by Haas, Roland and Shcherbakov, Roman V. and Bode, Tanja and Laguna, Pablo
15 pages, 17 figures, submitted to Astrophysical Journal

We present numerical relativity results of tidal disruptions of white dwarfs from ultra-close encounters with a spinning, intermediate mass black hole. These encounters require a full general relativistic treatment of gravity. We show that the disruption process and prompt accretion of the debris strongly depend on the magnitude and orientation of the black hole spin. However, the late-time accretion onto the black hole follows the same decay, $latex \dot{M}$ ~ t^{-5/3}, estimated from Newtonian gravity disruption studies. We compute the spectrum of the disk formed from the fallback material using a slim disk model. The disk spectrum peaks in the soft X-rays and sustains Eddington luminosity for 1-3 yrs after the disruption. For arbitrary black hole spin orientations, the disrupted material is scattered away from the orbital plane by relativistic frame dragging, which often leads to obscuration of the inner fallback disk by the outflowing debris. The disruption events also yield bursts of gravitational radiation with characteristic frequencies of ~3.2 Hz and strain amplitudes of ~10^{-18} for galactic intermediate mass black holes. The optimistic rate of considered ultra-close disruptions is consistent with no sources found in ROSAT all-sky survey. The future missions like Wide-Field X-ray Telescope (WFXT) could observe dozens of events.

by Hayasaki, Kimitake and Yagi, Kent and Tanaka, Takahiro and Mineshige, Shin
10 pages, 2 figures, submitted to ApJ

When binary black holes are embedded in a gaseous environment, a rotating disk surrounding them, the so-called circumbinary disk, will be formed. The binary exerts a gravitational torque on the circumbinary disk and thereby the orbital angular momentum is transferred to it, while the angular momentum of the circumbinary disk is transferred to the binary through the mass accretion. The binary undergoes an orbital decay due to both the gravitational wave emission and the binary-disk interaction. This causes the phase evolution of the gravitational wave signal. The precise measurement of the gravitational wave phase thus may provide information regarding the circumbinary disk. In this paper, we assess the detectability of the signature of the binary-disk interaction using the future space-borne gravitational wave detectors such as DECIGO and BBO by the standard matched filtering analysis. We find that the effect of the circumbinary disk around binary black holes in the mass range $latex 6M_sun\le{M}\lesssim3\times10^3M_sun$ is detectable at a statistically significant level in five year observation, provided that gas accretes onto the binary at a rate greater than $latex \dot{M}\sim1.4\times10^{17} [gs^{-1}] j^{-1}(M/10M_sun)^{33/23}$ with 10% mass-to-energy conversion efficiency, where j represents the efficiency of the angular momentum transfer from the binary to the circumbinary disk. We show that $latex O(0.1)$ coalescence events are expected to occur in sufficiently dense molecular clouds in five year observation. We also point out that the circumbinary disk is detectable, even if its mass at around the inner edge is by over 10 orders of magnitude less than the binary mass.

by King, Andrew
to appear in proceedings of `The Golden Age of Cataclysmic Variables’, Memorie Societa’ Astronomica Italiana, 2012 (F. Giovannelli and L. Sabau-Graziati eds.)

I briefly review the progress of accretion disc theory since the seminal paper of Shakura and Sunyaev.

by Nixon, Chris and King, Andrew
8 pages, 9 figures. Accepted for publication in MNRAS

We simulate the viscous evolution of an accretion disc around a spinning black hole. In general any such disc is misaligned, and warped by the Lense-Thirring effect. Unlike previous studies we use effective viscosities constrained to be consistent with the internal fluid dynamics of the disc. We find that nonlinear fluid effects, which reduce the effective viscosities in warped regions, can promote the breaking of the disc into two distinct planes. This occurs when the Shakura & Sunyaev dimensionless viscosity parameter alpha is ~ 45 degrees. The break can be a long-lived feature, propagating outwards in the disc on the usual alignment timescale, after which the disc is fully co- or counter-aligned with the hole. Such a break in the disc may be significant in systems where we know the inclination of the outer accretion disc to the line of sight, such as some X-ray binaries: the inner disc, and so any jets, may be noticeably misaligned with respect to the orbital plane.

by Barai, Paramita and Proga, Daniel and Nagamine, Kentaro
23 pages, 11 figures, submitted. Uploaded version contains low-resolution color figures. Version with high-resolution figures can be found at: http://www.physics.unlv.edu/~barai/AllPages/Images-Movies/BHaccr_MultiPhase.pdf

(Abridged) We investigate non-spherical behavior of gas accreting onto a central supermassive black hole performing simulations using the SPH code GADGET-3 including radiative cooling and heating by the central X-ray source. As found in earlier 1D studies, our 3D simulations show that the accretion mode depends on the X-ray luminosity (L_X) for a fixed density at infinity and accretion efficiency. In the low L_X limit, gas accretes in a stable, spherically symmetric fashion. In the high L_X limit, the inner gas is significantly heated up and expands, reducing the central mass inflow rate. The expanding gas can turn into a strong enough outflow capable of expelling most of the gas at larger radii. For some intermediate L_X, the accretion flow becomes unstable developing prominent non-spherical features, the key reason for which is thermal instability (TI) as shown by our analyses. Small perturbations of the initially spherically symmetric accretion flow that is heated by the intermediate L_X quickly grow to form cold and dense clumps surrounded by overheated low density regions. The cold clumps continue their inward motion forming filamentary structures; while the hot infalling gas slows down because of buoyancy and can even start outflowing through the channels in between the filaments. We found that the ratio between the mass inflow rates of the cold and hot gas is a dynamical quantity depending on several factors: time, spatial location, and L_X; and ranges between 0 and 4. We briefly discuss astrophysical implications of such TI-driven fragmentation of accreting gas on the formation of clouds in narrow and broad line regions of AGN, the formation of stars, and the observed variability of the AGN luminiosity.

by Berger, E. and Zauderer, A. and Pooley, G. G. and Soderberg, A. M. and Sari, R. and Brunthaler, A. and Bietenholz, M. F.
Submitted to ApJ; 22 pages, 2 tables, 9 figures

We present continued radio observations of the tidal disruption event SwiftJ164449.3+573451 extending to \sim216 days after discovery. The data are part of a long-term program to monitor the expansion and energy scale of the relativistic outflow, and to trace the parsec-scale environment around a previously-dormant supermassive black hole (SMBH). The new observations reveal a significant change in the radio evolution starting at \sim1 month, with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude, and an overall density profile around the SMBH of rho \propto r^{-3/2} (0.1-1.2 pc) with a significant flattening at r\sim0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an L \propto t^{-5/3} tail, which is observed in the X-rays. Instead, we conclude that the relativistic jet was launched with a wide range of Lorentz factors, obeying E(>Gamma) \propto Gamma^{-2.5}. The similar ratio of duration to dynamical timescale for Sw1644+57 and GRBs suggests that this result may be applicable to GRBs as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at r\sim0.5 pc in good agreement with the Bondi radius for a \sim10^6 M_sun black hole. The density at \sim0.5 pc is about a factor of 30 times lower than inferred for the Milky Way galactic center, potentially due to a smaller number of mass-shedding massive stars. From our latest observations (\sim216 d) we find that the jet energy is E_{iso}\sim5×10^{53} erg (E_j\sim2.4×10^{51} erg for theta_j=0.1), the radius is r\sim1.2 pc, the Lorentz factor is Gamma\sim2.2, the ambient density is n\sim0.2 cm^{-3}, and the projected size is r_{proj}\sim25 microarcsec. Assuming no future changes in the observed evolution we predict that the radio emission from Sw1644+57 should be detectable with the EVLA for several decades, and will be resolvable with VLBI in a few years.

