Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2009-12-16
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
8 pages, 8 figures. Invited talk at the meeting: Accretion and ejection in AGN: a global view (Como, June 2009). To be publish
Scientific paper
The LAT instrument, onboard the Fermi satellite, in its first three months of operation detected more than 100 blazars at more than the 10 sigma level. This is already a great improvement with respect to its predecessor, the instrument EGRET onboard the Compton Gamma Ray Observatory. Observationally, the new detections follow and confirm the so-called blazar sequence, relating the bolometric observed non-thermal luminosity to the overall shape of the spectal energy distribution. We have studied the general physical properties of all these bright Fermi blazars, and found that their jets are matter dominated, carrying a large total power that correlates with the luminosity of their accretion disks. We suggest that the division of blazars into the two subclasses of broad line emitting objects (Flat Spectrum Radio Quasars) and line-less BL Lacs is a consequence of a rather drastic change of the accretion mode, becoming radiatively inefficient below a critical value of the accretion rate, corresponding to a disk luminosity of ~1 per cent of the Eddington one. The reduction of the ionizing photons below this limit implies that the broad line clouds, even if present, cannot produce significant broad lines, and the object becomes a BL Lac.
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