Optical variability of the active galactic nucleus 1E 0754.6+3928 and reverberation-based mass estimate for the central black hole

We analyze the spectroscopic and photometric observations of the active galactic nucleus (AGN) 1E 0754.6+3928 performed at Crimean Astrophysical Observatory in 1998–2004. Based on formal spectroscopic criteria for the optical wavelength range, we can classify this object as a narrow-line Seyfert 1 (NLS1) galaxy. Over the period of its observations, the AGN 1E 0754.6+3928 showed a very low rms flux variability amplitude in both continuum and Hβ (～3%). The Hβ time lag relative to the continuum has been found from the cross-correlation function centroid to be τ _cent = 112 _?67 ⁺²¹⁵ days. This result and the Hβ line dispersion estimate have allowed us to determine the central black hole mass by the reverberation method: 1.05 × 10⁸ M _⊙. The position of 1E 0754.6+3928 in the black hole mass-luminosity diagram agrees with the positions of other NLS1 galaxies and can be explained by an enhanced mass accretion rate in the central source.     

The massive black hole at the galactic center

At the dynamic center of the Milky Way high spatial resolution, near-infrared imaging and spectroscopy have made it possible in the last few years to measure stellar velocities down to separations of less than five light days from the compact radio source SgrA* (in the constellation Sagittarius). These measurements make a compelling case for the presence of a compact, central dark mass of 2.6 × 10⁶ solar masses. Simple physical considerations show that this dark mass cannot consist of a stable cluster of stars, stellar remnants, substellar condensations or a degenerate gas of elementary particles. Energy equipartition requires that at least 105 solar masses must be associated with SgrA* itself and is enclosed within less than 8 light minutes (equivalent to 15 Schwarzschild radii of a million solar mass black hole). If one accepts these arguments it is hard to escape the conclusions that there must be a massive black hole at the core of the Milky Way.     

Formation and evolution of galactic nuclei, black holes

The formation and evolution of galactic nuclei is discussed, mainly from the viewpoint of the dense gaseous and stellar system in the central portions of young galactic nuclei. A review is given on models of galactic nuclei containing one or more black holes and how the standard historic Rees-Spitzer-Stone scenario of isolated nuclei formation has been changed by the cosmological structure formation scenario. The dynamical interaction of two or more black holes with their stellar environment and their merging possibilities is presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stellar velocity dispersion and mass estimation for galactic disks

Available velocity dispersion estimates for the old stellar population of galactic disks at galactocentric distances r?2L (where L is the photometric radial scale length of the disk) are used to determine the threshold local surface density of disks that are stable against gravitational perturbations. The mass of the disk M_d calculated under the assumption of its marginal stability is compared with the total mass M_t and luminosity L _B of the galaxy within r=4L. We corroborate the conclusion that a substantial fraction of the mass in galaxies is probably located in their dark halos. The ratio of the radial velocity dispersion to the circular velocity increases along the sequence of galactic color indices and decreases from the early to late morphological types. For most of the galaxies with large color indices (B–V)₀>0.75, which mainly belong to the S0 type, the velocity dispersion exceeds significantly the threshold value required for the disk to be stable. The reverse situation is true for spiral galaxies: the ratios M_d/L_B for these agree well with those expected for evolving stellar systems with the observed color indices. This suggests that the disks of spiral galaxies underwent no significant dynamical heating after they reached a quasi-equilibrium stable state.     

Rotation curves and mass distributions in flat self-gravitating disks

We have developed a new approach that allows the surface-density distribution in flat finite-radius galactic disk models to be reconstructed from an arbitrary smooth angular-velocity distribution. Upper limits for the disk mass and radius are shown to exist for a wide class of rotation curves analytically extended to the unseen part of the disk.     

Unipolar induction of a magnetized accretion disk around a black hole

The structure and magnitude of the electromagnetic field produced by a rotating accretion disk around a black hole were determined. The disk matter is assumed to be a magnetized plasma with a frozenin poloidal magnetic field. The vacuum approximation is used outside the disk.     

Angular momentum transport by thermal emission in black hole accretion disks

We calculate the amount of angular momentum that thermal photons carry out of a viscous black hole accretion disk, due to the strong Doppler shift imparted to them by the high orbital velocity of the radiating disk material. While thermal emission can not drive accretion on its own, we show that along with disk heating it does nonetheless result in a loss of specific angular momentum, thereby contributing to an otherwise viscosity‐driven accretion flow. In particular, we show that the fraction of the angular momentum that is lost to thermal emission at a radius r in a standard, multi‐color disk is ∼0.4r_s/r, where r_s is the Schwarzschild radius of the black hole. We briefly highlight the key similarties between this effect and the closely related Poynting‐Robertson effect (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observational evidence for massive black holes in the centers of active galaxies

Naturally occurring water vapor maser emission at 1.35 cm wavelength provides an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Because of the exceptionally fine angular resolution, 200 microarcseconds, obtainable with very long baseline interferometry, accompanied by high spectral resolution, <0.1 km s^-1, the dynamics and structures of these disks can be probed with exceptional clarity. The data on the galaxy NGC 4258 are discussed here in detail. The mass of the black hole binding the accretion disk is 3·9 × 10⁷ M_⊙. Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly measured with VLBI over a period of a few years. These measurements also allow the distance from the earth to the black hole to be estimated to an accuracy of 4 per cent. The status of the search for other maser/black hole candidates is also discussed.     

