Multiwavelength Properties of Blazars

Blazar spectral energy distributions (SEDs) are double peaked and follow a self-similar sequence in luminosity. The so-called "blue" blazars, whose first SED component peaks at X-ray energies, are TeV sources, although with a relatively small fraction of their bolometric luminosities. The "red" blazars, with SED peaks in the infrared-optical range, appear to emit relatively more power in the gamma-ray component but at much lower energies (GeV and below). Correlated variations across the SEDs (of both types) are consistent with the picture that a single electron population gives rise to the high-energy parts of both SED components, via synchrotron at low energies and Compton-scattering at high energies. In this scenario, the trends of SED shape with luminosity can be explained by electron cooling on ambient photons. With simple assumptions, we can make some estimates of the physical conditions in blazar jets of each "type" and can predict which blazars are the most likely TeV sources. Upper limits from a mini-survey of candidate TeV sources indicate that only ~10% of their bolometric luminosity is radiated in gamma-rays, assuming the two SED components peak near 1 keV and 1 TeV. Finally, present blazar samples are too shallow to indicate what kinds of jets nature prefers, i.e., whether the low-luminosity "blue" blazars or the high-luminosity "red" blazars are more common.Comment: 16 pages, including 4 figures. Invited talk at the Veritas Workshop on the TeV Astrophysics of Extragalactic Objects (October 1998), ed. T. C. Weekes and M. Catanese, (Elsevier, Astroparticle Physics), in press. Latex version uses Elsevier macros (elsart.sty) and psfig.st

Unified Schemes for Radio-Loud Active Galactic Nuclei

The appearance of active galactic nuclei (AGN) depends so strongly on orientation that our current classification schemes are dominated by random pointing directions instead of more interesting physical properties. Light from the centers of many AGN is obscured by optically thick circumnuclear matter and in radio-loud AGN, bipolar jets emanating from the nucleus emit light that is relativistically beamed along the jet axes. Understanding the origin and magnitude of radiation anisotropies in AGN allows us to unify different classes of AGN; that is, to identify each single, underlying AGN type that gives rise to different classes through different orientations. This review describes the unification of radio-loud AGN, which include radio galaxies, quasars, and blazars. We describe the classification and properties of AGN and summarize the evidence for anisotropic emission. We outline the two most plausible unified schemes for radio-loud AGN, one linking quasars and luminous radio galaxies and another linking BL~Lac objects and less luminous radio galaxies. Using the formalism appropriate to samples biased by relativistic beaming, we show the population statistics for two schemes are in accordance with available data. We analyze the possible connections between low- and high-luminosity radio-loud AGN. We review potential difficulties with unification and conclude that none currently constitutes a serious problem. We discuss likely complications to unified schemes that are suggested by realistic physical considerations; these will be important to consider when more comprehensive data for larger complete samples become available. We conclude with a list of the ten questions we believe are the most pressing in this field.Comment: 88 pages, latex file, uses aaspp.sty macro (available via ftp from ftp://aas.org/pubs/aastex/). Accompanying 22 figures and 3 tables available at http://itovf2.roma2.infn.it/padovani/review.html. (Abstract is abridged.) The only change is that the revised version indicates this paper is an invited review for PASP, in press, September 1995 issu

The Cosmic History of Black Hole Growth from Deep Multiwavelength Surveys

Significant progress has been made in the last few years on understanding how supermassive black holes form and grow. In this paper, we begin by reviewing the spectral signatures of Active Galactic Nuclei (AGN) ranging from radio to hard X-ray wavelengths. We then describe the most commonly used methods to find these sources, including optical/UV, radio, infrared and X-ray emission and optical emission lines. We then describe the main observational properties of the obscured and unobscured AGN population. Finally, we summarize the cosmic history of black hole accretion, i.e., when in the history of the Universe supermassive black holes were getting most of their mass. We finish with a summary of open questions and a description of planned and future observatories that are going to help answer them.Comment: 33 pages, 20 figures, review paper accepted for the Advances in Astronomy Special Issue "Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus Supermassive Black Holes

X-ray temporal and spectral studies of blazars with the Ginga satellite

This is the final report to the National Aeronautics and Space Administration (NASA) concerning NASA grant NAG8-697. This grant was awarded to Dr. C. Megan Urry of the Space Telescope Science Institute in response to a proposal, entitled 'X-Ray Temporal and Spectral Studies of Blazars with the Ginga Satellite', to collaborate with Japanese colleagues in using the Ginga X-ray satellite. The grant was originally awarded on 2/27/88 and expired on 3/31/94. The Ginga X-ray satellite had unprecedented sensitivity in the 2-20 keV energy band, allowing us to make detailed temporal and spectral studies of a large number of blazars, which are a kind of unusually luminous and variable active galactic nuclei. We were successful with several proposals and were able to observe a number of different active galactic nuclei. Our investigations under this grant fall broadly into two categories: (1) Ginga observations of blazars, usually in conjunction with simultaneous multiwavelength observations using other facilities; and (2) the application of calculated pair plasma spectra to the X-ray colors of active galactic nuclei. These are described in turn