7,781 research outputs found

    Multiwavelength Properties of Blazars

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    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

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    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

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    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?

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    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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