Sub-parsec structure of binary supermassive black holes in active galactic nuclei

Long-term, multi-frequency monitoring of the radio fluxes of the four BL Lac objects 3C 120, OJ 287, 1308+326, and BL Lac is considered. Harmonic components of the flux variability on scales from one year to decades are determined. The observational data used were obtained at the Radio Astronomy Laboratory of the Crimean Astrophysical Observatory (Ukraine) and the University of Michigan Radio Astronomy Observatory (USA). These data are used to construct kinematic models for active galactic nuclei using values for the orbital and precessional periods of binary systems consisting of supermassive black holes. The derived speeds of the companions in their orbits lie in the narrow range 3000–4000 km/s. The orbital radii for the binary supermassive black holes also lie in a narrow range, 10¹⁷–10¹⁸ cm, providing evidence that observed prominent examples of active galactic nuclei are fairly close binary systems. The parameters of the mediumin which the components of the binary systems are moving are estimated, as well as the rates at which the systems are losing orbital angular momentum and their lifetimes to coalescence.     

Quasi-simultaneous VLBI and RATAN-600 observations of active galactic nuclei

VLBI observations of several quasars and BL Lacertae objects were carried out at 1.66 GHz in November–December 1999 using six antennas (Medvezh'i Ozera, Svetloe, Pushchino, Noto, HartRAO, and Shanghai). Maps of six sources (0420+022, 0420-014, 1308+326, 1345+125, 1803+784, and DA 193) obtained with milliarcsecond resolution are presented and discussed, together with their broad-band (1–22 GHz) spectra obtained on the RATAN-600 radio telescope at epochs close to those of the VLBI observations. Comparison of the VLBI maps with maps of these sources obtained on standard VLBI networks and with the RATAN-600 quasisimultaneous total-flux measurements indicates the reliability of the results obtained on this Low Frequency VLBI Network and the good efficiency of this network.     

Non-uniform model for the synchrotron radiation of Sgr A* and other low-luminosity galactic nuclei

A model for non-uniform source of synchrotron radiation with a power-law radial distribution of the magnetic field and relativistic-electron density along one-or two-sided jets is described. Non-relativistic jets with both constant cross sections (collimated jets) and cross sections that are proportional to distance (conical jets) are considered. Formulas that can be used to determine source parameters from the spectral index, source size, and index of the relativistic-electron energy spectrum based on multi-frequency observations are obtained. In the case of a conical jet, these formulas coincide with the analogous formulas for a spherical source obtained by A.P. Marscher. Relations that can be used to estimate the magnetic-field strength from the brightness temperature in the self-absorbed region are also obtained. As examples, the inhomogeneous-source model is applied to the compact radio sources at the centers of the Milky Way, Sgr A*, and the low-luminosity galactic nuclei M81* and M87*, which are associated with supermassive black holes. The inner radius of the radiation region is determined. For Sgr A*, this distance turns out to be comparable to the gravitational radius, smaller than the radius of the last stable orbit for a non-rotating black hole, and consistent with the radius of the last stable orbit expected for a rotating black hole. The inner radii in M81* and M87* are ～15 R _S , an order of magnitude larger than for Sgr A*. Estimates of the magnetic field at the inner radius are 400 G for M81*, 0.65–5.3 kG for Sgr A*, and 20–100 kG for M87*. These magnetic fields and the Blandford-Znajek model for the radiation of a rotating black hole are used to estimate the rotational speed of the black holes, which are in agreement with the characteristic variability time scales for these three objects. However, the accuracy of these estimates is modest, and is limited primarily by the accuracy of interferometric measurements at millimeter wavelengths.     

Polarimetric differences between Schwarzschild and Kerr black holes in active galactic nuclei

If the linear polarization of the optical emission of active galactic nuclei (AGNs) arises in magnetized accretion disk (the Milne problem), the degree of polarization should depend strongly on the spin of the central black hole. For the same black hole luminosities and masses, the polarization is substantially higher for rotating Kerr than for non-rotating Schwarzschild black holes. Statistically, this means that the majority of AGNs displaying appreciable linear polarization should have Kerr black holes. The spin dependence of the polarization is due to the fact that the radius of the innermost stable circular orbit r _isco depends on the spin—this radius is three gravitational radii for a Schwarzschild black hole, and a factor of six smaller for a rapidly rotating black hole. This means that the magnetic field in the region of emergence of the optical emission, which decreases with distance from r _isco , is higher for a non-rotating than for a rapidly rotating black hole. This higher magnetic field gives rise to strong Faraday depolarization, explaining the effect considered here.     

