Next Contents Previous


According to Unified Schemes, blazars are that special class of radio-loud AGN with their jets pointing more or less towards the observer, and therefore constitute a relatively rare class of objects. Radio-loud AGN make up only ~ 10-15% of all AGN (e.g., Kellermann et al. 1989), while a generous upper limit to the fraction of blazars amongst radio sources is 50% (as inferred, for example, from the fraction of FSRQ and BL Lacs in the 1 Jy catalogue [Stickel et al. 1994] which, being a high-frequency radio catalogue, is biased towards flat-spectrum sources). It then follows that blazars make up at most 5% of all AGN, but more likely even less than that.

Mukherjee et al. (1997) have identified 51 high-confidence EGRET sources (mainly from the Second EGRET catalogue; Thompson et al. 1995, 1996) with AGN, all of them blazars. If the probability of detecting an AGN with EGRET were independent of the class, then in this list we would expect at maximum 3 blazars, with most sources being associated with radio-quiet AGN. Instead, we have 100% blazars and 0% other sources. In particular, no radio-quiet AGN has been detected so far by EGRET. Note that blazar gamma-ray luminosities are in the range 1045-1049 erg/s (under the assumption of isotropy) and in many cases the output in gamma-rays dominates the total (bolometric) luminosity.

To find out what is so special about blazars we need to have a closer look at their properties.