297 research outputs found
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
Hot absorbers and the x-ray spectra of BL Lac objects
This is the final report to the National Aeronautics and Space Administration (NASA) concerning NASA grant NAG5-1671. This grant was awarded to Dr. C. Megan Urry of the Space Telescope Science Institute in response to a proposal, entitled 'Hot Absorbers and the X-Ray Spectra of BL Lac Objects.' The grant was originally awarded in October 1991 and expired in February 1994. The purpose of this project was to detect spectral features in high signal-to-noise Rosat PSPC spectra of X-ray-bright BL Lac objects. We were granted observations of three targets: PKS2155-304, Mrk421, and Mrk501. All three were bright and yielded 5,000-30,000 photons in the good PSPC data
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