Extreme Ultraviolet Photometry and Spectroscopy of BL Hydri

Abstract

We have analyzed Extreme Ultraviolet Explorer (EUVE) observations of BL Hyi obtained during a high-accretion state in 1995 October. The light curve obtained with the Deep Survey instrument reveals a broad hump covering the phases in which both poles have been known to be active in X-ray emission. In contrast to the X-ray, both poles contribute a comparable extreme ultraviolet (EUV) flux. The short-wavelength spectrum can be fitted with either a relatively cool (5 eV), pure-hydrogen white dwarf or with a 17 eV blackbody; both fits imply a neutral hyrogen column density in the local interstellar medium of ~3 × 10¹⁹ cm^-2. However, nearly simultaneous IUE far-ultraviolet (FUV) spectroscopic measurements rule out pure-hydrogen white dwarf models with temperatures of less than 8 eV. BL Hyi has a cooler accretion area than AM Her and VV Pup, but it is similar in temperature to QS Tel. The total EUV energy is 2-4 × 10³² ergs s^-1, which is only slightly greater than the hard X-ray and cyclotron energy. This suggests that BL Hyi has a conventional accretion column and that the polar cap is primarily heated from above by radiation from the accretion shock, rather than from below by dense blobs that penetrate the photosphere.