Engineering topics

Sikora, Marek

Publications and source records attributed to Sikora, Marek.

15 records

DOE OSTI2021

The Dependence of the Fraction of Radio Luminous Quasars on Redshift and its Theoretical Implications

While radio emission in quasars can be contributed to by a variety of processes (involving star-forming regions, accretion disk coronas and winds, and jets), the powering of the radio loudest quasars must involve very strong jets, presumably launched by the Blandford–Znajek mechanism incorporating the magnetically arrested disk (MAD) scenario. Here, we focus on the latter and investigate the dependence of their fraction on redshift. We also examine the dependence of the radio-loud fraction (RLF) on BH mass ( M BH ) and Eddington ratio (λ Edd ), while excluding the redshift bias by narrowing its range. In both of these investigations, we remove the bias associated with: (1) the diversity of source selection by constructing two well-defined, homogeneous samples of quasars (first within 0.7 ≤ z ≤ 1.9, second within 0.5 ≤ z ≤ 0.7); and (2) a strong drop in the RLF of quasars at smaller BH masses by choosing those with BH masses larger than 10 8.5 M ⊙ . We confirm some of the previous results showing the increase in the fraction of radio-loud quasars with cosmic time and that this trend can be even steeper if we account for the bias introduced by the dependence of the RLF on BH mass, whereas the bias introduced by the dependence of the RLF on Eddington ratio is shown to be negligible. Assuming that quasar activities are triggered by galaxy mergers, we argue that such an increase can result from the slower drop with cosmic time of mixed mergers than of wet mergers.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1996

The Compton Mirror in NGC 4151

We show that the sharp cutoff in the hard X-ray spectrum of NGC 4151, unusual for Seyfert 1 galaxies, can be reconciled with the average Seyfert 1 spectrum if we assume that the central source is completely hidden from our line of sight by the thick part of the accretion disk or by the broad emission-line clouds. The observed X-ray radiation is produced by scattering of the Seyfert 1 type spectrum in the higher, cooler parts of the accretion disk corona, or in a wind. A sharp cutoff appears as a result of the Compton recoil effect. This model naturally explains a discrepancy regarding the inclination of the central source, inferred to be low (face-on) from observations of the iron K-alpha emission line, but inferred to be high on the basis of optical and UV observations.

Poutanen, Juri↗

NASA NTRS1995

A Large-Particle Monte Carlo Code for Simulating Non-Linear High-Energy Processes Near Compact Objects

High-energy radiation processes in compact cosmic objects are often expected to have a strongly non-linear behavior. Such behavior is shown, for example, by electron-positron pair cascades and the time evolution of relativistic proton distributions in dense radiation fields. Three independent techniques have been developed to simulate these non-linear problems: the kinetic equation approach; the phase-space density (PSD) Monte Carlo method; and the large-particle (LP) Monte Carlo method. In this paper, we present the latest version of the LP method and compare it with the other methods. The efficiency of the method in treating geometrically complex problems is illustrated by showing results of simulations of 1D, 2D and 3D systems. The method is shown to be powerful enough to treat non-spherical geometries, including such effects as bulk motion of the background plasma, reflection of radiation from cold matter, and anisotropic distributions of radiating particles. It can therefore be applied to simulate high-energy processes in such astrophysical systems as accretion discs with coronae, relativistic jets, pulsar magnetospheres and gamma-ray bursts.

Stern, Boris E.↗

NASA NTRS1994

Energetic and radiative constraints on highly relativistic jets

We examine constraints on the energetics and radiative efficiencies of highly relativistic, synchrotron-emitting jets. If the observed intraday radio variability of compact radio sources is intrinsic and results from incoherent synchrotron radiation, then the associated jets must have bulk Lorentz factors in the range Gamma varies approximately 30 to 100, several times larger than the largest values inferred from superluminal expansion, and larger even than the values required to avoid the synchrotron self-Compton catastrophe. We show that such highly relativistic jets produce synchrotron radiation with extremely low radiative efficiency. As a result they must carry enormous kinetic energy fluxes, L(sub j) approximately greater than 10(exp 47)(Delta Omega/0.1 sr), where Delta Omega is the solid angle subtended by the jet, in order to produce 'apparent' synchrotron brightness temperatures approximately greater than 10(exp 16) K. Energy losses by such jets should be strongly dominated by Compton scattering of diffuse ambient radiation, and they should produce large X-ray and gamma-ray fluxes.

Begelman, Mitchell C.↗

NASA NTRS1994

High-energy radiation from active galactic nuclei

Two recent findings concerning high-energy radiation properties of active galactic nuclei -- discovery of breaks in hard X-ray spectra of Seyfert galaxies, and discovery of huge fluxes of hard gamma rays from blazars -- seem to press us to change our standard views about radiation production in these objects. I review briefly the existing radiation models, confront them with the newest observations, and discuss newly emerging theoretical pictures which attempt to account for the discoveries.

Sikora, Marek↗

NASA NTRS1994

Comptonization of diffuse ambient radiation by a relativistic jet: The source of gamma rays from blazars?

