2,804 research outputs found
On the dependence of the spectral parameters on the observational conditions in homogeneous time dependent models of the TeV blazars
Most of current models of TeV blazars emission assume a Synchrotron
Self-Compton mechanism where relativistic particles emit both synchrotron
radiation and Inverse Compton photons. For sake of simplicity, these models
usually consider only steady state emission. The spectral features are thus
only related to the shape of the particle distribution, and do not depend on
the timing of observations. In this letter, we study the effect of, firstly,
the lag between the beginning of the injection of the fresh particles and the
trigger of the observation, and secondly, of a finite injection duration. We
illustrate these effects considering an analytical time-dependent model of the
synchrotron emission by a monoenergetic distribution of leptons. We point out
that the spectral shape can be in fact very dependent on observational
conditions if the particle injection term is time-dependent, particularly
taking into account the effect of the time averaging procedure on the final
shape of the SED. Consequences on the acceleration process are also discussed.Comment: Letter to Editor, accepted for publication in A&
A theoretical unifying scheme for gamma-ray bright blazars
The phenomenology of gamma-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: 1) a decrease in
the frequencies of the synchrotron and inverse Compton peaks; 2) 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, highly polarized
quasars and lowly polarized quasars. The proposed scheme is based on the
correlations among the physical parameters derived in the present paper by
applying to 51 gamma 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, and explains the observational trends presented by
Fossati et al. (1998) in a companion paper, dealing with the spectral energy
distributions of all blazars. This gives us confidence that our scheme applies
to all blazars as a class.Comment: 25 pages, 11 figures, uses mn.sty and psfig.tex. Accepted for
publication in MNRA
For a global approach to audiovisual heritage: A plea for North/South exchange in research and practice.
BeppoSAX Observations of Mkn 421: clues on the particle acceleration ?
Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. We present
highlights of the results of the thorough temporal and spectral analysis
discussed by Fossati et al. (1999) and Maraschi et al. (1999), focusing on the
flare of April 1998, which was simultaneously observed also at TeV energies.
The detailed study of the flare in different energy bands reveals a few very
important new results: (a) hard photons lag the soft ones by 2-3 ks *a behavior
opposite to what is normally found in High energy peak BL Lacs X-ray spectra*;
(b) the flux decay of the flare can be intrinsically achromatic if a stationary
underlying emission component is present. Moreover the spectral evolution
during the flare has been followed by extracting X-ray spectra on few ks
intervals, allowing to detect for the first time the peak of the synchrotron
component shifting to higher energies during the rising phase, and then
receding. The spectral analysis confirms the delay in the flare at the higher
energies, as above a few keV the spectrum changes only after the peak of the
outburst has occurred. The spectral and temporal information obtained challenge
the simplest models currently adopted for the (synchrotron) emission and most
importantly provide clues on the particle acceleration process. A theoretical
picture accounting for all the observational constraints is discussed, where
electrons are injected at low energies and then progressively accelerated
during the development of the flare.Comment: 5 pages, LaTeX, uses aipproc.sty, 3 PostScript figures, to appear in
the Proceedings of the "Fifth Compton Symposium", held in Portsmouth (NH),
1999 Septembe
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