SIMBAD references

In this paper the detection and detailed analysis of orbital modulation in X-rays (with ROSAT ) is presented for two supersoft sources, the 4.1 hour SMC binary source 1E0035.4-7230 and the galactic classical nova GQ Mus with an orbital period of 85.5 minutes. These are besides Nova Cygni 1992 those supersoft binary sources for which the shortest orbital periods have been deduced. In both systems the X-ray light curve is strongly modulated, asymmetric and in phase with the optical orbital light curve. This could be in favor for a polar like system with a magnetic nature of the underlying white dwarf (WD). For 1E0035.4-7230 a correlation of the hardness ratio with the count rate has been found, which is consistent with a varying mean temperature of the X-ray emitting region from ∼4x10⁵K to ∼5x10⁵K. Such a temperature variation could be due to a changing aspect of an extended accretion column above the polar caps of the magnetic WD. Emission from this column may arise due to scattering and heating by the soft X-ray flux released in the envelope of the steadily nuclear burning WD. Alternatively the hardness ratio variation could be due to an absorbing hydrogen column of N_H<4x10²⁰cm^–2 attributed to cold matter distributed in the binary system (e.g. a wind driven from the donor star). In order to explain the intensity modulation a strongly ionized component with an electron density n_e∼10¹⁴cm^–3 is required. Only for GQ Mus has the magnetic nature of the WD been inferred from optical data and an optically thick accretion column is required in order to model the light curve. Strong orbital modulation in X-rays has also been found in the LMC supersoft source CAL 87 and moderate modulation in the galactic source RX J0019.8+2156. For further 5 supersoft sources which have a known orbital period no X-ray modulation has been detected. In total ∼30% of the supersoft binary systems show strong orbital modulation in X-rays and only for one system a polar like nature has been shown. As ∼20% of the close binary systems are expected to be eclipsing, these observations could also be explained by inclination effects and the modulation could be due to an extended atmosphere of the donor star, a strong wind driven from the donor or structure at the rim of the accretion disk. Further observations mainly in other wavelength regimes are required in order to test the proposed models.