Recent Energy Gamma Ray Experiment Telescope (EGRET) observations of blazars have revealed strong, variable gamma-ray fluxes with no signatures of gamma-ray absorption by pair production. This radiation probably originates from the inner parts of relativistic jets which are aimed nearly toward us. On sub-parsec scales, the jet will be pervaded by radiation from the broad-line region, as well as by photons from the central continuum source (some of which will be scattered by thermal plasma). In a frame moving with the relativistic outflow, the energy of this ambient radiation would be enhanced. This radiation would be Comptonized by both cold and relativistic electrons in the jet, yielding (in the observer's frame) a collimated beam of X-rays and gamma rays. On the assumption that this process dominates self-Comptonization of synchrotron radiation, we develop a self-consistent model for variable gamma-ray emission, involving a single population of relativistic electrons accelerated by a disturbance in the jet. The spectral break between the X-ray and gamma-ray band, observed in 3C 279 and deduced for other blazars, results from inefficient radiative cooling of lower energy electrons. The existence of such a break strongly favors a model involving Comptonization of an external radiation field over a synchrotron self-Compton model. We derive constraints on such model parameters as the location and speed of the source, its dimensions and internal physical parameters, the maximum photon energies produced in the source, and the density and distribution of ambient radiation. Finally, we discuss how observations might discriminate between our model and alternative ones invoking Comptonization of ambient radiation.

Sikora, Marek↗

NASA NTRS1992

Reaction rate and energy-loss rate for photopair production by relativistic nuclei

The process of e(+/-) pair production by relativistic nuclei on ambient photons is considered. The process is important for cosmic-ray nuclei in interstellar and intergalactic space as well as in galactic and extragalactic compact objects. The rate of this process is given by an integral of the cross section over the photon angular and energy distribution. In the case of isotropic photons, the angular integration is performed to provide an expression for the rate at given photon energy in the nucleus rest frame. The total rate then becomes a single integral of that rate over the photon energy distribution. Formulas are also given for the fractional energy loss of a relativistic nucleus colliding with a photon of a given energy in the rest frame. The nucleus energy-loss rate is integrated over the photon angular distribution in the case of isotropic photons, and simple fits are provided.

Chodorowski, Michal J.↗

NASA NTRS1992

Does an orbiting star cause periodic modulation of X-rays from NGC6814?

An obvious candidate for the phenomenon underlying the periodicity in the X-ray emission from the Seyfert galaxy NGC6814 is the orbital motion of a star or low-mass compact object around the central black hole. It is shown here that the presence of an orbiting star could be easily verified by looking for the effects of Lense-Thirring precession of the orbital plane caused by the dragging of inertial frames around a rotating black hole. Precession-induced variations in the waveform and in the phase of the observed periodicity should have a period of between a month and a year. Such variations could account for the different waveforms present in the Ginga and Exosat data set from observations of NGC6814 and may be detectable in existing Ginga and future Rosat, OSSE/BRO, and Astro-D data.

Sikora, Marek↗

NASA NTRS1991

Outflows driven by cosmic-ray pressure in broad absorption line QSOs

It is shown how the escape of ultrarelativistic neutrons from the central engine of an AGN can lead to the production of a strong fast wind with most of the acceleration occurring outside the broad emission line region. This feature is required in order to explain observations of broad absorption line (BAL) QSOs, in which the blue wings of the broad emission lines are often absorbed. The dynamical and thermal structure of the wind is determined, and the formation of BALs in such an outflow is modeled, deriving line profiles and constraining the rate at which the energy or relativistic protons has be be converted to thermal energy in order to provide pressure confinement for the BAL clouds. The problem of acceleration and survival of the clouds is discussed.

Begelman, Mitchell↗

NASA NTRS1990

Consequences of relativistic proton injection in active galactic nuclei

The processes are analyzed by which extremely relativistic protons lose energy under the conditions thought to apply in the central engines of active galactic nuclei. Analytic formulae are derived which permit the consequences of relativistic proton injection under a variety of conditions to be predicted. Different proton cooling mechanisms are compared for a power-law proton injection function and two types of background radiation spectra: power-law and power-law plus blackbody. The kinetic equations for neutrons and protons are examined, and it is shown how to calculate the energy distributions of primary gamma rays and escaping neutrons and neutrinos.

Begelman, Mitchell C.↗

NASA NTRS1989

Relativistic neutrons in active galactic nuclei

The acceleration of protons to relativistic energies in active galactic nuclei leads to the creation of relativistic neutrons which escape from the central engine. The neutrons decay at distances of up to 1-100 pc, depositing their energies and momenta in situ. Energy deposition by decaying neutrons may inhibit spherical accretion and drive a wind, which could be responsible for the velocity fields in emission-line regions and the outflow of broad absorption line systems. Enhanced pressure in the neutron decay region may also help to confine emission line clouds. A fraction of the relativistic proton energy is radiated in gamma-rays with energies which may be as large as about 100,000 GeV.

Sikora, Marek↗

NASA NTRS1989

Ultra-high-energy photons from active galactic nuclei - Theory

First-order Fermi acceleration in collisionless shocks is supposed to operate in the central regions of AGN, initiating processes which ultimately result in ultrahigh energy (UHE) photons. This paper analyzes the formation of the UHE photon spectrum inside the central source. The escape probability of the UHE photons, the energy spectra of created electron-positron pairs, and their synchrotron radiation in the external region are calculated.

Sikora, Marek↗

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

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.↗