Using EUV photometry obtained with the Extreme Ultraviolet Explorer ( EUVE ) satellite and UBVR optical photometry obtained with the 2.7-m telescope at McDonald Observatory , we have detected quasi-coherent oscillations ( so-called “ dwarf nova oscillations ” ) in the EUV and optical flux of the dwarf nova SS Cygni during its 1996 October outburst .
There are two new results from these observations .
First , we have for the first time observed “ frequency doubling : ” during the rising branch of the outburst , the period of the EUV oscillation was observed to jump from 6.59 s to 2.91 s. Second , we have for the first time observed quasi-coherent oscillations simultaneously in the optical and EUV .
We find that the period and phase of the oscillations are the same in the two wavebands , finally confirming the long-held assumption that the periods of the optical and EUV/soft X-ray oscillations of dwarf novae are equal .
The UBV oscillations can be simply the Rayleigh-Jeans tail of the EUV oscillations if the boundary layer temperature kT _ { bb } { \lower 3.225 pt \hbox { $ < $ } \atop \raise 2.15 pt \hbox { $ \sim$ } } 15 eV and hence the luminosity L _ { bb } { \lower 3.225 pt \hbox { $ > $ } \atop \raise 2.15 pt \hbox { $ \sim$ } } 1.2 % \times 10 ^ { 34 } ( d / { 75 ~ { } pc } ) ^ { 2 } ~ { } erg~ { } s ^ { -1 } ( comparable to that of the accretion disk ) .
Otherwise , the lack of a phase delay between the EUV and optical oscillations requires that the optical reprocessing site lies within the inner third of the accretion disk .
This is strikingly different from other cataclysmic variables , where much or all of the disk contributes to the optical oscillations .