by Garcia-Senz, Domingo and Cabezon, Ruben M. and Escartin, Jose Antonio
15 pages, 12 figures, accepted for publication in Astronomy & Astrophysics

In this paper we develop and check a fully conservative SPH scheme based on a tensor formulation which can be applied to simulate astrophysical systems. In the proposed scheme derivatives are calculated from an integral expression which leads to a tensor, rather than vectorial, estimation of gradients and reduces to the standard formulation in the continuum limit. The new formulation improves the interpolation of physical magnitudes, leading to a set of conservative equations which looks similar to those of standard SPH. The resulting scheme was checked using a variety of well known tests, all of them simulated in two dimensions. An application of the proposed tensor method to astrophysics was also discussed by simulating the stability of a sun-like polytrope calculated in three dimensions.

by Rice, W. K. M. and Forgan, D. H. and Armitage, P. J.
9 pages, 11 figures, MNRAS in press

Recent simulations of self-gravitating accretion discs, carried out using a three-dimensional Smoothed Particle Hydrodynamics (SPH) code by Meru and Bate, have been interpreted as implying that three-dimensional global discs fragment much more easily than would be expected from a two-dimensional local model. Subsequently, global and local two-dimensional models have been shown to display similar fragmentation properties, leaving it unclear whether the three-dimensional results reflect a physical effect or a numerical problem associated with the treatment of cooling or artificial viscosity in SPH. Here, we study how fragmentation of self-gravitating disc flows in SPH depends upon the implementation of cooling. We run disc simulations that compare a simple cooling scheme, in which each particle loses energy based upon its internal energy per unit mass, with a method in which the cooling is derived from a smoothed internal energy density field. For the simple per particle cooling scheme, we find a significant increase in the minimum cooling time scale for fragmentation with increasing resolution, matching previous results. Switching to smoothed cooling, however, results in lower critical cooling time scales, and tentative evidence for convergence at the highest spatial resolution tested. We conclude that precision studies of fragmentation using SPH require careful consideration of how cooling (and, probably, artificial viscosity) is implemented, and that the apparent non-convergence of the fragmentation boundary seen in prior simulations is likely a numerical effect. In real discs, where cooling is physically smoothed by radiative transfer effects, the fragmentation boundary is probably displaced from the two-dimensional value by a factor that is only of the order of unity.

by Laibe, Guillaume and Price, Daniel J.
Accepted for publication in MNRAS

In a companion paper (Laibe & Price 2011b), we have presented an algorithm for simulating two-fluid gas and dust mixtures in Smoothed Particle Hydrodynamics (SPH). In this paper, we develop an implicit timestepping method that preserves the exact conservation of the both linear and angular momentum in the underlying SPH algorithm, but unlike previous schemes, allows the iterations to converge to arbitrary accuracy and is suited to the treatment of non- linear drag regimes. The algorithm presented in Paper I is also extended to deal with realistic astrophysical drag regimes, including both linear and non-linear Epstein and Stokes drag. The scheme is benchmarked against the test suite presented in Paper I, including i) the analytic solutions of the dustybox problem and ii) solutions of the dustywave, dustyshock, dustysedov and dustydisc obtained with explicit timestepping. We find that the implicit method is 1- 10 times faster than the explicit temporal integration when the ratio r between the the timestep and the drag stopping time is 1 < r < 1000.

by Laibe, Guillaume and Price, Daniel J.
Accepted for publication in MNRAS

We present a new algorithm for simulating two-fluid gas and dust mixtures in Smoothed Particle Hydrodynamics (SPH), systematically addressing a number of key issues including the generalised SPH density estimate in multi-fluid systems, the consistent treatment of variable smoothing length terms, finite particle size, time step stability, thermal coupling terms and the choice of kernel and smoothing length used in the drag operator. We find that using double-hump shaped kernels improves the accuracy of the drag interpolation by a factor of several hundred compared to the use of standard SPH bell-shaped kernels, at no additional computational expense. In order to benchmark our algorithm, we have developed a comprehensive suite of standardised, simple test problems for gas and dust mixtures: dustybox, dustywave, dustyshock, dustysedov and dustydisc, the first three of which have known analytic solutions. We present the validation of our algorithm against all of these tests. In doing so, we show that the spatial resolution criterion \Delta < cs ts is a necessary condition in all gas+dust codes that becomes critical at high drag (i.e. small stopping time ts) in order to correctly predict the dynamics. Implicit timestepping and the implementation of realistic astrophysical drag regimes are addressed in a companion paper.

by Krolik, Julian H. and Piran, Tsvi

Tidal disruption of main sequence stars by black holes has generally been thought to lead to a signal dominated by UV emission. If, however, the black hole spins rapidly and the poloidal magnetic field intensity on the black hole horizon is comparable to the inner accretion disk pressure, a powerful jet may form whose luminosity can easily exceed the thermal UV luminosity. When the jet beam points at Earth, its non-thermal luminosity can dominate the emitted spectrum. The thermal and non-thermal components decay differently with time. In particular, the thermal emission should remain roughly constant for a significant time after the period of maximum accretion, beginning to diminish only after a delay, whereas after the peak accretion rate, the non-thermal jet emission decays, but then reaches a plateau. When the newly-found flare source Swift J2058 is analyzed in terms of this model, it is found to be consistent with an event in which a main sequence solar-type star is disrupted by a black hole of mass at least $latex \sim 10^7 M_{\odot}$. Swift may have already observed the beginning of the flat phase in the non-thermal emission from this source. Optical photometry over the first $latex \simeq 40$ d of this flare is also consistent with this picture, but there is a large uncertainty in the bolometric correction. We suggest that future searches for main sequence tidal disruptions use methods sensitive to jet radiation as well as to thermal UV radiation.

by Zubovas, Kastytis and Nayakshin, Sergei and Markoff, Sera
11 pages. MNRAS submitted

It is theoretically expected that a supermassive black hole (SMBH) in the centre of a typical nearby galaxy disrupts a Solar-type star every ~ 10^5 years, resulting in a bright flare lasting for months. Sgr A*, the resident SMBH of the Milky Way, produces (by comparison) tiny flares that last only hours but occur daily. Here we explore the possibility that these flares could be produced by disruption of smaller bodies – asteroids. We show that asteroids passing within an AU of Sgr A* could be split into smaller fragments which then vaporise by bodily friction with the tenuous quiescent gas accretion flow onto Sgr A*. The ensuing shocks and plasma instabilities may create a transient population of very hot electrons invoked in several currently popular models for Sgr A* flares, thus producing the required spectra. We estimate that asteroids larger than ~ 10 km in size are needed to power the observed flares, with the maximum possible luminosity of the order 10^39 erg s^-1. Assuming that the asteroid population per parent star in the central parsec of the Milky Way is not too dissimilar from that around stars in the Solar neighbourhood, we estimate the asteroid disruption rates, and the distribution of the expected luminosities, finding a reasonable agreement with the observations. We also note that planets may be tidally disrupted by Sgr A* as well, also very infrequently. We speculate that one such disruption may explain the putative increase in Sgr A* luminosity ~ 300 yr ago.

