Physics
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........33s&link_type=abstract
Thesis (PH.D.)--THE JOHNS HOPKINS UNIVERSITY, 1994.Source: Dissertation Abstracts International, Volume: 55-03, Section: B, page
Physics
Scientific paper
The effects of stimulated scattering on high brightness temperature radiation are studied in two important contexts. In the first case, we assume that the radiation is confined to a collimated beam traversing a relativistically streaming magnetized plasma. When the plasma is cold in the bulk frame, stimulated scattering is only significant if the angle between the photon motion and the plasma velocity is less than gamma^{-1} , where gamma is the bulk Lorentz factor. Under the assumption that the center of the photon beam is parallel to the bulk motion, we calculate the scattering rate as a function of the angular spread of the beam and gamma. Magnetization changes the photon recoil, without which stimulated scattering has no effect. It also introduces a strong dependence on frequency and polarization: if the photon frequency matches the electron cyclotron frequency, the scattering rate of photons polarized perpendicular to the magnetic field can be substantially enhanced relative to Thomson, and if the photon frequency is much less than the cyclotron frequency the scattering is suppressed. Applying these calculations to pulsars, we find that stimulated scattering of the radio beam in the magnetized wind believed to exist outside the light cylinder can substantially alter the spectrum and polarization state of the radio signal. We suggest that the scattering rate is so high in some pulsars that the ability of the radio signal to penetrate the pulsar magnetosphere requires modification of either the conventional model of the magnetosphere or assumptions about the effects of stimulated scattering upon a beam. In the second case, we present a model of the radio emission from synchrotron self-absorbed sources, including the effects of induced Compton scattering by the relativistic electrons in the source. Order of magnitude estimates show that stimulated scattering becomes the dominant absorption process when (kTB/m ec^2)tau_{T }_sp{~}> 0.1. Numerical simulations demonstrate that relativistic induced Compton scattering limits the brightness temperature of a self-absorbed synchrotron source to T_ {B} _sp{~ }< few times 1011K and can significantly flatten the radio spectrum. The radio spectrum of the core of 3C279 is well matched by a model in which stimulated scattering is important, and the additional constraint T_{B } _sp{~}< 2 times 10 11K may be important to the interpretation of the broad band spectra in variable extra -galactic compact radio sources (Landau, et al. 1986). Stimulated scattering reduces the amplitude of the radio frequency variability relative to the x-ray variability, an effect which can be detected by multi-wavelength variability studies. Data for a sample of resolved compact radio cores (Ghisellini, et al. 1993) indicate that it is inconsistent to neglect induced Compton scattering when inferring the physical parameters of the sources. The necessary generalizations to the standard synchrotron self-Compton theory are presented. Relativistic induced Compton scattering is very sensitive to the number of mildly relativistic electrons in the source, and so may be a useful probe of this portion of the electron energy distribution.
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