A Physical Model for Active Galactic Nuclei with Double‐peaked Broad Emission Lines

Abstract

The double-peaked broad emission lines are usually thought to be linked to accretion disks; however, the local viscous heating in the line-emitting disk portion is usually insufficient for the observed double-peaked broad-line luminosity in most sources. It was suggested that the X-ray radiation from an ion-supported torus in the inner region of the disk can photoionize the outer line-emitting disk region. However, our calculations show that only a small fraction (≲2.3%) of the radiation from the radiatively inefficient accretion flow (RIAF) in the inner region of the disk can photoionize the line-emitting disk portion, because the solid angle of the outer disk portion subtended to the inner region of the RIAF is too small. We propose a physical model for double-peaked line emitters, in which only those AGNs with sufficient matter above the disk (slowly moving jets or outflows) can scatter enough photons radiated from the inner disk region to the outer line-emitting disk portion extending from several hundred to more than 2000 gravitational radii. Our model predicts a power-law r-dependent line emissivity Hα ∝ R, where β ~ 2.5, which is consistent with β ~ 2-3 required by the model fittings for double-peaked line profiles. Using a sample of radio-loud AGNs with double-peaked emission lines, we show that the outer disk regions can be efficiently illuminated by the photons scattered from slow or mild relativistic electron-positron jets with γj ≲ 2. It is consistent with the fact that no double-peaked emission line is present in strong radio quasars with relativistic jets. For radio-quiet double-peaked line emitters, slow outflows with Thomson scattering depth ~ 0.2 instead of jets can scatter sufficient photons to (illuminate) the line-emitting regions. This model can therefore solve the energy budget problem for double-peaked line emitters.