Astron. Astrophys. 324, L73-L76
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Letter to the Editor
A 2.8-second quasi-coherent oscillation
in the soft
X-ray flux of the dwarf nova SS Cygni
André van Teeseling
Geismarlandstr. 11, D-37083 Göttingen, Germany
Received 23 May 1997 / Accepted 11 June 1997
During maximum optical brightness of a wide
outburst, the soft X-ray flux of the dwarf nova SS Cyg was measured
with the ROSAT satellite. The soft X-ray flux is modulated
with a highly coherent sinusoidal oscillation with a period near 2.8 s.
This is, by more than a factor of 2.5, the shortest period measured in a dwarf
nova and, if identified with the Kepler period, provides a lower limit for
the mass of the white dwarf in SS Cyg of 1.24 solar masses.
Except for the much shorter period and the higher phase stability, the
properties of the oscillation are reminiscent of oscillations observed
previously in SS Cyg, which suggests a similar physical origin.
The shorter period may be due to a significantly faster rotation, near
break-up, of the white-dwarf surface layers or a location in
the accretion disk much closer to the white dwarf than before.
Key words: accretion disks - stars: individual: SS Cygni - cataclysmic
variables - oscillations - white dwarfs - X-rays: stars
© European Southern Observatory (ESO) 1997