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

V. I. Slysh 《Astronomy Reports》2008,52(5):343-351

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.  相似文献   

The supermassive black hole at the center of our galaxy: Determination of its main physical parameters     

A. A. Kisselev  Yu. N. Gnedin  E. A. Grosheva  N. A. Shakht  D. L. Gorshanov  M. Yu. Piotrovich 《Astronomy Reports》2007,51(2):100-108

We have used two astrometric methods developed at the Main Astronomical Observatory of the Russian Academy of Sciences—the method of apparent-motion parameters (AMP) and a direct geometrical method (DGM)—to derive the orbit of the star S2 around the Galactic center, and thereby the mass of the supermassive black hole at the Galactic center. The AMP method, which is based on measurements of the curvature of a fairly short orbital arc, is efficient if observational data on the relative radial velocity are available. The mass of the supermassive black hole was also estimated using astrophysical methods, based on the empirical relation between the masses of the supermassive black holes at the centers of galaxies and quasars and the radio and X-ray luminosities of these regions. We estimate the magnetic-field strength near the event horizon of the supermassive black hole at the Galactic center using a synchrotron self-absorption model.  相似文献   

Spectral distribution of the polarized radiation from standard accretion disks in Active Galactic Nuclei: Observational analysis     

V. L. Afanas’ev  N. V. Borisov  Yu. N. Gnedin  S. D. Buliga  T. M. Natsvlishvili  M. Yu. Piotrovich 《Astronomy Reports》2014,58(10):725-732

Spectropolarimetric observations of a number of Active Galactic Nuclei obtained using the SCORPIO-2 aperture focal reducer installed on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences are used to estimate physical parameters of these objects. The measured polarization and its wavelength dependence are consistent with the expectations of a standard accretion-disk model taking into account the effect of Faraday depolarization over the mean free path of the emitted photons. Estimates of the magnetic field in the accretion disk near the innermost stable orbit and the spin of the accreting central black hole are obtained. It is concluded that supermassive black holes with standard accretion disks and equal magnetic and radiative pressures are primarily Kerr black holes.  相似文献   

Determination of the Spins of Supermassive Black Holes in FR I and FR II Radio Galaxies     

A. G. Mikhailov  Yu. N. Gnedin 《Astronomy Reports》2018,62(1):1-8

The spins of supermassive black holes in FR I and FR II radio galaxies are estimated using two models for the generation of the relativisitic jets, based on the Blandford–Znajek and Blandford–Payne mechanisms: the hybrid model of Meier and a flux-trapping model. The magnetic field at the event horizon is estimated assuming equipartition between the energy densities of the magnetic field and the accreting material. The magnetic field near the inner edge of the accretion disk is estimated assuming equipartition between the magnetic pressure and the radiation pressure, and also assuming proportionality between the magnetic field and the spin. In the case of FR I objects, both mechanisms for the generation of the jets (the hybrid model of Meier and a flux-trapping model) are efficient. For the FR II objects, equipartition between the energy densities of the magnetic field and the accretion flow facilitates stronger retrograde rotation of the supermassive black hole. Plots of spin versus mass suggest a predominantly chaotic character for the accretion in both types of radio galaxies.  相似文献   

The nature hypervelocity stars     

A. V. Tutukov  A. V. Fedorova 《Astronomy Reports》2009,53(9):839-849

We list and analyze the main currently known mechanisms for accelerating the space motions of stars. A high space velocity of a star can be a consequence of its formation in the early stages of the evolution of a massive galaxy, when it was spheroidal and non-stationary, so that stars were born with velocities close to the escape velocity for the galaxy. Another possibility is that the star arrived from another galaxy with a velocity that is high for our Galaxy. The decay of unstable close multiple stars or supernova explosions in close binaries can also provide velocities of up to several hundreds of km/s to main-sequence stars and velocities of up to ∼1000 km/s to degenerate stars, neutron stars, and stellar-mass black holes. The merger of components of a binary system containing two neutron stars or a neutron star and a black hole due to gravitational-wave radiation can accelerate the nascent black hole to a velocity∼1000 km/s. Hypervelocity relativistic stars can be born due to asymmetric neutrino ejection during a supernova explosion. Stars can be efficiently accelerated by single and binary supermassive black holes (with masses from several millions to several billions of solar masses) in the nuclei of galaxies. Thanks to their gravitational field and fast orbital motion (in the case of binary objects), supermassive black holes are able to accelerate even main-sequence stars to relativistic velocities.  相似文献   