Study of the physical conditions in active galactic nuclei. A method for estimating the physical parameters of radio sources

A new method for estimating the physical parameters of active galactic nuclei involving the analysis of observations of the compact radio sources in them is proposed. The method is based on an inhomogeneous model for a synchrotron radio source. Theoretical spectra of the radio sources are obtained via numerical solution of the transfer equation. Due to the paucity of observational data, only interval estimates of the magnetic field strength and the energy densities of the magnetic field and relativistic particles can be obtained. A mechanism for the formation of flat radio spectra is proposed.     

The possibility of observing resonance gamma-ray absorption in the spectra of active galactic nuclei

The Geant 4.8.1 program package has been used to carry out Monte Carlo simulations of resonance features arising in gamma-ray spectra due to the passage of the photons through a large amount of neutral, stationary material under conditions corresponding to those in active galactic nuclei (AGNs). The dependences of the depth and shape of resonance features on the optical depth and geometry of the absorbing material, as well as parameters of the initial spectrum, are analyzed. Special attention is paid to the dependence of the spectrum on the viewing angle for the observations relative to the symmetry axis of a beam of gamma-rays, for viewing angles from fractions of a degree to several tens of degrees. It is shown that Δ-resonance absorption with appreciable depths (>10%) can appear in the gamma-ray spectra of AGNs only if observations are made at small angles (<0.2°) to the beam axis.     

Linear polarization of the radiation from active galactic nuclei and the redshift dependence of their main parameters

We consider the observed continuum linear polarization of extragalactic objects with various redshifts z, most of which have degrees of polarization p ≤ 10%. We propose that this polarization is due to multiple scattering of the radiation in magnetized accretion disks around the Active Galactic Nuclei (AGN; the Milne problem in an optically thick atmosphere). The structure of the accretion disks and the polarization of the emergent radiation depend on the main parameters of the AGN—the mass of the central body M _BH , accretion rate $ \dot M $ \dot M , magnetic field at the black-hole event horizon B _H , angular momentum a _*, and the explicit form of the magnetic-field distribution in the accretion disk. Theoretical expressions for the degree of polarization are averaged over all angles of the disks to the line of sight, and the resulting formula compared with the mean observed polarizations in redshift intervals Δz = 0.25. The dependence of the observed degree of polarization and the main parameters on the redshift z is derived. The degrees of polarization of 305 objects from the catalog of Hutsemekers et al. with redshifts from zero to z = 2.25 are used for the analysis.     

Study of the physical conditions in active galactic nuclei. Physical conditions in the cores of two nearby radio galaxies

We have carried out a search for compact radio sources in the cores of 16 nearby radio galaxies. We detected compact components in four radio galaxies, and found upper limits for the flux density in compact components in ten radio galaxies. VLBI observations enabled the detection of a turnover in the spectra of the two nearby radio galaxies 3C 111 and 3C 465. Using a method based on an inhomogeneous model for a synchrotron source, we estimate the magnetic-field strength and the energy densities in the magnetic field and relativistic electrons in the cores of these radio galaxies. Strong inhomogeneity in the distribution of the magnetic fields in the cores of 3C 111 and 3C 465 is implied by our analysis. The magnetic-field strengths in the central regions of these galactic nuclei, on scales of ～0.1 pc, exceed the mean strength by four to five orders of magnitude, and lie in the range 10² G < H < 10⁴ G.     

Black holes in close binary systems and galactic nuclei

After 50 years of observational studies of black holes, great progress has been achieved in this branch of astrophysics. Several dozen stellar-mass black holes have been discovered in X-ray binaries, and several hundred supermassive black holes in galactic nuclei. The remarkable recent discovery of gravitational waves from merging black holes in a binary system by LIGO marks the beginning of a new stage in black-hole research. It is quite possible that gravitational-wave studies will provide definitive evidence for the existence of event horizons in black holes in the near future. On the other hand, the development of methods for space and ground-based radio-interferometry observations provides hope that it will be possible to obtain images of “shadows” of supermassive black holes in galactic nuclei, and to observe directly processes occurring in the vicinities of the event horizons of supermassive black holes. This is important for tests of general relativity in extremely strong gravitational fields.     

