| I. The spectra of Type IIn supernovae indicate the presence of a pre-existing slow, dense circumstellar wind (CSW). If the CSW extends sufficiently far from the progenitor star, then dust formation should occur in the wind. The light from the supernova explosion will scatter off this dust and produce a light echo. We calculate the brightness of the light echo as a function of time for a range of dust shell geometries, and use our calculations to fit to the light curves of SN 1988Z and SN 1997ab, the two slowest declining IIn supernovae on record. While we find that the light curves of both objects can be reproduced by the echo model, more subtle properties of the emission cannot. We conclude that light echoes cannot properly account for the slow decline seen in some IIn's, and that a shock interaction is likely to dominate the continuum emission.; II. IT is currently not known whether the Universe has an intergalactic magnetic field IGMF If such a field does exist, it would have had an effect on a wide range of astrophysical processes. High energy (>20 GeV) photons emitted by cosmologically distant blazars will pair-produce with photons from the IR/optical background. These pairs cool by Compton upscattering cosmic microwave background (CMB) and optical photons, producing an extended photon halo about a high energy source as a result. If an IGMF is present, the electrons and positrons will be deflected by the Lorentz force as they traverse intergalactic space and the size of the extended halo will increase. We calculate the effect of the IGMF on the expected cascade halo and show that this effect could be used to improve the current constraints on the IGMF by as much as 13 orders of magnitude.; III. We present the results of a search for an X-ray cascade halo in a Chandra observation of the region surrounding the blazar 1156 + 295. We show that the observed emission is consistent with a point source, i.e. no cascade halo is observed. While firm predictions are hard to make, estimates for the intrinsic TeV flux of the blazar from broadband spectral fits imply that the IGMF should exceed 10-21 G to explain the non-detection. This implies that some form of field amplification must have occurred in the Universe.; IV. Polarization effects are likely to be important in calculations involving induced (simulated) Compton scattering. The non-linear coupling to the radiation field, however, complicates the introduction of these effects into an equation of radiative transfer. A quantum mechanical derivation is presented for the case of low frequency radiation scattering off cold electrons. We reproduce, with a slight modification, the equation of Wilson (1982). We proceed to generalize the result of Wilson by relaxing the assumptions of low photon energy by considering the problem in a relativistic framework. |
