Recent Science Highlights

1 August 1995

(following information provided by Dr. Antonella Fruscione)

S. Rosen (Leicester), J. Mittaz (Mullard Space Science Laboratory), D. Buckley (South African Astronomical Observatory), A. Layden (Cerro Tololo Inter-American Observatory), C. McCain (Mt. Stromlo Observatory), J.P. Osborne (Leicester) and M. Watson (Leicester) report on the first detailed EUVE observations of the AM Her star, QS Telescopii.

• This AM Her binary contains a strongly magnetic white dwarf accreting material from a cool companion star. Our EUVE observations of the star found it in a bright state, displaying a double peaked EUV light curve (see figure), indicative of accretion onto two poles. This contrasts with the single peaked (single pole) light curves of earlier ROSAT observations and the very faint, weakly accreting state observed in early (IOC and survey) EUVE data. The change in accretion geometry is probably related to a higher mass transfer rate and may reflect the ability of dense filaments within the accretion flow to penetrate the magnetic field and reach the second pole which is inaccessible when the accretion rate (filament density) is lower.
• The EUVE data also uncovered a deep orbital dip in the light curve, most probably caused when the accretion flow between the stars occults the EUV emission region. The lack of a strong energy dependence in the dip profile disfavours photoelectric absorption by cold material but may be consistent with partial covering by opaque blobs/filaments in the flow.
• The EUVE spectrum is consistent with a low temperature (~15eV) blackbody-like distribution, suggesting a very large bolometric soft X-ray flux and hence a large ratio of the fluxes of the soft and hard spectral components. However, we also find tentative evidence of an edge at 85 Å and absorption lines at 98 Å and 116 Å due, perhaps, to NeVI, NeVIII and NeVII respectively. If confirmed, the range of ionization states observed and the absence of a strong OVI edge raises important questions about the heating mechanism in the accretion region.

Further monitoring of this star by EUVE and X-ray satellites may provide improved insights into the cause of accretion mode changes in AM Her stars and on the properties of the accretion flow.