Energy accumulating accretion disk orbiting a Kerr black hole

A geometrically thin, energy accumulating $aL-disk is suggested which orbits a Kerr black hole. With increasing internal forces, the “standard” disks develop into energy accumulating disks. These accumulating disks are geometrically thin as long as their internal forces remain below a certain bound, allowing nearly geodesic orbits.     

Neutrino radiation from a collapsing galactic nucleus

We discuss the possible observational manifestation of the formation of massive black holes in galactic nuclei in the form of an intense high-energy neutrino flux. A short-lived (≤10 yr) hidden neutrino source results from the natural dynamicalal evolution of a central star cluster in the galactic nucleus before its gravitational collapse. The central star cluster at the final evolutionary stage consists of degenerate compact stars (neutron stars and stellar-mass black holes) and is embedded in a massive gaseous envelope produced by destructive collisions of normal stars. Multiple fireballs from frequent collisions of neutron stars give rise to a tenuous quasi-stationary cavity in the central part of the massive envelope. The cavity is filled with shock waves on which an effective cosmic-ray acceleration takes place. Allthe accelerated particles, except the secondary high-energy neutrinos, are absorbed in the dense envelope. The neutrino signal that carries information on the dynamicals of the collapsing galactic nucleus can be recorded by a neutrino detector with an effective area S∼1 km².     

Are 3C 120 and Other Active Galactic Nuclei Overweight Microquasars?

The appearance of superluminal radio knots follows drops in the X-ray flux in the FR1 radio galaxy 3C 120 and possibly the FR2 source 3C 111. This corresponds in a very general way to the behavior of the X-ray binary GRS 1915 + 105, but the light curves of the microquasar are much richer in detail. Starting in 2003.7, the character of the radio and X-ray light curves of 3C 120 changed, perhaps signaling a new stage of activity. I discuss here what one might expect when a microquasar is scaled up to AGN dimensions, and compare this with what we see in 3C 120. There is a mismatch between expectations and observations.     

Gas flow in a two component galactic disk

Numerical simulations of two-component (stars + gas) self-gravitating galactic disks show that the interstellar gas can significantly affect the dynamical evolution of the disk even if its mass fraction (relative to the total galaxy mass) is as low as several percent. Aided by efficient energy dissipation, the gas becomes gravitationally unstable onlocal scale and forms massive clumps. Gravitational scattering of stars by these clumps leads to suppression of bar instability usually seen in heavy stellar disks. In this case, gas inflow towards the galactic center is driven by dynamical friction which gas clumps suffer instead of bar forcing.     

Runaway instabilities in geometrically thin accretion disks around Kerr black holes

The stability of the innermost disk region orbiting a Kerr black hole is investigated for geometrically thin accretion disks. The infalling matter transports mass and angular momentum into the Kerr hole. This affects the inner disk boundary and leads to runaway instabilities in some cases.     

Dynamical properties of a centrifugally driven outflow from a disc near a black hole

The inclination of the field lines at the surface of the disc plays a crucial role on the nature of the magnetically driven flows. For the non-relativistic case, a centrifugally driven outflow of matter from the disc is possible, if the poloidal component of the magnetic field makes an angle of less than a critical 60° with the disc surface (Blandford and Payne 1982). We investigate the dynamical properties of the magnetically driven flows from the disc near a non-rotating black hole and find that the critical angle becomes larger than 60° when the flows start from the region near the black hole.     

Why do we see so few black holes in massive binaries?

We offer a simple explanation for the small number of black holes observed in pairs with massive stars. In detached massive binaries, spherically symmetric accretion takes place. This accretion could result in effective energy release in the hard band only if the equipartition of the gravitational and magnetic energy of plasma is established (Shvartsman’s theorem). However, we show that due to the magnetic exhaust effect this equilibrium is virtually never established for the actual magnetic fields observed on massive stars: Shvartsman’s theorem does not work. As a result, it is virtually impossible to detect black holes in detached massive binaries by currently available means (mainly, through X-ray observations).     