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

N. A. Silant’ev  M. Yu. Piotrovich  Yu. N. Gnedin  T. M. Natsvlishvili 《Astronomy Reports》2011,55(8):683-688

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.  相似文献   

Determining the spins of supermassive black holes in Active Galactic Nuclei based on spectropolarimetric observations     

Yu. N. Gnedin  V. L. Afanasiev  N. V. Borisov  M. Yu. Piotrovich  T. M. Natsvlishvili  S. D. Buliga 《Astronomy Reports》2012,56(8):573-577

Strong constraints are obtained for the spins of supermassive black holes in a number of Active Galactic Nuclei. These estimates are based on spectropolarimetric data, obtained mainly on the 6-m telescope of the Special Astrophysical Observatory, as well as data on the kinetic power of relativistic jets. The magnetic fields at the innermost stable Keplerian orbit in the accretion disk and at the event horizon of the supermassive black hole are estimated. These data are used to place strong constraints on the spins of supermassive black holes in Active Galactic Nuclei.  相似文献   

The probability of forming hyperveloicty stars in the Galaxy     

Dremova  G. N.  Dremov  V. V.  Orlov  V. V.  Tutukov  A. V.  Shirokova  K. S. 《Astronomy Reports》2015,59(11):1019-1035

The probability of forming a Galactic hypervelocity star is estimated for the scenario of Hills, which describes the dynamical capture of one component of a binary star by the gravitational field of the supermassive black hole in the Galactic center, leading to the ejection of the other component. Ten thousand initial orientations of the binary orbits were considered, and the semi-major axes of the binary orbits were varied in a wide range from 11.3 R _⊙ to 425 R _⊙. Two series of computations were carried out, in which the mass of the supermassive black hole was taken to be 10⁶ M _⊙ and 3.4 × 10⁶ M _⊙. Numerical simulations of encounters of the binary and black hole in the framework of the three-body and N-body problems are used to localize regions favorable for the formation of hypervelocity stars. The motion of the ejected star in the regular field of the Galaxy is calculated, and the conditions under which the star escapes the Galaxy defined. The probability of escaping the Galaxy is caluclated as a function of various parameters the initial separation of the binary components and the distance of the binary from the black hole. On average, the probability of forming a hypervelocity star is higher for closer encounters and more tightly bound binary pairs.

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Stars with relativistic speeds in the Hills scenario     

G. N. Dremova  V. V. Dremov  A. V. Tutukov 《Astronomy Reports》2017,61(7):573-590

The dynamical capture of a binary system consisting of a supermassive black hole (SMBH) and an ordinary star in the gravitational field of a central (more massive) SMBH is considered in the three-body problem in the framework of a modified Hills scenario. The results of numerical simulations predict the existence of objects whose spatial speeds are comparable to the speed of light. The conditions for and constraints imposed on the ejection speeds realized in a classical scenario and the modified Hills scenario are analyzed. The star is modeled using an N-body approach, making it possible to treat it as a structured object, enabling estimation of the probability that the object survives when it is ejected with relativistic speed as a function of the mass of the star, the masses of both SMBHs, and the pericenter distance. It is possible that the modern kinematic classification for stars with anomalously high spatial velocities will be augmented with a new class—stars with relativistic speeds.  相似文献   

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

N. A. Silant’ev  M. Yu. Piotrovich  Yu. N. Gnedin  T. M. Natsvlishvili 《Astronomy Reports》2010,54(11):974-982

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.  相似文献   

Circular polarization—Another difference between quasars and BL Lac objects?     

