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Search Results: 1 - 10 of 316555 matches for " Paul J. Wiita "
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Cosmic Radio Jets
Paul J. Wiita
Physics , 2001,
Abstract: Extragalactic radio sources, including quasars, are now typically understood as being produced by a pair of nearly symmetric, oppositely directed relativistic jets. While some these sources span megaparsecs, and are thus the largest physically connected structures in the universe, emitting regions identified as jets have now been found on all scales down to fractions of a parsec, and jets appear to be a common element of most (maybe all) types of active galactic nuclei (AGN). We first summarize key observations of different classes of cosmic radio jets, and describe how they may be connected. Theoretical models for the launching and propagation of extragalactic jets are briefly described. All of these models assume a magnetized plasma, which typically amounts to only a small fraction of the accreted gas, is ejected from the vicinity of a supermassive black hole. The extreme complexity of the relevant physics has demanded numerical simulations to examine non-linear effects on the stability of propagating jets, and some recent results from these efforts are summarized.
A Multidimensional Relativistic Hydrodynamics Code with a General Equation of State
Eunwoo Choi,Paul J. Wiita
Physics , 2010, DOI: 10.1088/0067-0049/191/1/113
Abstract: The ideal gas equation of state with a constant adiabatic index, although commonly used in relativistic hydrodynamics, is a poor approximation for most relativistic astrophysical flows. Here we propose a new general equation of state for a multi-component relativistic gas which is consistent with the Synge equation of state for a relativistic perfect gas and is suitable for numerical (special) relativistic hydrodynamics. We also present a multidimensional relativistic hydrodynamics code incorporating the proposed general equation of state, based on the HLL scheme, which does not make use of a full characteristic decomposition of the relativistic hydrodynamic equations. The accuracy and robustness of this code is demonstrated in multidimensional calculations through several highly relativistic test problems taking into account nonvanishing tangential velocities. Results from three-dimensional simulations of relativistic jets show that the morphology and dynamics of the relativistic jets are significantly influenced by the different equation of state and by different compositions of relativistic perfect gases. Our new numerical code, combined with our proposed equation of state is very efficient and robust, and unlike previous codes, it gives very accurate results for thermodynamic variables in relativistic astrophysical flows.
Modeling the Emission from Turbulent Relativistic Jets in Active Galactic Nuclei
Victoria Calafut,Paul J. Wiita
Physics , 2014, DOI: 10.1007/s12036-015-9324-2
Abstract: We present a numerical model developed to calculate observed fluxes of relativistic jets in active galactic nuclei. The observed flux of each turbulent eddy is dependent upon its variable Doppler boosting factor, computed as a function of the relativistic sum of the individual eddy and bulk jet velocities and our viewing angle to the jet. The total observed flux is found by integrating the radiation from the eddies over the turbulent spectrum. We consider jets that contain turbulent eddies that have either standard Kolmogorov or recently derived relativistic turbulence spectra. We also account for the time delays in receiving the emission of the eddies due to their different simulated positions in the jet, as well as due to the varying beaming directions as they turn over. We examine these theoretical light curves and compute power spectral densities (PSDs) for a range of viewing angles, bulk velocities of the jet, and turbulent velocities. These PSD slopes depend significantly on the turbulent velocity and are essentially independent of viewing angle and bulk velocity. The flux variations produced in the simulations for realistic values of the parameters tested are consistent with the types of variations observed in radio-loud AGN as, for example, recently measured with the Kepler satellite, as long as the turbulent velocities are not too high.
Radio Galaxies and the Magnetization of the IGM
Gopal-Krishna,Paul J. Wiita
Physics , 2003,
Abstract: Observed radio galaxies had a much higher comoving density during the `quasar era', at z ~ 2-3, but these sources are only detectable for small fractions of their active lifetimes at such high z due to expansion losses and increased inverse Compton losses against the cosmic microwave background. Using recent models for the evolution of the size and luminosity of powerful double radio sources, as well as LCDM simulations of the cosmic web of baryonic material, we argue that during the quasar era a high volume fraction of this web was occupied by the lobes of double radio sources. They could have seeded the IGM with an average magnetic field approaching 10^{-8} G. Further, these advancing overpressured lobes could compress the denser interstellar gas clouds of the galaxies engulfed by them and thus trigger starbursts. This can probably account for much of the intense star-formation activity witnessed beyond z ~ 1.5. Also, the sweeping up of the ISM of the gas-rich galaxies by the rapidly advancing radio lobes may well be responsible for the widespread metal pollution of the IGM and proto-galaxies at high redshifts.
Effect of Beam-Plasma Instabilities on Accretion Disk Flares
Vinod Krishan,Paul J. Wiita,S. Ramadurai
Physics , 2000,
Abstract: We show that a certain class of flare models for variability from accretion disk coronae are subject to beam-plasma instabilities. These instabilities can prevent significant direct acceleration and greatly reduce the variable X-ray emission argued to arise via inverse Compton scattering involving relativistic electrons in beams and soft photons from the disk.