by Stone, Nicholas and Loeb, Abraham
4 pages, 4 figures

When a star is tidally disrupted by a supermassive black hole (SMBH), the streams of liberated gas form an accretion disk after their return to pericenter. We demonstrate that Lense-Thirring precession in the spacetime around a rotating SMBH can produce significant time evolution of the disk angular momentum vector, due to both the periodic precession of the disk and the nonperiodic, differential precession of the bound debris streams. Jet precession and periodic modulation of disk luminosity are possible consequences. The persistence of the jetted X-ray emission in the Swift J164449.3+573451 flare suggests that the jet axis was aligned with the spin axis of the SMBH during this event.

by Moesta, Philipp and Alic, Daniela and Rezzolla, Luciano and Zanotti, Olindo and Palenzuela, Carlos
4 pages, 3 figures

We revisit the suggestion that dual jets can be produced during the inspiral and merger of supermassive black holes when these are immersed in a force-free plasma threaded by a uniform magnetic field. By performing independent calculations and by computing the electromagnetic emission in a way which is consistent with estimates using the Poynting flux, we show that a dual-jet structure is present but energetically subdominant with respect to a non-collimated and predominantly quadrupolar emission, which is similar to the one computed when the binary is in electrovacuum. While our findings set serious restrictions on the detectability of dual jets from coalescing binaries, they also increase the chances of detecting an EM counterpart from these systems.

by Fabian, A. C. and Zoghbi, A. and Wilkins, D. and Dwelly, T. and Uttley, P. and Schartel, N. and Miniutti, G. and Gallo, L. and Grupe, D. and Komossa, S. and Santos-Lleo, M.
9 pages, 19 figures, MNRAS in press

The Narrow Line Seyfert 1 Galaxy 1H0707-495 went in to a low state from 2010 December to 2011 February, discovered by a monitoring campaign using the X-Ray Telescope on the Swift satellite. We triggered a 100 ks XMM-Newton observation of the source in 2011 January, revealing the source to have dropped by a factor of ten in the soft band, below 1 keV, and a factor of 2 at 5 keV, compared with a long observation in 2008. The sharp spectral drop in the source usually seen around 7 keV now extends to lower energies, below 6 keV in our frame. The 2011 spectrum is well fit by a relativistically-blurred reflection spectrum similar to that which fits the 2008 data, except that the emission is now concentrated solely to the central part of the accretion disc. The irradiating source must lie within 1 gravitational radius of the event horizon of the black hole, which spins rapidly. Alternative models are briefly considered but none has any simple physical interpretation.

by Zanotti, Olindo
6 pages, 4 figures. Accepted by New Astronomy

We show the results of two dimensional general relativistic inviscid and isothermal hydrodynamical simulations comparing the behavior of co-rotating (with respect to the black hole rotation) and counter-rotating circumbinary quasi-Keplerian discs in the post merger phase of a supermassive binary black hole system. While confirming the spiral shock generation within the disc due to the combined effects of mass loss and recoil velocity of the black hole, we find that the maximum luminosity of counter-rotating discs is a factor ~(2-12) higher than in the co-rotating case, depending on the spin of the black hole. On the other hand, the luminosity peak happens ~10 days later with respect to the co-rotating case, for a binary with a total mass M~10^6 M_\odot. Although the global dynamics of counter-rotating discs in the post merger phase of a merging event is very similar to that for co-rotating discs, an important difference has been found. In fact, increasing the spin of the central black hole produces more luminous co-rotating discs while less luminous counter-rotating ones.

by Sadowski, A.
Ph.D. thesis written under the supervision of Prof. Marek Abramowicz and defended on July 1, 2011 at the Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences; 206 pages

In this thesis, I study hydrodynamical models of slim accretion disks — advective, optically thick disks which generalize the standard models of radiatively efficient thin disks to all accretion rates. I start with a general introduction to the theory of accretion onto compact objects. It is followed by a derivation of the commonly-used standard models of thin disks. In the subsequent section I introduce the equations describing slim disks, explain the numerical methods I used to solve them and discuss properties of such solutions. I also give a general derivation of non-stationary equations and present the time evolution of thermally unstable accretion disks. I introduce a state-of-the-art approach coupling the radial and vertical structures of an advective accretion disk and discuss the improvements it brings to vertically-averaged solutions. I also present a numerical model of self-illuminated slim accretion disks. Finally, I present and discuss applications of slim accretion disks: estimating of spin of the central black hole in LMC X-3 through X-ray continuum fitting basing on high-luminosity data, spinning-up of black holes by super-critical accretion flows and normalizing of magnetohydrodynamical global simulations.

by Dubois, Yohan and Devriendt, Julien and Slyz, Adrianne and Teyssier, Romain
24 pages, 18 figures, 1 table, submitted to MNRAS

We develop a new sub-grid model for the growth of supermassive Black Holes (BHs) and their associated Active Galactic Nuclei (AGN) feedback in hydrodynamical cosmological simulations. Assuming that BHs are created in the early stages of galaxy formation, they grow by mergers and accretion of gas at a Eddington-limited Bondi accretion rate. However this growth is regulated by AGN feedback which we model using two different modes: a quasar-heating mode when accretion rates onto the BHs are comparable to the Eddington rate, and a radio-jet mode at lower accretion rates. In other words, our feedback model deposits energy as a succession of thermal bursts and jet outflows depending on the properties of the gas surrounding the BHs. We assess the plausibility of such a model by comparing our results to observational measurements of the coevolution of BHs and their host galaxy properties, and check their robustness with respect to numerical resolution. We show that AGN feedback must be a crucial physical ingredient for the formation of massive galaxies as it appears to be the only physical mechanism able to efficiently prevent the accumulation of and/or expel cold gas out of halos/galaxies and significantly suppress star formation. Our model predicts that the relationship between BHs and their host galaxy mass evolves as a function of redshift, because of the vigorous accretion of cold material in the early Universe that drives Eddington-limited accretion onto BHs. Quasar activity is also enhanced at high redshift. However, as structures grow in mass and lose their cold material through star formation and efficient BH feedback ejection, the AGN activity in the low-redshift Universe becomes more and more dominated by the radio mode, which powers jets through the hot circum-galactic medium.

by Rice, W. K. M. and Armitage, P. J. and Mamatsashvili, G. R. and Lodato, G. and Clarke, C. J.
MNRAS, in press

Self-gravity becomes competitive as an angular momentum transport process in accretion discs at large radii, where the temperature is low enough that external irradiation likely contributes to the thermal balance. Irradiation is known to weaken the strength of disc self-gravity, and can suppress it entirely if the disc is maintained above the threshold for linear instability. However, its impact on the susceptibility of the disc to fragmentation is less clear. We use two-dimensional numerical simulations to investigate the evolution of self-gravitating discs as a function of the local cooling time and strength of irradiation. In the regime where the disc does not fragment, we show that local thermal equilibrium continues to determine the stress – which can be represented as an effective viscous alpha – out to very long cooling times (at least 240 dynamical times). In this regime, the power spectrum of the perturbations is uniquely set by the effective viscous alpha and not by the cooling rate. Fragmentation occurs for cooling times tau < beta_crit / Omega, where beta_crit is a weak function of the level of irradiation. We find that beta_crit declines by approximately a factor of two, as irradiation is increased from zero up to the level where instability is almost quenched. The numerical results imply that irradiation cannot generally avert fragmentation of self-gravitating discs at large radii; if other angular momentum transport sources are weak mass will build up until self-gravity sets in, and fragmentation will ensue.

by Tanaka, Takamitsu and Haiman, Zoltán and Menou, Kristen
16 pages with 5 figures, submitted to MNRAS

