摘要:

Gamma-ray emission from AGN provides us with unprecedented insights into the physics of extragalactic jets. The emission from these jets fits naturally into the existing theoretical framework of relativistic jets as inferred from parsec scale and kiloparsec scale observations. Models of the &ggr;-ray fluxes give us important knowledge of jet parameters, in particular magnetic fields and the size of the emitting region. For example, in the case of Markarian 501, the jet plasma is subequipartiion and the energy flux is particle dominated. Discussion of the physics of jets on the subparsec scale leads naturally to questions of jet composition. The dynamics of jets are consistent with conventional electron-proton composition if the minimum Lorentz factors,&ggr;min>100.Jets can be electron-position if&ggr;min≈100.©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370796

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

In our review of the blazar phenomenon, we discuss blazar models, with a focus on the following issues: sub-parsec jets and their environment; energy dissipation and particle acceleration; and radiative processes. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370797

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The objectives of my talk are to indicate the requirements for particle acceleration for both electrodynamic models and hadronic models in blazars, then to propose a relativistic Fermi acceleration process to achieve the goal of UHE Cosmic Ray production in relativistic flows. Relativistic magnetic fronts, when they cross each other, produce an efficient Fermi process. It is also indicate how forward and backward fronts can be generated. Most of the involved physics also apply to Gamma Ray Bursts provided that they are produced by a magnetically collimated relativistic flow. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370798

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Some general features of ideal MHD plasma outflows from rotating and magnetized central gravitating objects such as AGN are briefly discussed. The asymptotic structure, morphology and degree of collimation of the outflow are analyzed in the light of a family of exact solutions produced via a nonlinear separation of the variables in the full set of the MHD equations. Among the interesting features of this solution is a quantitative energetic criterion for the transition of the morphologies of the outflows from highly collimated jets to uncollimated winds. It is proposed that in a space where the two main variables are the energy of the magnetic rotator and the angle between the line of sight and the ejection axis, some observed characteristics of AGN can be understood. Thus, while the horizontal AGN classification from Type 0 to Types 1 and 2 is mainly an orientation effect, as in the standard model, the vertical AGN classification as those with uncollimated outflows (radio-quiet sources: Seyferts, QSO’s) and collimated outflows (radio-loud sources: radio galaxies and Blazars) depends on the efficiency of the magnetic rotator and the environment in which the outflows propagate. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370799

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

It has been suggested that relativistic outflows may power the jets of active galactic nuclei and gamma-ray bursts. We address the important issue how the kinetic energy of collimated outflows is converted into radiation. It is shown that swept-up ambient matter is quickly isotropized in the outflow region by a relativistic two-stream instability, which provides relativistic particles in the jet without invoking any acceleration process. The fate of the outflow and the spectral evolution of the photon and neutrino emission of the energetic particles is therefore solely determined by the initial condition. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370800

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The EGRET instrument on the Compton Gamma Ray Observatory (1991–2000) has positively detected high energy &ggr;-ray emission from more than 67 active galaxies of the blazar class. The majority of the EGRET blazars are flat-spectrum radio quasars, which are characterized by inferred isotropic luminosities often as high as3×1049&hthinsp;ergs&hthinsp;s−1.The remainder are BL Lac objects, some of which have been detected at TeV energies (>250 GeV) by ground-based atmospheric Cherenkov telescopes. One of the remarkable characteristics observed in blazars is that the &ggr;-ray luminosity often dominates the bolometric power in these sources. The detection of blazars by EGRET has undoubtedly been one of the highlights of the mission, and has forever impacted our understanding of the emission mechanisms in these objects. In this article, we summarize the properties of EGRET blazars, and review the constraints that the EGRET observations place on the various models of &ggr;-ray production in these sources. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370801

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Being dominated by relativistic jets oriented close to the line of sight, blazars are rare laboratories to study jet physics. The first clues are provided by their broad-band Spectral Energy Distributions (SEDs) and the observed spectral trends with bolometric luminosity. These show that a rich variety of SED shapes exist, with so-called red and blue sources forming the two extremes of a continuous distribution. These trends can be understood in terms of different preferred cooling mechanisms in the jet, which in turn depend on source power and the jet’s ambient radiation. Here I summarize nearly one decade of study of blazars SEDs, pointing out possible selection biases affecting current datasets. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370802

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The non-thermal spectra of blazars, observed from radio to GeV/TeV &ggr;-rays, reveal two pronounced components, both produced by radiation from energetic particles. One peaks in the IR to soft X-ray band, radiating via the synchrotron process; the other, peaking at high &ggr;-ray energies, is produced by the Compton process. The X-ray regime is important, as it is where the emission due to both processes overlap. In this paper, results from recent multi-frequency observations of blazars are reported with emphasis on X-ray observations. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370803

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Blazars are the only astrophysical objects that have been studies in all wavelength ranges accessible for astrophysical investigations. Their broad band spectral energy distributions cover more than 19 orders of magnitude in the most extreme examples. Most of the emission is due to non-thermal radiation mechanisms. The intensity is variable in all energy bands studied and time-scales span up to nine orders of magnitude. Variability has been detected on all time-scales probed. Spectral and variability characteristics are reviewed. Correlations between variations in different energy bands are studied in statistical approaches and in direct correlation analyzes. All well-studied sources exhibit bimodal spectral energy distributions. The lower energy component, due to synchrotron emission, extends up to near-IR frequencies in classical radio-loud Blazars but may reach up to 1 MeV in the most extreme source Mrk 501. Selection effects introduce an apparent dichotomy with objects whose synchrotron power peaks above or below the optical-UV regime. Number-densities, luminosity functions and cosmological evolution appear to be different in these two classes. The high energy component is less well studied but exhibits comparable characteristics. Correlations between intensity and cut-off frequency of any of the two spectral components as well as correlations between either of these properties are strongly affected by selection effects caused by temporal variability. The power spectra of variability are broad band. There is only little evidence for periodic signals. A pronounced break in the structure function on time-scales of 100&hthinsp;000 sec has been identified in different energy bands. This introduces a quasi-periodic signature in campaigns of finite temporal extent. The physical implication of this break is not understood. It may either signal two different processes being responsible for the variations on longer and shorter time-scales or it may be a measure of a characteristic length scale. The break is independent of photon energy, suggesting that it is not due to acceleration or cooling processes. Variability on all time-scales modulates the spectral properties. In any individual source the cut-off frequency of the synchrotron component may change by more than two orders of magnitude on short time-scales. Both, the low- and high-energy components are found to display fast variations in intensity. This introduces changes in the Compton dominance and illustrates the inhomogeneity of the emitting regions in the jets. Multifrequency campaigns can constrain the acceleration and radiation mechanisms producing the bulk of the non-thermal radiation in Blazars, but temporal and frequency coverage have not been sufficient in previous experiments. In particular, detections in the gamma-ray bands have only been made during bright states. This calls for more sensitive instruments and proper temporal coverage. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370804

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The great penetrating power of neutrinos makes them ideal probe of astrophysical sites and conditions inaccessible to other forms of radiation. These are the centers of stars (collapsing or not) and the centers of Active Galactic Nuclei (AGN). It has been suggested that AGN presented a very promising source of high energy neutrinos, possibly detectable by underwater neutrino detectors. This paper reviews the evolution of ideas concerning the emission of neutrinos from AGN in view of the more recent developments in gamma-ray astronomy and their implications for the neutrino emission from these class of objects. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1370805

出版商:AIP    

年代:1901

数据来源: AIP