A model for the explosive synthesis of p nuclei

A model for the synthesis of p nuclei during a supernova explosion of a 25 M _⊙ star is presented. Its main distinguishing feature is the requirement that the yield of s nuclei after the explosion fully correspond to the observed solar-system abundances. A fully self-consistent system of kinetic equations including (n, γ), (p, γ), (α, γ), (p, n), (α, n), and (α, p) reactions and their reverse reactions was solved for the mass fractions of various nuclei, taking into account the dynamics of the propagation of the supernova shock. The absolute abundances of the p nuclei are calculated, and the role of helium layers in the supernova in the synthesis of r nuclei is analyzed. The calculated abundances of p nuclei are in overall good agreement with observations. This makes it possible to synthesize ¹¹³In, ¹³⁸La, and some other problematic isotopes, although the problem of synthesizing molybdenum and ruthenium remains.     

A possible relation between the masses of black holes in galactic nuclei and the parameters of the host galaxies

Data on about forty virialized galaxy clusters with bright central galaxies, for which both the galactic velocity dispersion (?? _gal) and the stellar velocity dispersion in the brightest galaxies (??*) are measured, have been used to obtain several approximate relations between ?? _gal, ??*, the absolute B magnitude of the brightest central galaxyM _B ^BCG , and the mass of the central massive black holeM _BH: $\begin{gathered} \log \sigma _* = (0.12 \pm 0.14)\log \sigma _{gal} + (2.1 \pm 0.4), \hfill \\ \log \sigma _* = - (0.15 \pm 0.02)M_B^{BCG} + (0.85 \pm 0.5), \hfill \\ \log M_{BH} = 0.51\log \sigma _{gal} + 7.28. \hfill \\ \end{gathered} $ . These relations can be used to derive crude estimates ofMBH in the nuclei of the brightest galaxies using the parameters of the both host galaxies and the host galaxy clusters. The last relation above confirms earlier suggestions of a quadratic relation between the masses of the coronas of the host systems and the masses their central objects: M _hg ^halo ?? M _cent ² . The relations obtained are consistent with the common evolution of subsystems with different scales and masses formed in the process of hierarchical clustering.     

A unified theory for the formation of galactic structures

A new theory for the formation of the main structures of galaxies is proposed: these structures are viewed as low-frequency normal modes in disks consisting of precessing stellar orbits. Mathematically, the theory is based on an integral equation in the form of a classical eigenvalue problem, with the eigenvalues being equal to the angular velocities Ω_p of the modes. Analysis of the general properties of the master integral equation (without finding concrete solutions) shows that it admits two types of solutions: barlike and spiral. The numerical algorithms are discussed and particular solutions of the integral equation are presented. If resonance interaction can be neglected, the bar mode represents a neutral perturbation of the disk. This mode can be amplified by the effect of the long-range gravitational field of the mode on stars located in the vicinity of the corotation and outer-Lindblad resonances. Spiral perturbations are waves with zero total angular momentum, and spiral modes are excited at the inner-Lindblad resonance. The approach proposed is compared to currently accepted mechanisms for the formation of galactic structures. In particular, Toomre's application of the swing amplification mechanism to explain the formation of global modes is critically discussed. In addition, we show that it is not correct to simulate the real stellar velocity dispersion in a galaxy using softened gravity.     

Acceleration of spatial motions of stars by close-binary supermassive black holes in galactic nuclei

The conditions for the acceleration of the spatial motions of stars by close-binary supermassive black holes (SMBHs) in galactic nuclei are analyzed in order to derive the velocity distribution for stars ejected from galaxies by such black holes. A close binary system consisting of two SMBHs in circular orbits was subject to a spherically symmetrical “barrage” of solar-mass stars with various initial velocities. The SMBHs were treated as point objects with Newtonian gravitational fields. Models with binary component-mass ratios of 1, 0.1, 0.01, and 0.001 were studied. The results demonstrate the possibility of accelerating neutron stars, stellar-mass black holes, and degenerate dwarfs to velocities comparable to the relative orbital velocities of the binary-SMBH components. In the stage when the binary components are merging due to the action of gravitational-wave radiation, this velocity can approach the speed of light. The most massive binary black-holes (M ? 10⁹M_⊙) can also accelerate main-sequence stars with solar or subsolar masses to such velocities.     

High-velocity stars as a result of encounters between stars and massive binary black holes in galactic nuclei

Numerical simulations of the motions of stars in the gravitational fields of binary black holes with various component mass ratios have been carried out. Two models are considered: (1) the two-body problem with two fixed centers; (2) the general three-body problem. The first model is applicable only over short times Δt ? T, where T is the period of the binary system. The second model is applicable at all times except for during close encounters of stars with one of the binary components, r ≤ 0.00002 pc, where r is the distance from the star to the nearer black hole. In very close passages, relativistic corrections must be taken into account. Estimates of the probability of formation of high-velocity stars as a result of such interactions are obtained. It is shown that this mechanism is not suitable for the nucleus of our Galaxy due to the probable absence of a second massive black hole in the central region of the Galaxy.     