Pulsar timing arrays (PTAs) are expected to detect gravitational waves (GWs) from individual low-redshift (z10^9 Msun) black hole (SMBH) binaries with orbital periods of approx. 0.1 – 10 yrs. Identifying the electromagnetic (EM) counterparts of these sources would provide confirmation of putative direct detections of GWs, present a rare opportunity to study the environments of compact SMBH binaries, and could enable the use of these sources as standard sirens for cosmology. Here we consider the feasibility of such an EM identification. We show that because the host galaxies of resolved PTA sources are expected to be exceptionally massive and rare, it should be possible to find unique hosts of resolved sources out to redshift z=0.2. At higher redshifts, the PTA error boxes are larger, and may contain as many as 100 massive-galaxy interlopers. The number of candidates, however, remains tractable for follow-up searches in upcoming wide-field EM surveys. We develop a toy model to characterize the dynamics and the thermal emission from a geometrically thin, gaseous disc accreting onto a PTA-source SMBH binary. Our model predicts that at optical and infrared frequencies, the source should appear similar to a typical luminous active galactic nucleus (AGN). However, owing to the evacuation of the accretion flow by the binary’s tidal torques, the source might have an unusually low soft X-ray luminosity and weak UV and broad optical emission lines, as compared to an AGN powered by a single SMBH with the same total mass. For sources near z=1, the decrement in the rest-frame UV should be observable as an extremely red optical color. These properties would make the PTA sources stand out among optically luminous AGN, and could allow their unique identification.

by Sesana, A. and Roedig, C. and Reynolds, M. T. and Dotti, M.
19 pages, 11 figures, submitted to MNRAS

We demonstrate that very massive (>10^8\msun), cosmologically nearby (z10^-13 erg s^-1 cm^-2 will be in the reach of upcoming X-ray observatories. Double relativistic K\alpha lines may be observable in a handful of low redshift (z<0.3) sources by proposed deep X-ray probes, such as Athena. (Abridged)

by Ponce, Marcelo and Faber, Joshua A. and Lombardi, James C.
Submitted to ApJ. Movies available as ‘ancillary files’

Numerical calculations of merging black hole binaries indicate that asymmetric emission of gravitational radiation can kick the merged black hole at up to thousands of km/s, and a number of systems have been observed recently whose properties are consistent with an active galactic nucleus containing a supermassive black hole moving with substantial velocity with respect to its broader accretion disk. We study here the effect of an impulsive kick delivered to a black hole on the dynamical evolution of its accretion disk using a smoothed particle hydrodynamics code, focusing attention on the role played by the kick angle with respect to the orbital angular momentum vector of the pre-kicked disk. We find that for more vertical kicks, for which the angle between the kick and the normal vector to the disk $latex \theta\lesssim 30^\circ$, a gap remains present in the inner disk, in accordance with the prediction from an analytic collisionless Keplerian disk model, while for more oblique kicks with $latex \theta\gtrsim 45^\circ$, matter rapidly accretes toward the black hole. There is a systematic trend for higher potential luminosities for more oblique kick angles for a given black hole mass, disk mass and kick velocity, and we find large amplitude oscillations in time in the case of a kick oriented $latex 60^\circ$ from the vertical.

by Zhang, Xue-Guang
17 pages, 11 figures, MNRAS accepted

In this manuscript, the structure of broad emission line regions (BLRs) of well-mapping double-peaked emitter (AGN with broad double-peaked low-ionization emission lines) 3C390.3 is studied. Besides the best fitted results for double-peaked broad optical balmer lines of 3C390.3 by theoretical disk model, we try to find another way to further confirm the origination of double-peaked line from accretion disk. Based on the long-period observed spectra in optical band around 1995 collected from AGN WATCH project, the theoretical disk parameters of disk-like BLRs supposed by elliptical accretion disk model (Eracleous et al. 1995) have been well determined. Through the theoretical disk-like BLRs, characters of observed light-curves of broad double-peaked H$latex \alpha$ of 3C390.3 can be well reproduced based on the reverberation mapping technique. Thus the accretion disk model is preferred as one better model for BLRs of 3C390.3. Furthermore, we can find that different disk parameters should lead to some different results about size of BLRs of 3C390.3 from the one measured through observational data, which indicates the measured disk parameters are significantly valid for 3C390.3. After that, the precession of theoretical elliptical disk-like BLRs being considered, we can find that the expected line profile in 2000 by theoretical model is consistent with the observed line profile by HST around 2000. Based on the results, we can further believe that the origination of broad double-peaked balmer emission lines of 3C390.3 are from accretion disk around central black hole.

by Wong, Ka-Wah and Irwin, Jimmy A. and Yukita, Mihoko and Million, Evan T. and Mathews, William G. and Bregman, Joel N.
5 pages, 4 figures, accepted for publication in the Astrophysical Journal Letters

Gas undergoing Bondi accretion onto a supermassive black hole (SMBH) becomes hotter toward smaller radii. We searched for this signature with a Chandra observation of the hot gas in NGC 3115, which optical observations show has a very massive SMBH. Our analysis suggests that we are resolving, for the first time, the accretion flow within the Bondi radius of a SMBH. We show that the temperature is rising toward the galaxy center as expected in all accretion models in which the black hole is gravitationally capturing the ambient gas. There is no hard central point source that could cause such an apparent rise in temperature. The data support that the Bondi radius is at about 4 arcsec-5 arcsec (188-235 pc), suggesting a SMBH of 2 x 10^9 M_sun that is consistent with the upper end of the optical results. The density profile within the Bondi radius has a power law index of 1.03^{+0.23}_{-0.21} which is consistent with gas in transition from the ambient medium and the accretion flow. The accretion rate at the Bondi radius is determined to be {\dot M}_B = 2.2 x 10^{-2} M_sun yr^{-1}. Thus, the accretion luminosity with 10% radiative efficiency at the Bondi radius (10^{44} ergs s^{-1}) is about six orders of magnitude higher than the upper limit of the X-ray luminosity of the nucleus.

by Amaro-Seoane, Pau and Miller, M. Coleman and Kennedy, Gareth F.
Submitted to MNRAS

Several galaxies have exhibited X-ray flares that are consistent with the tidal disruption of a star by a central supermassive black hole. In theoretical treatments of this process it is usually assumed that the star was initially on a nearly parabolic orbit relative to the black hole. Such an assumption leads in the simplest approximation to a $latex t^{-5/3}$ decay of the bolometric luminosity and this is indeed consistent with the relatively poorly sampled light curves of such flares. We point out that there is another regime in which the decay would be different: if a binary is tidally separated and the star that remains close to the hole is eventually tidally disrupted from a moderate eccentricity orbit, the decay is slower, typically $latex \sim t^{-1.2}$. As a result, careful sampling of the light curves of such flares could distinguish between these processes and yield insight into the dynamics of binaries as well as single stars in galactic centres. We explore this process using three-body simulations and analytic treatments and discuss the consequences for present-day X-ray detections and future gravitational wave observations.

by Krolik, Julian H. and Piran, Tsvi

We propose that the remarkable object Swift J1644+57, in which multiple recurring hard X-ray flares were seen over a span of several days, is a system in which a white dwarf was tidally disrupted by an intermediate mass black hole. Disruption of a white dwarf rather than a main sequence star offers a number of advantages in understanding the multiple, and short, timescales seen in the light curve of this system. In particular, the short internal dynamical timescale of a white dwarf offers a more natural way of understanding the short rise times (~100s) observed. The relatively long intervals between flares (~5 x 10^4 s) may also be readily understood as the period between successive pericenter passages of the remnant white dwarf. In addition, the expected jet power is larger when a white dwarf is disrupted. If this model is correct, the black hole responsible must have mass < 10^5 M_{odot}.

