X-ray and optical observations of the ultrashort period dwarf nova SW Ursae Majoris - A likely new DQ Herculis star

Abstract

Optical photometric and spectroscopic and X-ray observations of the dwarf nova SW UMa are presented. Moderate resolution spectroscopic data have established an orbital period of 81.8 min and have revealed that the Balmer emission lines have a multiple structure. A narrow component, similar to the s-wave component which is sometimes seen in cataclysmic variables, has been identified in SW UMa. The narrow component is almost in phase with the Balmer line wings, implying that the emission arises in a region of the disk facing away from the white dwarf. Photometric observations have revealed the presence of a hump in the light curve. Changes in the hump amplitude are seen from night to night and a 15.9 min periodicity is evident in the light curve. Low-energy EXOSAT observations show identical orbital and 15.9 min modulations. The presence of the 15.9 min optical and X-ray periodicities suggest that SW UMa, in addition to being a dwarf nova, is also a member of the DQ Her class of cataclysmic variables.