9,146 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
Coordinated observations of X-ray bright BL Lacertae objects
No new International Ultraviolet Explorer (IUE) observations were scheduled. The work on the spectral extraction routine was continued. The computer code was adapted from the RDAF PDP 11/44 computer to a VAX 11/750, in order that it run more quickly. A chapter on ultraviolet observations of blazars for a book celebrating the scientific accomplishment of IUE was written in colloboration. A copy of the final draft is attached
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
Multiwavelength Properties of Blazars
The multiwavelength spectra of blazars appear to be dominated by nonthermal
emission from a relativistic jet oriented close to the line of sight. The
recent detection of many blazars at gamma-ray energies strongly supports this
scenario. High quality multiwavelength monitoring data for the brightest one or
two blazars suggest the optical through X-ray continuum is synchrotron emission
from an inhomogeneous jet. The gamma-rays are likely due to Compton scattering
of lower energy photons, either from within the jet or from the surrounding
gas. The physical properties of the jet and the way in which it is produced are
still largely a mystery but are probably related in some way to accretion onto
a central supermassive black hole. There is little direct observational
evidence for accretion disks in blazars, although there is evidence for winds
which might emanate from disks.Comment: 12 pages, 6 figures, latex file with encapsulated postscript. Invited
review presented at the 1996 COSPAR Scientific Assembly (Birmingham UK 14-21
July), in session E1.4 on The Variability of AGN and its Relation to
Accretion Disk Phenomenology; paper to appear in Adv. Space Scienc
Debate: Can Sustainable Tourism include Flying?
Can Sustainable tourism include flying? was the question addressed by the debate at the University of Central Lancashire (University of Central Lancashire), UK on March 30th 2012. There was an unexpected degree of consensus among the speakers that tourism had severe environmental impacts, mainly from flying and that many countries currently benefitting from tourism development were likely to suffer from climate change. They also agreed that corporate tourism often diverted funds from destination areas into their own profits. Those arguing for flying stressed the balance of environmental, social and economic benefits, while those arguing against prioritised the environmental damage of increasing aviation. The pro-fliers were optimistic about technological advances in aircraft and fuels but the others doubted their potential to reduce emissions from current or increased levels of aviation. Both sides admitted behavioural change was difficult, but saw hope in recent trends
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