by Stone, Nicholas and Loeb, Abraham
15 pages, 8 figures

We analyze stellar tidal disruption events as a possible observational signature of gravitational wave induced recoil of supermassive black holes. As a black hole wanders through its galaxy, it will tidally disrupt bound and unbound stars at rates potentially observable by upcoming optical transient surveys. To quantify these rates, we explore a broad range of host galaxy and black hole kick parameters. We find that emission from a transient accretion disk can produce ~1 event per year which LSST would identify as spatially offset, while super-Eddington tidal flares, if they exist, are likely to produce ~10 spatially offset events per year. A majority of tidal disruption flares, and a large majority of flares with an observable spatial offset, are due to bound rather than unbound stars. The total number of disruption events due to recoiled black holes could be almost 1% of the total stellar tidal disruption rate.

by Noble, Scott C. and Krolik, Julian H. and Schnittman, Jeremy D. and Hawley, John F.
Submitted to ApJ, 26 pages, 12 figures (some in color), AASTEX

Recent general relativistic magneto-hydrodynamic (MHD) simulations of accretion onto black holes have shown that, contrary to the basic assumptions of the Novikov-Thorne model, there can be substantial magnetic stress throughout the plunging region. Additional dissipation and radiation can therefore be expected. We use data from a particularly well-resolved simulation of accretion onto a non-spinning black hole to compute both the radiative efficiency of such a flow and its spectrum if all emitted light is radiated with a thermal spectrum whose temperature matches the local effective temperature. This disk is geometrically thin enough (H/r ~= 0.06) that little heat is retained in the flow. In terms of light reaching infinity (i.e., after allowance for all relativistic effects and for photon capture by the black hole), we find that the radiative efficiency is at least ~=6-10% greater than predicted by the Novikov-Thorne model (complete radiation of all heat might yield another ~6%). We also find that the spectrum more closely resembles the Novikov-Thorne prediction for a/M ~= 0.2–0.3 than for the correct value, a/M=0. As a result, if the spin of a non-spinning black hole is inferred by model-fitting to a Novikov-Thorne model with known black hole mass, distance, and inclination, the inferred a/M is too large by ~= 0.2–0.3.

by Armijo, Matias Montesinos and Pacheco, José A. de Freitas
14 pages, 11 figures, Submitted to the ApJ

The evolution of an accretion disk, formed as a consequence of the disruption of a star by a black hole, is followed by solving numerically the hydrodynamic equations. The present investigation aims to study the dependence of resulting light curves on dynamical and physical properties of such a transient disk during its existence. One of main results derived from our simulations is that black body fits of X-ray data tend to overestimate the true mean disk temperature. The temperature derived from black body fits should be identified with the color X-ray temperature rather than the average value derived from the true temperature distribution along the disk. The time interval between the beginning of the circularization of the bound debris and the beginning of the accretion process by the black hole is determined by the viscous timescale, which fixes also the raising part of the resulting light curve. The luminosity peak coincides with the beginning of matter accretion by the black hole and the late evolution of the light curve depends on the evolution of the debris fallback rate. Peak bolometric luminosities are in the range 10^45-10^46 erg s^-1 whereas peak luminosities in soft X-rays (0.2-2.0 keV) are typically one order of magnitude lower. The timescale derived from our preferred models for the flare luminosity to decay by two orders of magnitude is about 3-4 years. Predicted soft X-ray light curves were fitted to data on galaxies in which a variable X-ray emission, related to tidal events, was detected.

by Abramowicz, Marek A. and Fragile, P. Chris
58 pages, 22 figures, submitted to Living Reviews in Relativity

In this review, we summarize the present status of knowledge of black hole accretion disks by discussing in some details the fundamental concepts on which the theory rests, and by describing the most often used analytic and semi-analytic models of them. We also describe numerical simulations of black hole accretion.

by Burrows, D. N. and Kennea, J. A. and Ghisellini, G. and Mangano, V. and Zhang, B. and Page, K. L. and Eracleous, M. and Romano, P. and Sakamoto, T. and Falcone, A. D. and Osborne, J. P. and Campana, S. and Beardmore, A. P. and Breeveld, A. A. and Chester, M. M. and Corbet, R. and Covino, S. and Cummings, J. R. and D’Avanzo, P. and D’Elia, V. and Esposito, P. and Evans, P. A. and Fugazza, D. and Gelbord, J. M. and Hiroi, K. and Holland, S. T. and Huang, K. Y. and Im, M. and Israel, G. and Jeon, Y. and Jeon, Y. -B. and Kawai, N. and Krimm, H. A. and Mészáros, P. and Negoro, H. and Omodei, N. and Park, W. -K. and Perkins, J. S. and Sugizaki, M. and Sung, H. -I. and Tagliaferri, G. and Troja, E. and Ueda, Y. and Urata, Y. and Usui, R. and Antonelli, L. A. and Barthelmy, S. D. and Cusumano, G. and Giommi, P. and Marshall, F. E. and Melandri, A. and Perri, M. and Racusin, J. L. and Sbarufatti, B. and Siegel, M. H. and Gehrels, N.
Submitted to Nature. 4 pages, 3 figures (main paper). 26 pages, 13 figures (supplementary information)

Massive black holes are believed to reside at the centres of most galaxies. They can be- come detectable by accretion of matter, either continuously from a large gas reservoir or impulsively from the tidal disruption of a passing star, and conversion of the gravitational energy of the infalling matter to light. Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be variable but have never been observed to turn on or off. Tidal disruption of stars by dormant massive black holes has been inferred indirectly but the on- set of a tidal disruption event has never been observed. Here we report the first discovery of the onset of a relativistic accretion-powered jet in the new extragalactic transient, Swift J164449.3+573451. The behaviour of this new source differs from both theoretical models of tidal disruption events and observations of the jet-dominated AGN known as blazars. These differences may stem from transient effects associated with the onset of a powerful jet. Such an event in the massive black hole at the centre of our Milky Way galaxy could strongly ionize the upper atmosphere of the Earth, if beamed towards us.

by Rödig, Constanze and Dotti, Massimo and Sesana, Alberto and Cuadra, Jorge and Colpi, Monica
10 pages, 7 figures, accepted for publication in MNRAS

We study the dynamics of supermassive black hole binaries embedded in circumbinary gaseous discs, with the SPH code Gadget-2. The sub-parsec binary (of total mass M and mass ratio q=1/3) has excavated a gap and transfers its angular momentum to the self–gravitating disc (M_disc=0.2 M). We explore the changes of the binary eccentricity e, by simulating a sequence of binary models that differ in the initial eccentricity e_0, only. In initially low-eccentric binaries, the eccentricity increases with time, while in high-eccentric binaries e declines, indicating the existence of a limiting eccentricity e_crit that is found to fall in the interval [0.6,0.8]. We also present an analytical interpretation for this saturation limit. An important consequence of the existence of e_crit is the detectability of a significant residual eccentricity e_LISA} by the proposed gravitational wave detector LISA. It is found that at the moment of entering the LISA frequency domain e_LISA ~ 10^{-3}-10^{-2}; a signature of its earlier coupling with the massive circumbinary disc. We also observe large periodic inflows across the gap, occurring on the binary and disc dynamical time scales rather than on the viscous time. These periodic changes in the accretion rate (with amplitudes up to ~100%, depending on the binary eccentricity) can be considered a fingerprint of eccentric sub-parsec binaries migrating inside a circumbinary disc.

