The anomalous flux ratio of iron K lines in NGC 3783 and its possible Cerenkov origin

Abstract

Abstract When thermal relativistic electrons with an isotropic distribution of velocities move through a dense gas region, or impinge upon the surface of a cloud of dense gas, the Cerenkov effect will produce peculiar atomic or ionic emission lines, which we call ‘Cerenkov line-like radiation’. This prediction has been verified by laboratory experiments in the optical waveband. The newly recognized line emission mechanism can find wide applications in high-energy astrophysics. The first candidates are active galactic nuclei (AGNs), e.g. Seyfert 1 galaxies, for which iron Kα at ∼6.4–6.5 keV and Kβ at ≳7.0 keV with peculiar properties have been detected in observations. The traditional ‘photoelectric absorption–fluorescence line emission’ mechanism seems insufficient to explain some of the observational properties of the iron K lines. However, the Cerenkov line-like radiation mechanism provides solutions for these puzzles. In particular, the updated observed ratio of photon number fluxes of iron K lines fobsKα/fobsKβ≈ 4.0 is very difficult to understand from the prevailing fluorescence mechanism in classical physics. A solution can be obtained when the Cerenkov line mechanism is taken into consideration. If our suggestion were further supported by observations, the conventional scenario around the central supermassive black hole of an AGN would need to be modified to accommodate a more energetic, more violent and much denser environment than previously thought.