Relationship between the thickness of stellar disks and the relative mass of a dark galactic halo

We analyze the R-and K _s-band photometric profiles for two independent samples of edge-on galaxies. The thickness of old stellar disks is shown to be related to the relative masses of the spherical and disk components of galaxies. The radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with increasing total mass-to-light ratio of the galaxies, which reflects the relative contribution of the dark halo to the total mass, and with decreasing central deprojected disk brightness (density). Our results are in good agreement with numerical models of collisionless disks that evolved to a marginally stable equilibrium state. This suggests that, in most galaxies, the vertical stellar-velocity dispersion, on which the equilibrium-disk thickness depends, is close to a minimum value that ensures disk stability. The thinnest edge-on disks appear to be low-brightness galaxies in which the dark-halo mass far exceeds the stellar-disk mass.     

General relativistic MHD simulations of black hole accretion disks and jets

Accretion disks orbiting black holes power high-energy systems such as X-ray binaries and Active Galactic Nuclei. Observations are providing increasingly detailed quantitative information about such systems. This data has been interpreted using standard toy-models that rely on simplifying assumptions such as regular flow geometry and a parameterized stress. Global numerical simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions and, in principle, lead to a genuinely predictive theory. In recent years we have developed a fully three-dimensional general relativistic magnetohydrodynamic simulation code that evolves time-dependent inflows into Kerr black holes. Although the resulting global simulations of black hole accretion are still somewhat simplified, they have brought to light a number of interesting results. These include the formation of electro-magnetically dominated jets powered by the black hole’s rotation, and the presence of strong stresses in the plunging region of the accretion flow. The observational consequences of these features are gradually being examined. Increasing computer power and increasingly sophisticated algorithms promise a bright future for the computational approach to black hole accretion.     

Nodal and periastron precession of inclined orbits in the field of a rotating black hole

The inclination of low-eccentricity orbits is shown to significantly affect orbital parameters, in particular, the Keplerian, nodal precession, and periastron rotation frequencies, which are interpreted in terms of observable quantities. For the nodal precession and periastron rotation frequencies of low-eccentricity orbits in a Kerr field, we derive a Taylor expansion in terms of the Kerr parameter at arbitrary orbital inclinations to the black-hole spin axis and at arbitrary radial coordinates. The particle radius, energy, and angular momentum in the marginally stable circular orbits are calculated as functions of the Kerr parameter j and parameter s in the form of Taylor expansions in terms of j to within O[j ⁶]. By analyzing our numerical results, we give compact approximation formulas for the nodal precession frequency of the marginally stable circular orbits at various s in the entire range of the Kerr parameter.     

Magnetic field and radius of the innermost stable circular orbit near super‐massive black holes in active galactic nuclei

Magnetic fields in an accretion disk around the central black hole can modify the size of the innermost stable circular orbit (ISCO) and can produce a difference to the classical Novikov‐Thorne radius. We estimated the ISCO magnetic field strength from the polarimetric observations of the accretion‐disk radiation. This estimate is obtained taking into account the effect of the Faraday rotation of the polarization plane at the distance of the mean free path of photons between successive electron scattering events. We present the new method for estimating the ISCO radius in the accretion disk, i.e. in the nearest vicinity of a central black hole. Our estimates confirmed the Frolov, Shoom & Tzounis (2014) and Ranea‐Sandoval & Garcia (2015) conclusion that the magnetic field in the accretion disk decreases the size of the innermost stable circular orbit. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The tidal disruption rate in dense galactic cusps containing a supermassive binary black hole

We consider the problem of tidal disruption of stars in the centre of a galaxy containing a supermassive binary black hole with unequal masses. We assume that over the separation distance between the black holes, the gravitational potential is dominated by the more massive primary black hole. Also, we assume that the number density of stars is concentric with the primary black hole and has a power-law cusp. We show that the bulk of stars with a small angular-momentum component normal to the black hole binary orbit can reach a small value of total angular momentum through secular evolution in the gravitational field of the binary, and hence they can be tidally disrupted by the larger black hole. This effect is analogous to the so-called Kozai effect well known in celestial mechanics. We develop an analytical theory for the secular evolution of the stellar orbits and calculate the rate of tidal disruption. We compare our analytical theory with a simple numerical model and find very good agreement.
Our results show that for a primary black hole mass of  ∼10⁶–10⁷ M_⊙  , the black hole mass-ratio   q > 10⁻²  , cusp size ∼1 pc, the tidal disruption rate can be as large as  ∼10⁻²–1 M_⊙ yr⁻¹  . This is at least 10²–10⁴ times larger than estimated for the case of a single supermassive black hole. The duration of the phase of enhanced tidal disruption is determined by the dynamical-friction time-scale, and it is rather short: ∼10⁵ yr. The dependence of the tidal disruption rate on the mass ratio, and on the size of the cusp, is also discussed.