by Bloom, Joshua S. and Giannios, Dimitrios and Metzger, Brian D. and Cenko, S. Bradley and Perley, Daniel A. and Butler, Nathaniel R. and Tanvir, Nial R. and Levan, Andrew J. and Brien, Paul T. O’ and Strubbe, Linda E. and De Colle, Fabio and Ramirez-Ruiz, Enrico and Lee, William H. and Nayakshin, Sergei and Quataert, Eliot and King, Andrew R. and Cucchiara, Antonino and Guillochon, James and Bower, Geoffrey C. and Fruchter, Andrew S. and Morgan, Adam N. and van der Horst, Alexander J.
Submitted, 32 pages including supplemental online material

While gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, the vast majority of MBHs are considered dormant. Occasionally, a star passing too near a MBH is torn apart by gravitational forces, leading to a bright panchromatic tidal disruption flare (TDF). While the high-energy transient Swift J164449.3+573451 (”Sw 1644+57″) initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that the observations (Levan et al. 2011) suggest a sudden accretion event onto a central MBH of mass ~10^6-10^7 solar masses. We find evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 with a smaller-scale blazar. The phenomenologically novel Sw 1644+57 thus connects the study of TDFs and active galaxies, opening a new vista on disk-jet interactions in BHs and magnetic field generation and transport in accretion systems.

by Giannios, Dimitrios and Metzger, Brian D.
5 pages, 2 figures, submitted to MNRAS Letters

The tidal disruption of a star by a supermassive black hole provides us with a rare glimpse of these otherwise dormant beasts. It has long been predicted that the disruption will be accompanied by a thermal `flare’, powered by the accretion of bound stellar debris. Several candidate disruptions have been discovered in this manner at optical, UV and X-ray wavelengths. Here we explore the observational consequences if a modest fraction of the accretion power is channeled into an ultra-relativistic outflow. We show that a relativistic jet decelerates due to its interaction with the interstellar medium at sub-parsec distances from the black hole. Synchrotron radiation from electrons accelerated by the reverse shock powers a bright radio-infrared transient that peaks on a timescale ~1 yr after disruption. Emission from the forward shock may be detectable for several years after the peak. Deep radio follow-up observations of tidal disruption candidates at late times can test for the presence of relativistic ejecta. Upcoming radio transient surveys may independently discover tens to hundreds of tidal disruptions per year, complimenting searches at other wavelengths. Non-thermal emission from tidal disruption probes the physics of jet formation under relatively clean conditions, in which the flow parameters are independently constrained.

by Zamaninasab, M. and Eckart, A. and Dovciak, M. and Karas, V. and Schoedel, R. and Witzel, G. and Sabha, N. and Garcia-Marin, M. and Kunneriath, D. and Muzic, K. and Straubmeier, C. and Valencia-S, M. and Zensus, J. A.
12 pages, 13 figures, accepted for publication in MNRAS

Several massive black holes exhibit flux variability on time scales that correspond to source sizes of the order of few Schwarzschild radii. We survey the potential of near-infrared and X-ray polarimetry to constrain physical properties of such black hole systems, namely their spin and inclination. We have focused on a model where an orbiting hot spot is embedded in an accretion disk. A new method of searching for the time-lag between orthogonal polarization channels is developed and applied to an ensemble of hot spot models that samples a wide range of parameter space. We found that the hot spot model predicts signatures in polarized light which are in the range to be measured directly in the near future. However, our estimations are predicted upon the assumption of a Keplerian velocity distribution inside the flow where the dominant part of the magnetic field is toroidal. We also found that if the right model of the accretion flow can be chosen for each source (e.g. on the basis of magnetohydrodynamics simulations) then the black hole spin and inclination can be constrained to a small two-dimensional area in the spin-inclination space. The results of the application of the method to the available near-infrared polarimetric data of Sagittarius A* (Sgr A*) is presented. It is shown that even with the currently available data the spin and inclination of Sgr A* can be constrained. Next generations of near-infrared and X-ray polarimeters should be able to exploit this tool.

by Bode, Tanja and Bogdanovic, Tamara and Haas, Roland and Healy, James and Laguna, Pablo and Shoemaker, Deirdre
5 pages, 4 figures, 1 table

Modeling the late inspiral and merger of supermassive black holes is central to understanding accretion processes and the conditions under which electromagnetic emission accompanies gravitational waves. We use fully general relativistic, hydrodynamics simulations to investigate how electromagnetic signatures correlate with black hole spins, mass ratios, and the gaseous environment in this final phase of binary evolution. In all scenarios, we find some form of characteristic electromagnetic variability whose pattern depends on the spins and binary mass ratios. Binaries in hot accretion flows exhibit a flare followed by a sudden drop in luminosity associated with the plunge and merger, as well as quasi-periodic oscillations correlated with the gravitational waves during the inspiral. Conversely, circumbinary disk systems are characterized by a low luminosity of variable emission, suggesting challenging prospects for their detection.

by Chen, Xian and Sesana, Alberto and Madau, Piero and Liu, Fukun
16 pages, 20 figures, accepted for publication in the Astrophysical Journal

Tidal stellar disruptions have traditionally been discussed as a probe of the single, massive black holes (MBHs) that are dormant in the nuclei of galaxies. In Chen et al. (2009), we used numerical scattering experiments to show that three-body interactions between bound stars in a stellar cusp and a non-evolving “hard” MBH binary will also produce a burst of tidal disruptions, caused by a combination of the secular “Kozai effect” and by close resonant encounters with the secondary hole. Here we derive basic analytical scalings of the stellar disruption rates with the system parameters, assess the relative importance of the Kozai and resonant encounter mechanisms as a function of time, discuss the impact of general relativistic (GR) and extended stellar cusp effects, and develop a hybrid model to self-consistently follow the shrinking of an MBH binary in a stellar background, including slingshot ejections and tidal disruptions. In the case of a fiducial binary with primary hole mass M_1=10^7\msun and mass ratio q=M_2/M_1=1/81, embedded in an isothermal cusp, we derive a stellar disruption rate \dot{N_*} ~ 0.2/yr lasting ~ 3X10^5 yr. This rate is 3 orders of magnitude larger than the corresponding value for a single MBH fed by two-body relaxation, confirming our previous findings. For q<~10% of the tidal-disruption events may originate in MBH binaries.

by Wegg, Christopher and Bode, J. Nate
5 pages, 3 figures

We find that the majority of systems hosting multiple tidal disruptions are likely to contain hard binary SMBH systems, and also show that the rates of these repeated events are high enough to be detected by LSST over its lifetime. Therefore, these multiple tidal disruption events provide a novel method to identify super-massive black hole (SMBH) binary systems with parsec to sub-parsec separations. The rates of tidal disruptions are investigated using simulations of non-interacting stars initially orbiting a primary SMBH and the potential of the model stellar cusp. The stars are then evolved forward in time and perturbed by a secondary SMBH inspiraling from the edge of the cusp to its stalling radius. We find with conservative magnitude estimates that the next generation transient survey LSST should detect multiple tidal disruptions in approximately 3 galaxies over 5 years of observation, though less conservative estimates could increase this rate by an order of magnitude.

by Pang, Bijia and Pen, Ue-Li and Matzner, Christopher D. and Green, Stephen R. and Liebendörfer, Matthias

We conduct a survey of numerical simulations to probe the structure and appearance of non-radiative black hole accretion flows like the supermassive black hole at the Galactic centre. We find a generic set of solutions, and make specific predictions for currently feasible rotation measure (RM) observations, which are accessible to current instruments including the EVLA, GMRT and ALMA. The slow time variability of the RM is a key quantitative signature of this accretion flow. The time variability of RM can be used to quantitatively measure the nature of the accretion flow, and to differentiate models. Sensitive measurements of RM can be achieved using RM synthesis or using pulsars.

