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 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
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.
Figure 1 (57 Kb post-script): The double peaked EUV (EUVE DS) light curve of QS Tel. In
earlier ROSAT observations, the peak between phases 0.35 and 0.85 was
absent, being replaced by a weak, flat residual signal. Note also the
deep dip between phases 1.0 and 1.1.
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