X-ray and optical observations of polars

Abstract

The soft excess is a spectral feature shown by many polar cataclysmic variables, resembling a very luminous blackbody in soft X-rays. It is thought to arise from the accretion of discrete blobs of gas onto the WD, and was found in all polars discovered prior to the end of the ROSAT mission. All polars discovered since then seem to lack it, and the reason is still unknown. Here we present an XMM-Newton study of four polars discovered optically in recent years. All four lack the soft excess, and we test the hypothesis that the sought-after spectral feature might be concealed in the very soft X-ray regime unobservable by XMM-Newton.