X-ray energy spectra of the supersoft X-ray sources CAL 87 and RX J0925.7-4758 observed with ASCA

We report observation results of the supersoft X-ray sources CAL 87 and RX J0925.7-4758 with the X-ray CCD cameras (Solid-State Imaging Spectrometers [SISs]) on board ASCA. Because of the superior energy resolution of the SIS (DeltaE/E similar to 10% at 1 keV) relative to previous instruments, we co uld study detailed X-ray spectral structures of these sources for the first time. We have applied theoretical spectral models to CAL 87 and constraine d the white dwarf mass and intrinsic luminosity as 0.8-1.2 M. and 4 x 10(37 )-1.2 x 10(38) ergs s(-1), respectively. However, we have found the observe d luminosity is an order of magnitude smaller than the theoretical estimate , which indicates that the white dwarf is permanently blocked by the accret ion disk, and we are observing a scattering emission by a fully ionized acc retion disk corona (ADC) whose column density is similar to1.5 x 10(23) cm( -2). Through simulation we have shown that the orbital eclipse can be expla ined by the ADC model, such that a part of the extended X-ray emission from the ADC is blocked by the companion star filling its Roche lobe. We have f ound that very high surface gravity and temperature, similar to 10(10) cm s (-2) and similar to 100 eV, respectively, as well as a strong absorption ed ge at similar to1.02 keV, are required to explain the X-ray energy spectrum of RX J0925.7-4758. These values are only possible for an extremely heavy white dwarf near the Chandrasekhar limit. Although the supersoft source lum inosity should be similar to 10(38) ergs s(-1) at the Chandrasekhar limit, the observed luminosity of RX J0925.7-4758 is nearly 2 orders of magnitude smaller, even assuming an extreme distance of similar to 10 kpc. To explain the luminosity discrepancy, we propose a model in which very thick matter that was previously ejected from the system, as a form of jets, intervenes the line of sight and reduces the luminosity significantly because of Thoms on scattering.