity around the binary, and (ii) the leakage of gas into this cavity, occurring once per orbit via discrete streams on nearly radial trajectories. The first feature would reduce the emergent optical/UV flux of the system relative to active galactic nuclei powered by single SMBHs, while the second can trigger quasiperiodic fluctuations in luminosity. I argue that the quasiperiodic accretion signature may be much more dramatic than previously thought, because the infalling gas streams can strongly shock-heat via self-collision and tidal compression, thereby enhancing viscous accretion. Any optically thick gas that is circularized about either SMBH can accrete before the next pair of streams is deposited, fueling transient, luminous flares that recur every orbit. Due to the diminished flux in between accretion episodes, such cavity-accretion flares could plausibly be mistaken for the tidal disruptions of stars in quiescent nuclei. The flares could be distinguished from tidal disruption events if their quasiperiodic recurrence is observed, or if they are produced by very massive SMBHs that cannot disrupt solar-type stars. They may be discovered serendipitously in surveys such as LSST or eROSITA. I present a heuristic toy model as a proof of concept for the production of cavity-accretion flares, and generate mock light curves and specta. I also apply the model to the active galaxy OJ 287, whose production of quasiperiodic pairs of optical flares has long fueled speculation that it hosts a SMBH binary.

The effect of baryonic streaming motions on the formation of the first supermassive black holes

Observations of quasars at redshifts z > 6 reveal that 10^9 Msol supermassive black holes (SMBHs) had already formed when the Universe was 20. This is the regime where baryonic streaming motions--the relative velocities of baryons with respect to dark matter in the early Universe--most strongly inhibit star formation by suppressing gas infall and cooling. We investigate the impact of this effect on the growth of the first SMBHs using a suite of high-fidelity, ellipsoidal-collapse Monte Carlo merger-tree simulations. We find that the suppression of seed BH formation by the streaming motions significantly reduces the number density of the most massive BHs at z > 15, but the residual effect at lower redshifts is essentially negligible. The streaming motions can reduce by a factor of few the number density of the most luminous quasars at z ~ 10-11, where such objects could be detected by the James Webb Space Telescope. We conclude, with minor theoretical caveats, that baryonic streaming motions are unlikely to pose a significant additional obstacle to the formation of the observed high-redshift quasar SMBHs. Nor do they appreciably affect the heating and reionization histories of the Universe or the merger rates of nuclear BHs in the mass and redshift ranges of interest for proposed gravitational-wave detectors.

Driving the growth of the earliest supermassive black holes with major mergers of host galaxies

The formation mechanism of supermassive black holes (SMBHs) in general, and of

Witnessing the Birth of a Quasar

\sim 10^9\,{\rm M}_{\odot}

Intermediate-mass black holes from Population III remnants in the first galactic nuclei

SMBHs observed as luminous quasars at redshifts

Electromagnetic signatures of supermassive black hole binaries resolved by PTAs

z> 6