Optical Polarization of the Supernova in NGC 5253

Abstract

IF a type I supernova is the result of the explosion of a star with a degenerate nucleus (see, for example, Baglin¹) it is likely that a large magnetic field will be present in the radiating region for a short period following the outburst. Depending on the emission process a sufficiently large field could produce a measurable optical circular polarization. For example, for bremsstrahlung in an optically thick but very dilute plasma in an ordered magnetic field of H gauss, the fractional circular polarization in the optical continuum is q ∼ 10⁻⁹ H (see refs. 2 and 3). If the degenerate progenitor of the supernova is entirely destroyed in the explosion, conservation of magnetic flux dictates that q would reach the limit of detectability (q_LIM∼3 × 10⁻⁶ : H_LIM∼3,000 G) in only 20 s for either a white dwarf (initial radius r₀∼10⁹ cm, initial field H₀∼10⁶ G) or a neutron star (r₀∼10⁶ cm, H₀∼10¹² G) assuming an expansion velocity of 10⁴ km s⁻¹. If a sizable fragment survives, however, a measurable polarization might be seen for a much longer period. For synchrotron emission smaller fields can produce a detectable polarization. According to Legg and Westfold⁴ a hypothetical field H∼10⁶ G in the Crab nebula would produce q∼10% in the visible: since q ∝ H^½ a polarization q_LIM∼3 × 10⁻⁶ corresponds to H_LIM ∼ 10⁻³ G. However, the precise conditions strongly influence^5,6 the relations between q and H; the interpretation would therefore be uncertain even if this mechanism were shown to be dominant. The influence of radiative transfer in the dense layers near the star and of a possible fluorescence emission in the outer layers of the shell⁷ would make the interpretation for the synchrotron process even more difficult.