Our energy conserving ideal magneto-hydrodynamical simulations, which achieve high dynamical range by means of a deformed-mesh algorithm, stretch from several Bondi radii to about one thousandth of that radius, and continue for tens of Bondi times. Magnetized flows which lack outward convection possess density slopes around -1, almost independent of physical parameters, and are more consistent with observational constraints than are strongly convective flows We observe no tendency for the flows to become rotationally supported in their centres, or to develop steady outflow.

We support these conclusions with formulae which encapsulate our findings in terms of physical and numerical parameters. We discuss the relation of these solutions to other approaches. The main potential uncertainties are the validity of ideal MHD and the absence of a fully relativistic inner boundary condition. The RM variability predictions are testable with current and future telescopes.

by Bu, De-Fu and Yuan, Feng and Stone, James M
7 pages, 9 figures, submitted to MNRAS

For magnetized accretion flows with very low accretion rates such as that in the supermassive black hole in our Galactic center, $latex Sgr A^*$, the mean free path of electrons is much greater than the Larmor radius and is an appreciable fraction of the size of the system. In this case, the thermal conduction is anisotropic and dynamically important. Provided that the magnetic field is weak, magnetothermal instability (MTI) exists . It can amplify the magnetic field and align the field lines with the temperature gradient (i.e., the radial direction). If the accretion flow is differentially rotating, magnetorotational instability (MRI) also exists as well known. In this paper, we investigate the possible interaction of these two instabilities. We study a hot accretion flow around Bondi radius, where the infall timescale of gas is longer than the MTI and MRI growth timescales, thus MTI and MRI coexist. We focus on the interaction between MTI and MRI by examining the magnetic field amplification induced by the two instabilities. We find that MTI and MRI mainly amplify the radial and toroidal components of the magnetic field, respectively. Most importantly, we find that if MTI alone can amplify the magnetic field by a factor of $latex F_t$ and MRI alone by a factor of $latex F_r$, when MTI and MRI coexist, the magnetic field can be amplified by a factor of $latex F_t F_r$. We therefore conclude that MTI and MRI operate separately. The physical reason for the decouple of MTI and MRI is that they are two intrinsically different physical process. We also find that MTI helps to transfer angular momentum, because MTI can enhance the Maxwell stress (by amplifying the magnetic field) and Reynolds stress. Finally, we find that thermal conduction makes the temperature slope flatter by transporting energy outward. This makes the mass accretion rate smaller.

by Volonteri, Marta and Dotti, Massimo and Campbell, Duncan and Mateo, Mario
Submitted to ApJ

Only a small fraction of local galaxies harbor an accreting black hole, classified as an active galactic nucleus (AGN). However, many stellar systems are plausibly expected to host black holes, from globular clusters to nuclear star clusters, to massive galaxies. The mere presence of stars in the vicinity of a black hole provides a source of fuel via mass loss of evolved stars. In this paper we assess the expected luminosities of black holes embedded in stellar systems of different sizes and properties, spanning a large range of masses. We model the distribution of stars and derive the amount of gas available to a central black hole through a geometrical model. We estimate the luminosity of the black holes under simple, but physically grounded, assumptions on the accretion flow. Finally we discuss the detectability of ‘quiescent’ black holes in the local Universe.

by Jiang, Yanfei and Goodman, Jeremy
34 pages, 8 figures. Submitted to ApJ

Using a version of the ZEUS code, we carry out two-dimensional simulations of self-gravitating shearing sheets, with application to QSO accretion disks at a few thousand Schwarzschild radii, corresponding to a few hundredths of a parsec for a 10^8 solar-mass black hole. Radiation pressure and optically thick radiative cooling are implemented via vertical averages. We determine dimensionless versions of the maximum surface density, accretion rate, and effective viscosity that can be sustained by density-wave turbulence without fragmentation. Where fragments do form, we study the final masses that result. The maximum Shakura-Sunyaev viscosity parameter is approximately 0.4. Fragmentation occurs when the cooling time is less than about twice the shearing time, as found by Gammie and others, but can also occur at very long cooling times in sheets that are strongly radiation-pressure dominated. For accretion at the Eddington rate onto a 10^8 solar-mass black hole, fragmentation occurs beyond four thousand Schwarzschild radii, r_s. Near this radius, initial fragment masses are several hundred suns, consistent with estimates from linear stability; final masses after merging increase with the size of the sheet, reaching several thousand suns in our largest simulations. With increasing black-hole mass at a fixed Eddington ratio, self-gravity prevails to smaller multiples of r_s, where radiation pressure is more important and the cooling time is longer compared to the dynamical time; nevertheless, fragmentation can occur and produces larger initial fragment masses. We also find energy conservation is likely to be a challenge for all eulerian codes in self-gravitating regimes where radiation pressure dominates.

by Nixon, C. J. and Cossins, P. J. and King, A. R. and Pringle, J. E.
8 pages, 4 figures. Accepted for publication in MNRAS. Movies of the simulations can be found at: http://www.astro.le.ac.uk/users/cjn12/RetroBinaryMovies.html

We investigate whether a circumbinary gas disc can coalesce a supermassive black hole binary system in the centre of a galaxy. This is known to be problematic for a prograde disc. We show that in contrast, interaction with a retrograde circumbinary disc is considerably more effective in shrinking the binary because there are no orbital resonances. The binary directly absorbs negative angular momentum from the circumbinary disc by capturing gas into a disc around the secondary black hole, or discs around both holes if the binary mass ratio is close to unity. In many cases the binary orbit becomes eccentric, shortening the pericentre distance as the eccentricity grows. In all cases the binary coalesces once it has absorbed the angular momentum of a gas mass comparable to that of the secondary black hole. Importantly, this conclusion is unaffected even if the gas inflow rate through the disc is formally super–Eddington for either hole. The coalescence timescale is therefore always $latex \sim M_2/\dot M$, where $latex M_2$ is the secondary black hole mass and $latex \dot M$ the inflow rate through the circumbinary disc.

by Clausen, Drew and Eracleous, Michael
19 pages, 9 figures, accepted for publication in The Astrophysical Journal

We calculate the emission line spectrum produced by the debris released when a white dwarf (WD) is tidally disrupted by an intermediate-mass black hole (IMBH; $latex M\sim 10^{2}-10^{5}\msun$) and we explore the possibility of using the emission lines to identify such events and constrain the properties of the IMBH. To this end, we adopt and adapt the techniques developed by Strubbe & Quataert to study the optical emission lines produced when a main sequence (MS) star is tidally disrupted by a supermassive black hole. WDs are tidally disrupted outside of the event horizon of a $latex < 10^{5}\msun$ black hole, which makes these tidal disruption events good signposts of IMBHs. We focus on the optical and UV emission lines produced when the accretion flare photoionizes the stream of debris that remains unbound during the disruption. We find that the spectrum is dominated by lines due to ions of C and O, the strongest of which are \ion{C}{4} $latex \lambda$1549 at early times and [\ion{O}{3}] $latex \lambda$5007 at later times. Furthermore, we model the profile of the emission lines in the [\ion{O}{3}] $latex \lambda\lambda$4959, 5007 doublet and find that it is highly asymmetric with velocity widths of up to $latex \sim 2500 \rm{\;km\;s^{-1}}$, depending on the properties of the WD-IMBH system and the orientation of the observer. Finally, we compare the models with observations of X-ray flares and optical emission lines in the cores of globular clusters and propose how future observations can test if these features are due to a WD that has been tidally disrupted by an IMBH.

