EUV Observations of EX Hydrae: 107K Gas near a White Dwarf Surface

Abstract

We have observed the intermediate polar EX Hydrae for 180,000 s with the Extreme Ultraviolet Explorer. The EUV flux is strongly modulated at both the binary period (98 minutes) and the white dwarf spin period (67 minutes). The secondary star eclipse is total, with a duration of 41 ± 2 s. The ingress and egress of the secondary eclipse have a duration <3 s (68% confidence). The centroid of the secondary eclipse is independent of the white dwarf spin phase to within 6 s. The eclipse attributed to the disk bulge has an energy dependence consistent with photoelectric absorption through a neutral column of about 1.3 × 1020 cm-2. Folded on the white dwarf spin period, the EUV flux is modulated by a factor of 3.7. The shape of the light curve can be accurately matched with a simple geometrical absorption model, although the variation in modulation depth with energy (e.g., EUV vs. X-rays) requires a more sophisticated model. The EUV spectrum reveals many narrow emission lines characteristic of a plasma around 107 K, and possibly features indicating temperatures as low as 106 K. Assuming optically thin emission, we find a volume emission measure around 3 × 1054 cm-3 for the 107 K plasma, and an accretion rate around 3 × 1016 g s-1. By direct geometrical arguments we constrain the EUV emission region to a region extending over less than 4% of the white dwarf surface area located within 0.5 white dwarf radii (in the orbital plane) and 4.5 white dwarf radii (perpendicular to the plane) relative to the center of the white dwarf. Two independent arguments lead to the conclusion that the electron density is >1013 cm-3, probably >1015 cm-3, in the EUV-emitting region.