ASCA observation of the supersoft X-ray source CAL 87

We have investigated the energy spectrum and the orbital modulation of the supersoft X-ray source CAL 87 using the ASCA satellite. We have detected a deep absorption edge at 0.85(-0.01)(+0.02) keV superposed on a continuum that can be approximated with an absorbed blackbody (kT = 0.05(-0.01)(+0.06) keV). The edge structure may be produced by the highly ionized species of O VIII and O VII. Such deep edge structures from highly ionized heavy elements are expected for X-ray emission from a white dwarf with an envelope of stable hydrogen burning and strongly supports the model that a white dwarf with a large mass accretion rate is involved in the supersoft X-ray sources. Epoch-folding analysis reveals the presence of a partial X-ray eclipse with broad wings at orbital phase 0; the eclipse covers a quarter of the orbital phase and the X-ray flux decreases down to similar to 40% of the out-of-eclipse level. We discuss the possible cause of the partial eclipse, including the extended emission region due to the accretion disk corona, and the X-ray scattering site formed by the gas escaping from the binary system, where the gas is highly ionized by the large X-ray luminosity from the white dwarf.