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The recent discovery, by the Chandra satellite, that jets of blazars are strong X-ray emitters at large scales , lends support to the hypothesis that emitting plasma is still moving at highly relativistic speeds on these scales. In this case in fact the emission via inverse Compton scattering off cosmic background photons is enhanced and the resulting predicted X-ray spectrum accounts well for the otherwise puzzling observations. Here we point out another reason to favour relativistic large-scale jets, based on a minimum power argument: by estimating the Poynting flux and bulk kinetic powers corresponding to, at least, the relativistic particles and magnetic field responsible for the emission, one can derive the value of the bulk Lorentz factor for which the total power is minimized. It is found that both the inner and extended parts of the jet of PKS satisfy such a condition. 相似文献
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Annalisa Celotti 《Astrophysics and Space Science》2005,300(1-3):23-30
This contribution focuses on the evidence for a bimodality in the luminosity believed to be associated with the accretion
process in AGN. In particular, it will be stressed that this behavior does seem to be present in an analogous way both in
radio-loud and radio-quiet AGN, as inferred from samples selected in an independent way. The found bimodality can be naturally
– although not uniquely – interpreted in the frame of the ADIOS solution for radiative inefficient accretion flows. The (so
far) qualitative analogy with the behavior of XRB provides an interesting perspective to find a unique framework for the accretion
and jet production in accreting black hole systems. 相似文献
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A theoretical unifying scheme for gamma-ray bright blazars 总被引：1，自引：0，他引：1
G. Ghisellini A. Celotti G. Fossati L. Maraschi & A. Comastri 《Monthly notices of the Royal Astronomical Society》1998,301(2):451-468
The phenomenology of γ-ray bright blazars can be accounted for by a sequence in the source power and intensity of the diffuse radiation field surrounding the relativistic jet. Correspondingly, the equilibrium particle distribution peaks at different energies. This leads to a trend in the observed properties: an increase of the observed power corresponds to: (i) a decrease in the frequencies of the synchrotron and inverse Compton peaks, and (ii) an increase in the ratio of the powers of the high- and low-energy spectral components. Objects along this sequence would be observationally classified respectively as high-frequency BL Lac objects, low-frequency BL Lac objects, high-polarization quasars and low-polarization quasars. The proposed scheme is based on the correlations among the physical parameters derived in the present paper by applying to 51 γ-ray loud blazars two of the most accepted scenarios for the broad-band emission of blazars, namely the synchrotron self-Compton and external Compton models. This also explains the observational trends presented by Fossati et al., dealing with the spectral energy distributions of all blazars. This gives us confidence that our scheme applies to all blazars as a class. 相似文献
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Gabriele Ghisellini Davide Lazzati Annalisa Celotti Martin J. Rees 《Monthly notices of the Royal Astronomical Society》2000,316(4):L45-L49
Recently several studies have jointly analysed data from different cosmological probes with the motivation of estimating cosmological parameters. Here we generalize this procedure to allow freedom in the relative weights of various probes. This is done by including in the joint χ 2 function a set of 'hyper-parameters', which are dealt with using Bayesian considerations. The resulting algorithm, which assumes uniform priors on the log of the hyper-parameters, is very simple: instead of minimizing (where is per data set j ) we propose to minimize (where N j is the number of data points per data set j ). We illustrate the method by estimating the Hubble constant H 0 from different sets of recent cosmic microwave background (CMB) experiments (including Saskatoon, Python V, MSAM1, TOCO and Boomerang ). The approach can be generalized for combinations of cosmic probes, and for other priors on the hyper-parameters. 相似文献
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