Does the Iron K$_α$ Line of Active Galactic Nuclei Arise from the Cerenkov Line-like Radiation?

Abstract

When thermal relativistic electrons with isotropic distribution of velocities move in a gas region, or impinge upon the surface of a cloud that consists of a dense gas or doped dusts, the Cerenkov effect produces peculiar atomic or ionic emission lines -- the Cerenkov line-like radiation. This newly recognized emission mechanism may find wide applications in high-energy astrophysics. In this paper, we tentatively adopt this new line emission mechanism to discuss the origin of iron K$_α$ feature of AGNs. Motivation of this research is to attempt a solution to a problem encountered by the ``disk-fluorescence line'' model, i.e. the lack of temporal response of the observed iron K$_α$ line flux to the changes of the X-ray continuum flux. If the Cerenkov line emission is indeed responsible significantly for the iron K$_α$ feature, the conventional scenario around the central supermassive black holes of AGNs would need to be modified to accommodate more energetic, more violent and much denser environments than previously thought.