ROSAT Observations of An-Ursae and Mr-Serpentis - the Status of the Soft X-Ray Excess in Am-Herculis Stars

Details ROSAT Observations of An-Ursae and Mr-Serpentis - the Status of the Soft X-Ray Excess in Am-Herculis Stars ROSAT Observations of An-Ursae and Mr-Serpentis - the Status of the Soft X-Ray Excess in Am-Herculis Stars

Oct 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994mnras.270..692r&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Vol. 270, NO. 3/OCT1, P. 692, 1994

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88

Scientific paper

We present the results of ROSA T pointed observations of the AM Her stars AN UMa and MR Ser. AN UMa was in a high-brightness state. We examine its soft X-ray light curve, and fit its spectrum with a two-component model consisting of an absorbed soft blackbody and a harder thermal bremsstrahlung distribution. MR Ser was found to be in a low-brightness state, and had a spectrum which was well fitted with a single-component, thermal bremsstrahlung model. The question of the energy balance in AM Her stars is reviewed with a view to quantifying the incidence of the `soft X-ray excess'. The excess has been explained as due to the deposition of energy directly into the white dwarf photo sphere by the accretion stream, supplementing the energy of photons from the hot accretion column which irradiate the photo sphere. We have calculated the energy balance in AN UMa and 16 other AM Her stars observed with ROSAL We find that, although the majority of AM Her stars exhibit a soft X-ray excess, a number of objects are consistent with a model which includes radiative heating only. We examine possible factors which could determine the value of the soft X-ray excess. We find a correlation of soft X-ray excess with magnetic field strength or, alternatively, the radius at which material couples on to the magnetic field lines. Key words: stars: individual: AN UMa - stars: individual: MR Ser - stars: magnetic fields - stars: variables: other - X-rays: stars.

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