X-ray Energy Spectra of the Super-soft X-ray Sources CAL87 and RXJ0925.7-4758 Observed with ASCA

Abstract

We report observation results of the super-soft X-ray sources (SSS) CAL87 and RXJ0925.7-4758 with the X-ray CCD cameras (Solid-state Imaging Spectrometer; SIS) on-board the ASCA satellite. We have applied theoretical spectral models to CAL87, and constrained the white dwarf mass and intrinsic luminosity as 0.8 - 1.2 M_solar and 4 x 10^{37}- 1.2 x 10^{38} erg s^{-1}, respectively. However, we have found the observed luminosity is an order of magnitude smaller than the theoretical estimate, which indicates the white dwarf is permanently blocked by the accretion disk, and we are observing a scattering emission by a fully ionized accretion disk corona (ADC) whose column density is ~ 1.5 x 10^{23} cm^{-2}. Through simulation, we have shown that the orbital eclipse can be explained 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 found that very high surface gravity and temperature, ~ 10^{10} cm s^{-2} and ~ 100 eV respectively, as well as a strong absorption edge at ~ 1.02 keV, are required to explain the X-ray energy spectrum of RXJ0925.7-4758. These values are only possible for an extremely heavy white dwarf near the Chandrasekhar limit. Although the super soft source luminosity should be ~ 10^{38} erg s^{-1} at the Chandrasekhar limit, the observed luminosity of RXJ0925.7-4758 is nearly two orders of magnitude smaller even assuming an extreme distance of ~10 kpc. To explain the luminosity discrepancy, we propose a model that very thick matter which was previously ejected from the system, as a form of jets, intervenes the line of sight, and reduces the luminosity significantly due to Thomson scattering.