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Astron. Astrophys. 346, 125-133 (1999)


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Constraining the spectral parameters of RX J0925.7-4758 with the BeppoSAX LECS

H.W. Hartmann 1, J. Heise 1, P. Kahabka 2, C. Motch 3 and A.N. Parmar 4

1 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
2 Astronomical Institute `Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3 Observatoire Astronomique, UMR 7550 CNRS, 11 rue de l'Université, F-67000 Strasbourg, France
4 Astrophysics Division, Space Science Department of ESA, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands

Received 13 November 1998 / Accepted 12 March 1999

Abstract

The Super Soft Source RX J0925.7-4758 (RX J0925 hereafter) was observed by BeppoSAX LECS and MECS on January 25-26 1997. The source was clearly detected by the LECS but only marginally detected by the MECS. We apply detailed Non-Local Thermodynamic Equilibrium (Non-LTE) models including metal line opacities to the observed LECS spectrum. We test whether the X-ray spectrum of RX J0925 is consistent with that of a white dwarf and put constraints upon the effective temperature and surface gravity by considering the presence or absence of spectral features such as absorption edges and line blends in the models and the observed spectrum.

We find that models with effective temperatures above [FORMULA] or below [FORMULA] can be excluded. If we assume a single model component for RX J0925 we observe a significant discrepancy between the model and the data above the Ne IX edge energy at 1.19 keV. This is consistent with earlier observations with ROSAT and ASCA. The only way to account for the emission above [FORMULA] is by introducing a second spectral (plasma) component. This plasma component may be explained by a shocked wind originating from the compact object or from the irradiated companion star.

If we assume [FORMULA] then the derived luminosity is consistent with that of a nuclear burning white dwarf at a distance of [FORMULA] kpc.

Key words: stars: atmospheres – stars: individual: RX J0925.7-4758 – stars: white dwarfs – X-rays: stars

Send offprint requests to: w.hartmann@sron.nl

© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999

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