The ultraviolet spectrum of the magnetic cataclysmic variable RX J1802.1+1804

Abstract

We observed the counterpart to the X-ray source RX J1802.1+1804, an object recently discovered as part of a survey to identify the optical counterparts to ultrasoft X-ray sources in the ROSAT point-source catalog that was subsequently identified as a magnetic cataclysmic variable, with the ultraviolet spectrometers on IUE in low dispersion mode. We report on the results of these observations and interpret our data within the context of other UV spectroscopic studies of magnetic CVs (polars). RX J1802.1+1804 is a relatively bright UV source, with a UV-to-optical flux ratio and emission-line spectrum similar to other well-studied CVs. Our observations insufficiently sample the binary orbit to discern any phase-dependent effects. We found RX J1802.1+1804 to have an unusually large He II λ2733 emission-line strength, and we have also made probable detections of weaker He II lines at 2511, 3203, and possibly 2386Å, in addition to the more commonly detected He II at 1640Å. We suggest that RX J1802.1+1804 is thus anomalous in terms of its overall He II emission-line spectrum among the sample. Several lines of O III λλ3047 and 3133 are apparently detected as well, which is unusual for CVs at the signal-to-noise ratio levels obtainable with IUE long-wavelength spectrograph. Given the seemingly rich spectrum of helium and oxygen emission lines, and the known relationship between He II Lyα and the oxygen lines, i.e., the Bowen fluorescence mechanism, we can, in principle, constrain the size and thermodynamics of the emitting region. This analysis suggests that R~10⁹ cm and n_e~10¹¹ cm⁻³. This physical scale is consistent with the line-emission originating in the accretion column rather than in a hot spot on the white dwarf surface.