1998/06/11 by Markos Georganopoulos, Alan P. Marscher
Engineering · Physics and Astronomy · #Active galactic nucleus #Astrophysical jet #Astrophysics #Astrophysics and Cosmic Phenomena #BL Lac object #Blazar #Classical mechanics #Jet (fluid) #Kinetic energy #Luminosity #Mechanics #Neutrino Physics Research #Particle Accelerators and Free-Electron Lasers #Physics #astro-ph
paper · pdf · doi:10.1086/306273
32 pages, 8 figures, to be published in the ApJ (Oct 20, 1998)
arxiv created 1998/06/11 · openalex publication_date 1998/10/20 · arxiv updated 2016/08/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
We propose a unified classification for BL Lac objects (BLs), focusing on the synchrotron peak frequency ν s of the spectral energy distribution. The unification scheme is based on the angle Θ that describes the orientation of the relativistic jet and on the electron kinetic luminosity Λ kin of the jet. We assume that Λ kin scales with the size of the jet r in a self-similar fashion (Λ kin ∝ r 2 ), as supported by observational data. The jets are self-similar in geometry and have the same pressure and median magnetic field at the inlet, independent of size. The self-similarity is broken for the highest energy electrons, which radiate mainly at high frequencies, since for large sources they suffer more severe radiative energy losses over a given fraction of the jet length. We calculate the optically thin synchrotron spectrum using an accelerating inner jet model based on simple relativistic gasdynamics and show that it can fit the observed infrared-to-X-ray spectrum of PKS 2155-304. We couple the accelerating jet model to the unification scheme and compare the results to complete samples of BLs. The negative apparent evolution of X-ray-selected BLs is explained as a result of positive evolution of the jet electron kinetic luminosity Λ kin . We review observational arguments in favor of the existence of scaled-down accretion disks and broad emission-line regions in BLs. The proposed unification scheme can explain the lack of observed broad emission lines in X-ray-selected BLs, as well as the existence of those lines preferentially in luminous radio-selected BLs. Finally, we review observational arguments that suggest the extension of this unification scheme to all blazars.