V. M. Vitrishchak  I. N. Pashchenko  D. C. Gabuzda 《Astronomy Reports》2010,54(4):269-276

Analysis of VLBI data obtained on the VLBA reveals an absence of BL Lac objects with high degrees of circular polarization (>0.4%) detected among quasars. This cannot be due to observational selection effects: such objects should have been detected if the distributions of the degree of circular polarization for BL Lac objects and quasars were the same. This observational result provides new evidence for the existence of fundamental differences in the “central engines“ of objects in these two optical classes, which lead to different physical properties of their relativistic jets, such as the composition of the jet plasma (e ^??e ⁺ or e ^??p); the energy spectra of the particles; the magnitude, geometry and degree of order of the jet magnetic field; the angular momentum of the black hole; etc.  相似文献   

Dependence of the diffuse X-ray luminosities of virialized systems on the masses of their central objects     

B. V. Komberg 《Astronomy Reports》2006,50(6):432-438

Based on the available observational data on the diffuse X-ray emission from hot gas in virialized systems (the central regions of clusters of galaxies or the coronas of spherical galaxies) and the masses of the central objects in these systems (the central galaxies in clusters or massive compact objects—supermassive black holes—in the galactic nuclei), we show that the X-ray luminosity is proportional to the square of the mass of the central object. This is consistent with the dependence obtained earlier between X-ray luminosities of systems and the square of their optical luminosities (i.e., the luminosity of stars in these systems). The existence of such dependences for virialized systems on various scales may provide evidence that they are all formed by a single mechanism, such as hierarchical gravitational clustering. Although the times to achieve hydrostatic equilibrium between the gaseous and stellar components in systems on various scales differ, the relation between certain internal parameters of these systems may remain the same. This enables us to estimate certain parameters of virialized systems in terms of others, in particular, to estimate the masses of their central objects based on the diffuse X-ray luminosity of their coronas.  相似文献   

An estimate of the probability of capture of a binary star by a supermassive black hole     

G. N. Dremova  V. V. Dremov  A. V. Tutukov 《Astronomy Reports》2016,60(8):695-701

A simple model for the dynamics of stars located in a sphere with a radius of one-tenth of the central parsec, designed to enable estimation of the probability of capture in the close vicinity (r < 10^?3 pc) of a supermassive black hole (SMBH) is presented. In the case of binary stars, such a capture with a high probability results in the formation of a hyper-velocity star. The population of stars in a sphere of radius <0.1 pc is calculated based on data for the Galactic rotation curve. To simulate the distortion of initially circular orbits of stars, these are subjected to a series of random shock encounters (“kicks”), whose net effect is to “push” these binary systems into the region of potential formation of hyper-velocity stars. The mean crossing time of the border of the close vicinity of the SMBH (r < 10^?3 pc) by the stellar orbit can be used to estimate the probability that a binary system is captured, followed by the possible ejection of a hyper-velocity star.  相似文献   

Correlations between the development of a flare in the Blazar 3C 454.3 in the radio and optical     

A. E. Vol’vach  L. N. Vol’vach  M. G. Larionov  M. F. Aller  H. D. Aller  M. Villata  K. M. Raiteri 《Astronomy Reports》2008,52(11):867-874

