Astron. Astrophys. 362, 762-773 (2000)

3. Conclusion and discussion

The Cerenkov line-like emission of the relativistic electrons, passing through an optically thick dense gas, which we suggested early in 1980, has been verified by elegant laboratory experiments (Xu et al. 1981, 1988, 1989). In this paper, we give a detailed and clearer physical discussion and emphasize the potential importance of this new mechanism for high-energy astrophysics, and give the extended and improved formula system describing the emergent intensity, the line profile, the line width and the Cerenkov redshift of the Cerenkov line, among which the extension of formulae to the multi-electron complex atoms(ions) has special significance for the study of the broad lines of heavy elements in AGNs, particularly for lines in the X-ray band.

A possible application of the new line emission mechanism is in the exploration of the origin of the broad line of the low-ionized iron ions of Seyfert1 galaxies. Now the disk-line models, in which the line is regarded as one of the reflection components from the disk, strongly illuminated by the hard X-ray continuum, are widely accepted to explain the origin and characters of the broad -line with asymmetric profile. It is believed that the "Compton reflection and iron fluorescence features", provide a powerful probe of the accretion flow and the strong gravitational field (see e.g. Georgy & Fabian 1991; Reynolds et al. 1995). However, people now question the relativistically smeared disk-line interpretation. According to the disk-line model, both the line profile and the position of the peak are dependent on the inclination angle of the disk, and therefore are different from one sample to another. But the observations show similar profiles of the line for all Seyfert1 galaxies, even for Seyfert2 galaxies, with nearly unchanged peak at (Nandra, 1997a,b), which implies that the line might not be from the inner disk, as thought before. It seems more reasonable to replace the "cold disk" by "cold cloudlets" and/or "cold filaments" around the central massive black hole. Furthermore, the line emission mechanism might not, or rather, not only be photoionization-fluorescence. The photoionization-fluorescence model predicts positive correlations of both the light curves and the fluxes between the line and the X-ray continuum. But the observations do not confirm this (e.g. Lee et al. 1999). Besides, the prediction of a marked absorption dip at edge which always accompanies the fluorescent line is also not confirmed by the observations (Young et al. 1998).

We have shown that fluorescence is not a `unique' mechanism of the line-emission of the low-ionization iron in the X-ray band. Another line mechanism which can produce the line is the Cerenkov line-like emission, as described in this paper. For a very dense gas, optically thick for the continuum, the Cerenkov line becomes the unique emission line which can escape from the surface of the cloud of dense gas. The Cerenkov line will be strong to match the observation when the density of relativistic electrons in BLR is high enough. Therefore this kind of line emission might be a new possible mechanism to attack the problem of AGNs. We expect that some puzzles of line of iron could be resolved in this way (e.g. the observed strange correlations of the light curves and the fluxes between the line and X-ray continuum radiation), even though there remain a lot of problems to be solved.

© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000
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