ASCA, RXTE, EUVE, and optical observations of the high magnetic field cataclysmic variable AR Ursae Majoris

Abstract

The highest field polar AR UMa was observed with ASCA during a low state, and with simultaneous EUVE, RXTE, and ground-based optical photometry during a high state. The marginal detection at the low state places a limit on the hard X-ray emission, which is a factor of 5 below the high-state flux limit. The high-state EUV light curves are highly modulated with peak flux at phase 0.9, but the flux never entirely disappears, implying some view of a heated area at all times of the orbit. The spectra during bright phases suggest a temperature of 265,000 K while the fainter phases are cooler, ∼215,000 K, and the spectrum is hottest (320,000 K) during the stream dip phases. However, neither a blackbody nor a standard stellar atmosphere model provides a good fit to the data. In order for the EUV modulation to be consistent with geometrical parameters derived from polarimetry and spectroscopy, the primary heated area on the white dwarf must be geometrically extended from the southern magnetic pole while the northern pole is constantly in view. The temperatures of the heated regions are typical of polars, but the projected areas are small, which could be due to the lack of a good view of the main pole at the low inclination of the system. The RXTE light curve shows no modulation over the orbit and only a marginal detection in hard X-rays, implying a weak bremsstrahlung component that is typical for the highest magnetic field polars. In the optical, a low-amplitude sinusoidal modulation peaking at phase 0.5 is consistent with an origin from the irradiated secondary.