Radio and optical data are used to analyze the development of the flare in the blazar 3C 454.3 observed in 2004–2007. A detailed correspondance between the optical and radio flares is established, with a time delay that depends on the observing frequency. The variation of the delay of the radio flare relative to the optical flare is opposite to the dispersion delay expected for the propagation of radiation in the interstellar medium, testifying to an intrinsic origin for the observed outburst. Small-scale flux variations on time intervals of 5–10 days in the millimeter and optical are also correlated, with a time delay of about ten months. This may provide evidence for a single source generating the radiation at all wavelengths. Rapid flux fluctuations in the radio and optical that are correlated with the indicated time delays could be associated with inhomogeneities in the accretion disk. Detailed studies of the flux variations of Active Galactic Nuclei (AGN) can be used to analyze the structure of the accretion disk. A model for the energy release in AGN that is not associated purely with accretion onto supermassive black holes is proposed. As is the case for other active members of the AGN family, estimates of the lifetime of the binary black-hole system in 3C 454.3 suggest that this object is in a stage of its evolution that is fairly close to the coalescence of its black holes. The energy that is released as the companion of the central black hole loses orbital angular momentum is sufficient to explain the observed AGN phenomena. The source of primary energy release could be heating of the gas behind shock fronts that arise due to the friction between the companion black hole and the ambient gaseous medium. The orbit of the companion could be located at the periphery of the accretion disk of the central body at its apocenter and plunge more deeply into the accretion disk at its pericenter, inducing flares at all wavelengths. Energy-release parameters such as the temperature and density of the heated gas are estimated for 3C 454.3. The model considered assumes omnidirectional radiation of the medium in the presence of a magnetic field. The radiation corresponding to the minimum flux level (base level) could represent omnidirectional radiation due to the orbit of the moving companion. The fraction of the energy that is transferred to directed jets is small, comprising 1–2% of the total energy released due to the loss of orbital angular momentum by the companion.  相似文献   

VLBI of supernovae and gamma-ray bursts     

N. Bartel  B. Karimi  M. F. Bietenholz 《Astronomy Reports》2017,61(4):299-306

Supernovae and gamma-ray bursts (GRBs) are among the brightest events in the universe. Excluding Type Ia supernovae and short GRBs, they are the result of the core collapse of a massive star with material being ejectedwith speeds of several 1000 km/s to nearly the speed of light, and with a neutron star or a black hole left over as the compact remnant of the explosion. Synchrotron radiation in the radio is generated in a shell when the ejecta interact with the surrounding medium and possibly also in the central region near the compact remnant itself. VLBI has allowed resolving some of these sources and monitoring their expansion in detail, thereby revealing characteristics of the dying star, the explosion, the expanding shock front, and the expected compact remnant. We report on updates of some of the most interesting results that have been obtained with VLBI so far. Movies of supernovae are available from our website. They show the evolution from shortly after the explosion to decades thereafter, in one case revealing an emerging compact central source, which may be associated with shock interaction near the explosion center or with the stellar corpse itself, a neutron star or a black hole.  相似文献   

A prolonged flare in the blazar 3C 454.3     

A. E. Vol’vach  A. M. Kutkin  M. G. Larionov  L. N. Vol’vach  A. Lahteenmaki  M. Tornikoski  J. Tammi  J. Léon-Tavares  E. Järvelä  M. F. Aller  H. D. Aller 《Astronomy Reports》2013,57(1):46-51

We present an analysis of data from multi-frequency monitoring of the blazar 3C 454.3 in 2010–2012, when the source experienced an unusually prolonged flare with a duration of about two years. This corresponds to the orbital period of the companion in a scenario in which two supermassive black holes are present in the nucleus of 3C 454.3. The flare’s shape, duration, and amplitude can be explained as a result of precession, if the plane of the accretion disk and the orbital plane of the binary are coincident. We detected small-scale structure of the flare, on time scales of no more than a month. These features probably correspond to inhomogeneities in the accretion disk and surrounding regions, with sizes of the order of 10¹⁵ cm. We estimated the size of the accretion disk based on the dynamical and geometrical parameters of this binary system: its diameter is comparable to the size of the orbit of the supermassive binary black hole, and its thickness does not exceed the gravitational radius of the central black hole. The presence of characteristic small-scale features during the flare makes it possible to estimate the relative time delays of variations in different spectral ranges: from gamma-ray to millimeter wavelengths.  相似文献   

The pulsar J1852+0040 in Kes 79 and the nature of central compact objects in supernova remnants     

