Discovery of variable circular polarization from the remnant of Nova Chamaeleontis 1953 (RR Cha)

Abstract

We report on the discovery of variable circular polarization from the remnant of Nova Chamaeleontis 1953 (RR Cha). The circular polarization appears to be modulated on the primary's spin period and harmonics of the positive superhump period, with an amplitude peak-to-peak of almost ~10 per cent with both negative and positive polarization. A recent study by Woudt & Warner of RR Cha shows it to have both positive and negative superhumps, indicating the presence of a precessing/tilted accretion disc. In addition, they also find a stable period at 1950 s, characteristic of an Intermediate Polar. RR Cha also shows deep eclipses with variable depth. We propose a possible explanation for the origin of the polarized emission. We assume that it is of cyclotron origin. The variations of the circular polarization with the proposed spin period are caused by the rotation of the compact object. The long period variability arise as a result of a precessing/tilted accretion disc, periodically obscuring either of two accretion regions on the surface of the white dwarf. We also point out several similarities of RR Cha with the SW Sex stars. The lack of a time resolved spectroscopic study prevents to make any conclusion regarding its possible SW Sex nature. Hence, such spectroscopic study is encouraged. RR Cha is the second old nova exhibiting variable circular polarization and we therefore encourage polarimetric observations of other nova remnants.