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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

Universitäts-Sternwarte Göttingen, 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