by Schnittman, Jeremy D.
10 pages, 1 figure, submitted to Class. Quantum Grav. special issue: proceedings of 8th LISA Symposium

During the final moments of a binary black hole (BH) merger, the gravitational wave (GW) luminosity of the system is greater than the combined electromagnetic output of the entire observable universe. However, the extremely weak coupling between GWs and ordinary matter makes these waves very difficult to detect directly. Fortunately, the inspiraling BH system will interact strongly–on a purely Newtonian level–with any surrounding material in the host galaxy, and this matter can in turn produce unique electromagnetic (EM) signals detectable at Earth. By identifying EM counterparts to GW sources, we will be able to study the host environments of the merging BHs, in turn greatly expanding the scientific yield of a mission like LISA.

by Bogdanovic, Tamara and Bode, Tanja and Haas, Roland and Laguna, Pablo and Shoemaker, Deirdre
15 pages, 3 figures, submitted to Class. Quantum Grav. for proceedings of 8th LISA Symposium

What are the properties of accretion flows in the vicinity of coalescing supermassive black holes (SBHs)? The answer to this question has direct implications for the feasibility of coincident detections of electromagnetic (EM) and gravitational wave (GW) signals from coalescences. Such detections are considered to be the next observational grand challenge that will enable testing general relativity in the strong, nonlinear regime and improve our understanding of evolution and growth of these massive compact objects. In this paper we review the properties of the environment of coalescing binaries in the context of the circumbinary disk and hot, radiatively inefficient accretion flow models and use them to mark the extent of the parameter space spanned by this problem. We report the results from an initial, general relativistic, hydrodynamical study of the inspiral and merger of equal-mass, spinning black holes, motivated by the latter scenario. We find that correlated EM+GW oscillations can arise during the inspiral phase followed by the gradual rise and subsequent drop-off in the light curve at the time of coalescence. While there are indications that the latter EM signature is a more robust one, a detection of either signal coincidentally with GWs would be a convincing evidence for an impending SBH binary coalescence. The observability of an EM counterpart in the hot accretion flow scenario depends on the details of a model. In the case of the most massive binaries observable by the Laser Interferometer Space Antenna, upper limits on luminosity imply that they may be identified by EM searches out to z~0.1-1. However, given the radiatively inefficient nature of the gas flow, we speculate that a majority of massive binaries may appear as low luminosity AGN in the local universe.

by Ferreira, Barbara T.
PhD thesis, 199 pages, 32 figures. For higher quality images, please contact the author

This thesis investigates phenomena occurring in black-hole accretion discs which are likely to induce high-frequency quasi-periodic variability. Two classes of pseudo-relativistic theoretical models are studied. The first is based on the stability of transonic accretion flows and its connection to a disc instability that takes the form of propagating waves (viscous overstability). The second class of models looks at accretion-disc oscillations which are trapped due to the non-monotonic variation of the epicyclic frequency in relativistic flows. In particular, it focuses on inertial waves trapped below the maximum of the epicyclic frequency which are excited in deformed, warped or eccentric, discs. The influence of a transonic background on the propagation of these inertial modes is also investigated.

by Blecha, Laura and Cox, Thomas J. and Loeb, Abraham and Hernquist, Lars
29 pages, 18 figures. Submitted to MNRAS

Gravitational-wave (GW) recoil of merging supermassive black holes (SMBHs) may influence the co-evolution of SMBHs and their host galaxies. We examine this possibility using SPH/N-body simulations of gaseous galaxy mergers in which the merged BH receives a recoil kick. This enables us to follow recoiling BHs in self-consistent, evolving merger remnants. In contrast to recent studies on similar topics, we conduct a large parameter study, generating a suite of over 200 simulations with more than 60 merger models and a range of recoil velocities (vk). Our main results are as follows. (1) BHs kicked at nearly the central escape speed (vesc) may oscillate on large orbits for up to a Hubble time, but in gas-rich mergers, BHs kicked with up to ~ 0.7 vesc may be confined to the central few kpc of the galaxy, owing to gas drag and steep central potentials. (2) vesc in gas-rich mergers may increase rapidly during final coalescence, in which case trajectories may depend on the timing of the BH merger relative to the formation of the potential well. (3) Recoil events generally reduce the lifetimes of bright active galactic nuclei (AGN), but may actually extend AGN lifetimes at lower luminosities. (4) Kinematically-offset AGN (v > 800 km s^-1) may be observable for up to ~ 10 Myr either immediately after the recoil or during pericentric passages through a gas-rich remnant. (5) Spatially-offset AGN (R > 1 kpc) generally have low luminosities and lifetimes of ~ 1 – 100 Myr. (6) Rapidly-recoiling BHs may be up to ~ 5 times less massive than their stationary counterparts. This lowers the normalization of the M-sigma relation and contributes to both intrinsic and overall scatter. (7) Finally, the displacement of AGN feedback after a recoil event enhances central star formation rates, thereby extending the starburst phase of the merger and creating a denser stellar cusp. [Abridged.]

by Lusso, E. and Ciotti, L.
Accepted for publication in A&A, 12 pages, 5 figures

By means of a one-zone evolutionary model we study the co-evolution of supermassive black holes and their host galaxies, as a function of the accretion radiative efficiency, dark matter content and cosmological infall of gas. In particular, the radiation feedback is computed by using the self-regulated Bondi accretion. The models are characterized by strong oscillations when the galaxy is in the AGN state with a high accretion luminosity. We found that these one-zone models are able to reproduce two important phases of galaxy evolution, namely an obscured-cold phase when the bulk of star formation and black hole accretion occur, and the following quiescent hot phase in which accretion remains highly sub-Eddington. A Compton-thick phase is also found in almost all models, associated with the cold phase. An exploration of the parameter space reveals that the best agreement with the present day Magorrian relation is obtained, indipendently of the dark matter halo mass, for galaxies with low-mass seed black hole, and the accretion radiative efficiency ~0.1.

by Hayasaki, Kimitake
12 pages, 2 figures, submitted to ApJL

We study an accretion flow during the gravitational-wave driven evolution of binary massive black holes. After the binary orbit decays due to interacting with a massive circumbinary disk, the binary is decoupled from the circumbinary disk because the orbital-decay timescale due to emission of gravitational wave becomes shorter than the viscous timescale evaluated at the inner edge of circumbinary disk. During the subsequent evolution, the accretion disk, which is truncated at the tidal radius because of the tidal torque, also shrinks as the orbital decay. Assuming that the disk mass changed by this process is all accreted, the whole region of the disk completely becomes radiatively inefficient when the semi-major axis is several hundred Schwarzschild radii. The disk temperature can become comparable with the virial temperature there in spite of a low disk luminosity. The prompt high-energy emission is hence expected long before black hole coalescence as well as the gravitational wave signals. Binary massive black holes finally merge without accretion disks.