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 photo-ionize the outer line-emitting disk region .
However , our calculations show that only a small fraction ( \lesssim 2.3 per cent ) of the radiation from the radiatively inefficient accretion flow ( RIAF ) in the inner region of the disk can photo-ionize 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 two thousand gravitational radii .
Our model predicts a power-law r -dependent line emissivity \epsilon ^ { { H } \alpha } \propto R _ { d } ^ { - \beta } , where \beta \sim 2.5 , which is consistent with \beta \sim 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 \gamma _ { j } \lesssim 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 \sim 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 .