A simplified view of blazars: why BL Lacertae is actually a quasar in disguise

Abstract

We put forward a scenario where blazars are classified as flat-spectrum radio quasars, BL Lacs, low synchrotron, or high synchrotron peaked objects according to a varying combination of Doppler boosted radiation from the jet, emission from the accretion disk, the broad line region, and light from the host galaxy. We thoroughly test this new approach, which builds upon unified schemes, using Monte Carlo simulations and show that it can provide simple answers to a number of long-standing open issues. We also demonstrate that selection effects play a very important role in the diversity observed in radio and X-ray samples and in the correlation between luminosity and peak frequency of the synchrotron power (the so-called "blazar sequence"). It turns out that sources so far classified as BL Lacs on the basis of their observed weak, or undetectable, emission lines are of two physically different classes: intrinsically weak-lined objects, more common in X-ray selected samples, and heavily diluted broad-lined sources, more frequent in radio selected samples, which explains some of the confusion in the literature.