Bulk acceleration in relativistic jets and the spectral properties of blazars

Abstract

Models for the synchrotron and inverse Compton emission from relativistic jets with bulk acceleration are studied, under the assumption that the bulk velocity increases with increasing distance along the jet while the emission spectrum shifts to lower frequencies. The apparent simplicity of BL Lac spectra is interpreted as being due to the convoluted emission from a relativistic jet whose innermost regions at rest emit the highest frequencies, while outer regions, with increasing bulk velocity, emit lower frequencies. The observed broadband energy distributions strongly constrain the properties of such jets. The model predicts that rapid X-ray variability is associated with steep X-ray spectra, while objects with flat X-ray spectra are expected to be more variable at optical to radio frequencies. The absolute space density of radio-strong objects should be 0.01-0.05 of that of radio-weak objects with the same X-ray luminosity. 18 refs.