The Far‐Ultraviolet Spectrum of the Intermediate Polar EX Hydrae

Abstract

The intermediate polar EX Hya was observed in 1995 March with the Hopkins Ultraviolet Telescope on the Astro-2 space shuttle mission. The time-resolved, 820-1860 A data that were obtained are the first spectra of an intermediate polar to extend to the Lyman limit. They cover phases 67 = 0.05-0.50 of the 67 minute white dwarf rotational period. Fitted to a blackbody, the continuum has an effective temperature of about 25,000 K. At 67 = 0.0, the spectrum shows strong, broad (2500 km s-1 FWHM) lines of C IV λ1549, C III λ977, λ1176, N V λ1240, Si IV λ1394, and a blend of lines including Lyβ and O VI λ1034. The line ratios suggest a photoionized emission region with densities 1013 cm-3. As the phase increases to 0.5, the line fluxes drop by more than a factor of 4, but the continuum flux declines by only ~20%. There are ±30% fluctuations in the continuum flux, but not the lines, on timescales of less than a minute during the second half of the observation, which suggests that the lines and continuum arise in physically distinct regions of the accretion flow. These data are compatible with models of EX Hya in which the UV lines arise mainly in an optically thick accretion curtain and the UV continuum arises in the innermost part of the accretion curtain and/or the heated face of the white dwarf. The profile of the C IV λ1549 line changes considerably over the course of the observation, with a strong absorption appearing on the blue wing of the line from 67 = 0.36 to the end of the observation.