X-Ray Energy Spectra of the Supersoft X-Ray Sources CAL 87 and RX J0925.7-4758 Observed with ASCA

Details X-Ray Energy Spectra of the Supersoft X-Ray Sources CAL 87 and RX J0925.7-4758 Observed with ASCA X-Ray Energy Spectra of the Supersoft X-Ray Sources CAL 87 and RX J0925.7-4758 Observed with ASCA

Apr 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001apj...550.1007e&link_type=abstract

The Astrophysical Journal, Volume 550, Issue 2, pp. 1007-1022.

Astronomy and Astrophysics

Astronomy

15

Accretion, Accretion Disks, Stars: Individual: Alphanumeric: Cal 87, Stars: Individual: Alphanumeric: Rx J0925.7-4758, Stars: White Dwarfs, X-Rays: Stars

Scientific paper

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 (ΔE/E~10% at 1 keV) relative to previous instruments, we could study detailed X-ray spectral structures of these sources for the first time. We have applied theoretical spectral models to CAL 87 and constrained the white dwarf mass and intrinsic luminosity as 0.8-1.2 Msolar and 4×1037-1.2×1038 ergs s-1, respectively. However, we have found the observed luminosity is an order of magnitude smaller than the theoretical estimate, which indicates that 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 ~1.5×1023 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, ~1010 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 RX J0925.7-4758. These values are only possible for an extremely heavy white dwarf near the Chandrasekhar limit. Although the supersoft source luminosity should be ~1038 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 ~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 Thomson scattering.

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