On the Origin of the Fanaroff-Riley Dichotomy
Gopal-Krishna,Paul J. Wiita
Physics , 2000,
Abstract: A small fraction of double radio sources show a peculiar and striking hybrid morphology; they have a distinctly FR I structure on one side of the nucleus, and a FR II structure on the other. We argue that the mere existence of these HYMORS is quite incompatible with the theoretical explanations for the Fanaroff-Riley dichotomy that are based upon the nature of the jet plasma, or those invoking an intrinsic property of the central engine. Rather, these HYMORS strongly support models that explain the difference between FR I and FR II sources in terms of asymmetry of interaction of the jets with the external environments. We further show that a model for radio source dynamics we had earlier proposed can neatly reproduce the observed dependence of the radio power dividing the two FR classes on the optical luminosity of the host galaxy, as found by Owen & White and Ledlow & Owen
Was the Cosmic Web of Protogalactic Material Permeated by Lobes of Radio Galaxies During the Quasar Era?
Gopal-Krishna,Paul J. Wiita
Physics , 2001, DOI: 10.1086/324310
Abstract: Evidence for extended active lifetimes (> 10^8 yr) for radio galaxies implies that many large radio lobes were produced during the `quasar era', 1.5 < z < 3, when the comoving density of radio sources was 2 -- 3 dex higher than the present level. However, inverse Compton losses against the intense microwave background substantially reduce the ages and numbers of sources that are detected in flux-limited surveys. The realization that the galaxy forming material in those epochs was concentrated in filaments occupying a small fraction of the total volume then leads to the conclusion that radio lobes permeated much of the volume occupied by the protogalactic material during that era. The sustained overpressure in these extended lobes is likely to have played an important role in triggering the high inferred rate of galaxy formation at z > 1.5 and in the magnetization of the cosmic network of filaments.
Extragalactic radio sources with hybrid morphology: implications for the Fanaroff-Riley dichotomy
Gopal-Krishna,Paul J. Wiita
Physics , 2000,
Abstract: We provide observational and theoretical perspectives on the currently much debated issue of the Fanaroff-Riley (FR) morphological dichotomy of extragalactic radio sources. In this context we introduce a new, albeit rare, class of double radio sources in which the two lobes exhibit clearly different FR morphologies. It is argued that such `HYbrid MOrphology Radio Sources', or HYMORS, could be used to effectively constrain the theoretical mechanisms proposed for the FR dichotomy. Basically, the existence of HYMORS supports explanations for the FR dichotomy based upon jet interaction with the medium external to the central engine, and appears quite difficult to reconcile with the class of explanations that posit fundamental differences in the central engine, such as black hole spin or jet composition, to be responsible for the two FR classes of double radio sources.
Models for Accretion Disk Fluctuations through Self-Organized Criticality Including Relativistic Effects
Ying Xiong,Paul J. Wiita,Gang Bao
Physics , 2000, DOI: 10.1093/pasj/52.6.1097
Abstract: The possibility that some of the observed X-ray and optical variability in active galactic nuclei and galactic black hole candidates is produced in accretion disks through the development of a self-organized critical state is reconsidered. New simulations, including more complete calculations of relativistic effects, do show that this model can produce light-curves and power-spectra for the variability which agree with the range observed in optical and X-ray studies of AGN and X-ray binaries. However, the universality of complete self-organized criticality is not quite achieved. This is mainly because the character of the variations depend quite substantially on the extent of the unstable disk region. If it extends close to the innermost stable orbit then a physical scale is introduced and the scale-free character of self-organized criticality is vitiated. Significant dependence of the power spectrum density slope on the type of diffusion within the disk and a weaker dependence on the amount of differential rotation are noted. When general relativistic effects are incorporated in the models, additional substantial differences are produced if the disk is viewed from directions far from the accretion disk axis.
The Fanaroff-Riley Transition and the Optical Luminosity of the Host Elliptical Galaxy
Gopal-Krishna,Paul J. Wiita
Physics , 2001, DOI: 10.1051/0004-6361:20010572
Abstract: We show that a model for radio source dynamics we had earlier proposed can readily reproduce the relationship between the radio power division separating the two Fanaroff-Riley classes of extragalactic radio sources and the optical luminosity of the host galaxy, as found by Owen and Ledlow (1994). In our scenario, when less powerful jets eventually slow down to the point that the advance of the working surface (i.e., hotspot) becomes subsonic with respect to the external gas, the jet's collimation is severely weakened. This criterion distinguishes the powerful and well collimated FR II sources from the weaker sources producing the less collimated FR I type morphologies.
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