基于模式匹配法的明渠紊流涡结构分析

对涡结构进行识别是紊流涡动力学研究的难题,为了降低涡识别方法对流速测量空间分辨率的要求,消除阈值对涡识别结果可能产生的影响,提出了基于二维平面流场的涡结构模式匹配方法。应用该方法对明渠紊流中涡结构的密度、涡量、半径和强度等特征参数和涡动力学特征进行了分析。结果表明,涡结构的密度和涡量沿水深逐渐减小,涡半径沿水深逐渐增大,涡结构的环量在近壁区迅速增大而随后基本维持稳定;在相同水深处,顺向涡密度大于逆向涡且具有更强的旋转运动;顺向涡的平均流场具有典型的发夹涡群特征,部分逆向涡与顺向涡在空间上具有相关关系。     

Chemical composition of stars in the galactic halo

The chemical compositions of the atmospheres of six metal-poor stars are analyzed. Spectra with signal-to-noise ratios of no less than 100 and a resolution of R≈17 000 were obtained using the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The abundances of Li, O, α-process elements (Mg, Si, Ca, Ti), Na, K, Sc, iron-peak elements (Cr, Mn, Fe, Ni, Cu, Zn), and s-process elements (Y, Ba) are derived. The star G251-54 ([Fe/H]=?1.55, T _eff=5541 K, logg=3.58) is deficient in some elements compared to both stars with similar metallicities and the Sun. The atmosphere of G251-54 has the following elemental abundances relative to iron: [O/Fe]=+0.47, [α/Fe]≈?0.3, [Na/Fe]=?0.60, [Sc/Fe]=?0.57, [Cr, Ni, Fe]≈0, [Zn/Fe]=+0.16, [Cu/Fe]=?0.66, [Y/Fe]=?0.70, and [Ba/Fe]=?1.35. The remaining five stars have metallicities in the range ?1.6<[Fe/H]相似文献   

An active galactic nucleus as a massive coaxial pulsar: A massive central object with a Keplerian accretion disk and twisted MHD jet

The similarities in certain empirical relations derived for active galactic nuclei (AGN) and radio pulsars support the idea that the energy release in the jets of AGN is electrodynamical, associated with the presence of a massive central object (MCO) and circumnuclear, magnetized Keplerian disk. For example, in both types of object, the radio luminosities are proportional to characteristic time scales—for precession for AGN or rotation for pulsars—to the 0.8 power. In this approach to studying the nature of the activity, the $\dot P - P$ relation for pulsars can be considered analogous to the P _r-M _MCO relation for AGN, which has the form P _5GHz (W/Hz)=2.73 log(M _MCO/M _⊙)?2.87.     

A low-mode dynamo with meridional circulation and dipolar symmetry of the magnetic field

A dynamic system for the Parker dynamo including meridional circulation that is applicable to astrophysical objects is constructed. The meridional circulation is able to control the regimes for the generation of magnetic fields. If the meridional flows are weak, regimes with steady oscillations, dynamo bursts, fluctuations, and chaotic components are possible. When the meridional circulation is strengthened, the range of dynamo numbers required for fluctuations and dynamo bursts is reduced, and gradually vanishes; at the same time, the range required for oscillations is increased and raised to higher dynamo numbers. The latitude-time distributions of the toroidal and poloidal magnetic fields for steady oscillations are presented.     

主动围压下花岗岩动态力学特性与本构模型研究

为了研究花岗岩在不同围压、不同应变率下的动力学特性与本构行为,利用改进的分离式霍普金森压杆（SHPB）,对其进行了试验测试,并将损伤统计理论引入鲍埃丁模型,对所得到的本构模型与试验数据进行拟合分析,进而探讨模型中各参数对应力-应变曲线的影响。结果表明:围压和应变率都能提升花岗岩的抗压强度,两者均与试样动态强度增长因子呈正相关,且围压的存在提高了岩石的塑性特性,应力-应变曲线上出现塑性屈服平台;主动围压下,试样的弹性模量有一定的提高,但总体上未见明显的率效应和围压效应;本文所构建的模型预测结果与不同围压下试样的应力-应变曲线均有较高的吻合度,可为相关工程设计与施工提供一定参考。