Engineering topics

Begelman, Mitchell C.

Publications and source records attributed to Begelman, Mitchell C..

65 records · Page 4

NASA NTRS1989

Magnetic fields in the central engines of active galactic nuclei

Important physical processes which may occur in the central engines of active galactic nuclei and which rely on the presence of a strong magnetic field are discussed. These processes include those involved in the plasma physics of hot tenuous accretion flows, the production of nonthermal continuum radiation, and the radiative manifestation of hydromagnetic jet production. The main arguments which support the hypothesis that supermassive black holes are the prime movers in the central engines are reviewed, and some major deduction regarding the physical state of the accreting gas are pointed out.

Begelman, Mitchell C.↗

NASA NTRS1989

Physics of the central engine

Physical processes which may be important in the central engines of AGNs are discussed. Black hole accretion models have been elaborated during the past few years, and provide a plausible framework for jet and spectrum formation. However, the specific mechanisms responsible for producing the observed continuum radiation remain uncertain. Consideration is given to nonthermal models for the IR continuum, constraints on thermal plasma, and the production of relativistic electrons.

Begelman, Mitchell C.↗

NASA NTRS1988

Thermal coupling of ions and electrons by collective effects in two-temperature accretion flows

A plasma instability which may lead to the thermal coupling of ions and electrons in two-temperature accretion flows on a time scale shorter than the Coulomb coupling time has been found. The processes involved with the instability are described. It is shown that the instability has time to grow only in regions of large-amplitude, small-scale MHD turbulence.

Begelman, Mitchell C.↗

NASA NTRS1988

Molecular tori in Seyfert galaxies - Feeding the monster and hiding it

The principal properties of the tori of gas which surround Seyfert nuclei are discussed. The internal state of the clouds and their size distribution function are examined, and it is shown that the Jeans mass scale results in clouds which are individually sufficiently opaque to block out the nucleus, and that the balance of processes which controls their size distribution function also forces the covering factor to be of the order of or greater than unity. Where the gravitational potential is dominated by stars, cloud-cloud collisions keep the molecular clouds close to the equatorial plane. Stirring by stellar processes is never strong enough to compete with collisional losses. The position of the inner edge of the torus is determined by a balance between the inward flow of clouds and the rate at which the nuclear continuum can evaporate them.

Krolik, Julian H.↗

NASA NTRS1988

Accretion flows onto supermassive black holes

The radiative and hydrodynamic properties of an angular momentum-dominated accretion flow onto a supermassive black hole depend largely on the ratio of the accretion rate to the Eddington accretion rate. High values of this ratio favor optically thick flows which produce largely thermal radiation, while optically thin 'two-temperature' flows may be present in systems with small values of this ratio. Observations of some AGN suggest that thermal and nonthermal sources of radiation may be of comparable importance in the 'central engine'. Consideration is given to the possibilities for coexistence of different modes of accretion in a single flow. One intriguing possibility is that runaway pair production may cause an optically thick 'accretion annulus' to form at the center of a two-temperature inflow.

Begelman, Mitchell C.↗

NASA NTRS1988

The effects of X-rays from active galactic nuclei on the interstellar medium of the surrounding galaxy

The effects of an active nucleus on the large-scale properties of the host galaxy are examined, focusing on the effects of X-ray heating on the host galaxy's interstellar medium. The basic properties of AGNs and several questions concerning AGNs are reviewed. The relationship between X-ray heated winds and coronae is outlined. The case of X-ray heated winds in type 2 Seyfert galaxies is discussed.

Begelman, Mitchell C.↗

NASA NTRS1987

Inverse Compton scattering of ambient radiation by a cold relativistic jet - A source of beamed, polarized continuum in blazars?

A general formalism is developed for computing the intensity and polarization of unpolarized radiation scattered by a group of electrons with a specified distribution of momenta. The case of isotropic incident radiation with a power-law spectrum is addressed, and the 'head-on' approximation is developed to describe the nearly unidirectional character of the incident radiation in the electron rest frame. The formalism is implemented numerically, verifying the validity of the 'head-on' approximation and illustrating the polarization and intensity properties of radiation scattered by jets with different electron momentum distribution. The results are placed in an astrophysical setting, determining the optical depths and kinetic energies required for the scattered radiation to dominate over the isotropic component in blazars. It is argued that Comptonization of unbeamed AGN radiation by a relativistic jet can account for the polarization and the overall spectral shape of the IR-optical continuum in blazars.

Begelman, Mitchell C.↗

NASA NTRS1987

Electron injection by relativistic protons in active galactic nuclei

It is shown that protons with Lorentz factors larger than about 1,000,000 are cooled very rapidly by collisions with soft photons in the environment of an AGN. If the energy distribution of accelerated protons is sufficiently flat, then most of the energy contained in relativistic protons will be transformed to pairs, and then to radiation. Under these conditions, proton cooling due to p-gamma interactions is much more important than energy losses due to inelastic proton-proton collisions.

Sikora, Marek↗

NASA NTRS1987

Broad, variable absorption lines in the Seyfert Galaxy NGC 3516 - Probing the structure of the emission-line regions

Nineteen short-wavelength IUE archive images were analyzed in order to probe the structure of the active nucleus in NGC 3516, an SB0 Seyfert I galaxy. The measurements of these features are presented, and the strength, variability, and morphology of the emission and absorption lines and their relation to the continuum are analyzed. It is shown that the absorption must arise within 10 pc of the nonthermal continuum source and that its variability must stem from changes in the velocity space coverage of clouds optically thin to the Lyman continuum. Two possible sites of absorption are discussed: dense clouds in the narrow-line region and optically thin clouds in the broad-line region. A theoretical model that explains the anticorrelation of C IV absorption with continuum flux is described.

Voit, G. Mark↗

NASA NTRS1987

Thermal and dynamical effects of pair production on two-temperature accretion flows

The two-temperature criterion for quasi-spherical (ion-pressure supported) accretion onto a black hole, including the effects of electron-positron pair production is studied. For an interesting range of accretion rates, Coulomb interactions between protons and pairs can cool the innermost regions of the accretion flow. The cooled plasma, assumed to possess some angular momentum, will collapse onto the equatorial plane, forming an optically thick annulus. Hysteresis effects involved in the cooling criterion are expected to lead to bistability or time dependence, which may be associated with variability in certain classes of active galactic nuclei (AGNs). Radiation from the annulus may be responsible for the EUV/soft X-ray excess observed in some AGNs.

Begelman, Mitchell C.↗

NASA NTRS1986

Comptonization in supercritical winds. I - Spectral evolution. II - Dynamics and observational diagnostics

The physics governing spectral evolution in steady, fully ionized, radiatively driven winds from compact objects is examined. A largely analytic theory is developed to describe the effects of diffusion, adiabatic cooling, and repeated Compton scattering on the emergent radiation spectrum. Analytic forms for the wind temperature profile are derived, and spectral solutions obtained by considering a modified, time-independent Kompaneets equation are presented subject to a variety of initial conditions. A probabilistic formalism which describes the effect of diffusion photons travelling beyond the trapping radius is developed to allow the calculation of spectra observed at infinity. The second paper focuses on the dynamics of supercritical winds and on observational diagnostics which may be used to constrain model parameters in the contexts of specific astrophysical systems, including X-ray burst sources and mass-losing quasars.

Becker, Peter A.↗