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Ionisation Structure in Accretion Shocks with a Composite Cooling Function.pdf

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Ionisation Structure in Accretion Shocks with a Composite Cooling Function

a r X i v : a s t r o - p h / 0 1 0 6 0 5 2 v 1 4 J u n 2 0 0 1 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 1 February 2008 (MN LATEX style file v1.4) Ionisation Structure in Accretion Shocks with a Composite Cooling Function Kinwah Wu1,2, Mark Cropper2 Gavin Ramsay2 1 Research Centre for Theoretical Astrophysics, School of Physics A28, University of Sydney, NSW 2006, Australia 2 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT Received: ABSTRACT We have investigated the ionisation structure of the post-shock regions of magnetic cataclysmic variables using an analytic density and temperature structure model in which effects due to bremsstrahlung and cyclotron cooling are considered. We find that in the majority of the shock-heated region where H- and He-like lines of the heavy elements are emitted, the collisional-ionisation and corona-condition approximations are justified. We have calculated the line emissivity and ionisation profiles for Iron as a function of height within the post-shock flow. For low-mass white dwarfs, line emission takes place near the shock. For high-mass white dwarfs, most of the line emission takes place in regions well below the shock and hence it is less sensitive to the shock temperature. Thus, the line ratios are useful to determine the white-dwarf masses for the low-mass white dwarfs, but the method is less reliable when the white dwarfs are massive. Line spectra can, however, be used to map the hydrodynamic structure of the post-shock accretion flow. Key words: accretion — shock waves — X-rays: stars — stars: white dwarfs – methods: data analysis 1 INTRODUCTION When a supersonic accretion flow abruptly becomes sub- sonic near the surface of a compact star, a stand-off shock is formed. For the white dwarfs in magnetic cataclysmic vari- ables (mCVs) (see Warner 1995), the accretion-shock tem- perature Ts = 3 GμmHMw/8kRw (if xs Rw), where G is the gravitational consta