Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aipc..558..358w&link_type=abstract
HIGH ENERGY GAMMA-RAY ASTRONOMY: International Symposium. AIP Conference Proceedings, Volume 558, pp. 358-369 (2001).
Astronomy and Astrophysics
Astronomy
Active And Peculiar Galaxies And Related Systems
Scientific paper
Blazars are the only astrophysical objects that have been studies in all wavelength ranges accessible for astrophysical investigations. Their broad band spectral energy distributions cover more than 19 orders of magnitude in the most extreme examples. Most of the emission is due to non-thermal radiation mechanisms. The intensity is variable in all energy bands studied and time-scales span up to nine orders of magnitude. Variability has been detected on all time-scales probed. Spectral and variability characteristics are reviewed. Correlations between variations in different energy bands are studied in statistical approaches and in direct correlation analyzes. All well-studied sources exhibit bimodal spectral energy distributions. The lower energy component, due to synchrotron emission, extends up to near-IR frequencies in classical radio-loud Blazars but may reach up to 1 MeV in the most extreme source Mrk 501. Selection effects introduce an apparent dichotomy with objects whose synchrotron power peaks above or below the optical-UV regime. Number-densities, luminosity functions and cosmological evolution appear to be different in these two classes. The high energy component is less well studied but exhibits comparable characteristics. Correlations between intensity and cut-off frequency of any of the two spectral components as well as correlations between either of these properties are strongly affected by selection effects caused by temporal variability. The power spectra of variability are broad band. There is only little evidence for periodic signals. A pronounced break in the structure function on time-scales of 100 000 sec has been identified in different energy bands. This introduces a quasi-periodic signature in campaigns of finite temporal extent. The physical implication of this break is not understood. It may either signal two different processes being responsible for the variations on longer and shorter time-scales or it may be a measure of a characteristic length scale. The break is independent of photon energy, suggesting that it is not due to acceleration or cooling processes. Variability on all time-scales modulates the spectral properties. In any individual source the cut-off frequency of the synchrotron component may change by more than two orders of magnitude on short time-scales. Both, the low- and high-energy components are found to display fast variations in intensity. This introduces changes in the Compton dominance and illustrates the inhomogeneity of the emitting regions in the jets. Multifrequency campaigns can constrain the acceleration and radiation mechanisms producing the bulk of the non-thermal radiation in Blazars, but temporal and frequency coverage have not been sufficient in previous experiments. In particular, detections in the gamma-ray bands have only been made during bright states. This calls for more sensitive instruments and proper temporal coverage. .
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