I. F. Malov 《Astronomy Reports》2009,53(5):472-476

A possible model for the pulsar PSR J1852+0040 associated with the supernova remnant Kes 79 and detected in place of a central compact object in this remnant is discussed. The main observational properties of the pulsar can be understood as consequences of its weak surface magnetic field (B _s < 3 × 10¹¹ G) and short rotational period (P ～ 0.1 s). Its X-ray emission is thermal, and is generated in a small region near the surface of the neutron star due to cooling of the surface as the surface accretes matter from a relict disk surrounding the pulsar. The radio emission is generated in the outer layers of the pulsar magnetosphere by the synchrotron (cyclotron) mechanism. The optical luminosity of J1852+0040 is estimated to be L _opt < 10²⁸ erg/s. If the spectral features in another central compact object, 1E 1207.4+5209, are interpreted as electron cyclotron lines, this provides evidence for a weak surface magnetic field for this neutron star as well (B < 6 × 10¹⁰ G). The hypothesis that all central compact objects have weak surface fields makes it possible to explain the number of detected central compact objects, the absence of pulsar-wind nebulae associated with these objects, and the fact that no pulsar has yet been detected at the position of SN 1987a. We suggest that, after the supernova remnant has dissipated, the central compact object becomes a weak X-ray source (XDINS), whose weak emission is also due to the weakness of its magnetic field.  相似文献   

The dynamical evolution of galaxy clusters taking into account the deceleration of disk galaxies by the gaseous component of the cluster     

G. N. Dremova  A. V. Tutukov  V. V. Dremov 《Astronomy Reports》2010,54(8):704-718

Numerical simulations of the dynamical evolution of a galaxy cluster in the framework of the N-body problem taking into account dark matter are presented. These simulations are aimed at studying the role of intergalactic gas in the cluster (the ICM) in the formation of a central, supermassive cD galaxy. The numerical models indicate that deceleration of the galaxies by intergalactic gas supports the observed high temperature of this gas, and accelerates the formation of a supermassive cD galaxy in the cluster core. The accretion of interstellar gas by the cluster core can support a high accretion rate by the central, supermassive black hole associated with the nucleus of the cD galaxy. As a result, this nucleus harbors a bright quasar. The mass of the black hole can grow with time to values 10¹⁰ M _⊙, as are observed for the brightest quasars.  相似文献   

The evolution of close binary stars     

A. V. Tutukov  A. M. Cherepashchuk 《Astronomy Reports》2016,60(5):461-476

A review of our current understanding of the physics and evolution of close binary stars with various masses under the influence of the nuclear evolution of their components and their magnetic stellar winds is presented. The role of gravitational-wave radiation by close binaries on their evolution and the loss of their orbital angular momentum is also considered. The final stages in the evolution of close binary systems are described. The review also notes the main remaining tasks related to studies of the physics and evolution of various classes of close binaries, including analyses of collisions of close binaries and supermassive black holes in galactic nuclei. Such a collision could lead to the capture of one of the components by the black hole and the acceleration of the remaining component to relativistic speeds.  相似文献   

Spectropolarimetric Observations of Magnetic White Dwarfs with the SAO 6-m Telescope     

V. L. Afanas’ev  M. Yu. Piotrovich  Yu. N. Gnedin  S. D. Buliga  T. M. Natsvlishvili 《Astronomy Reports》2018,62(2):138-143

The results of spectropolarimetric observations of a number of magnetic white dwarfs obtained on the 6-m optical telescope of the Special Astrophysical Observatory are presented. The observations were carried out using the SCORPIO focal aperture-ratio reducer in a spectropolarimetric regime. Two characteristic dependences of the degree of polarization on the wavelength are observed. For one group of objects, the degree of linear polarization grows with wavelength, suggesting that the alignment of atoms and molecules in Rydberg states in the atmosphere of the white dwarf due to the action of its magnetic field influences scattering processes. The second group of objects displays an increase in the degrees of both linear and circular polarization with wavelength, providing evidence for the presence of protoplanetary disks around these magnetic white dwarfs, in which the alignment of circumstellar grains leads to the observed behavior.  相似文献