An effect of stimulated radiation processes on radio emission from extended sources
Both the standard theory of thermal radio emission and the synchrotron theory encounter some difficulties. The most crucial for the former one is nonpossibility to explain the radio spectrum of Venus in the decimeter range (Ksanfomality 1985). The radio spectra of planetary nebulae at high frequencies also are not comfortably consistent with the standard theory (Siodmiak & Tylenda 2001). Here we show that the account for an induced character of radiation processes sufficiently improves the predictions of the standard theory. Moreover, the developed here theory of radio emission from non-uniform gas gives the radio spectra of extended sources, such as supernova remnants and radio galaxies, which are normally attributed to the synchrotron emission. It is important, in this aspect, that the synchrotron self-absorption produces a change in the polarization position angle across the spectral peak. No such a change was detected in gigahertz-peaked spectrum sources (Mutoh et al. 2002). Besides, the flat or slightly inverted radio spectra of low-luminosity active galactic nuclei are not easily consistent with the synchrotron radiation mechanism (Nagar, Wilson & Falcke 2001, Ulvestad & Ho 2001). Thermal radiation in a magnetic field is polarized (see, e.g., Lang 1978). This is also true for radiation considered in the present paper, which can be described as incoherent induced radiation (Prigara 2001b).