Observations of quasi-coherent soft X-ray oscillations in U Geminorum and SS Cygni

Soft X-ray pulsations (0.1-0.5 keV) have been detected for the first time from U Geminorum and for the second time from SS Cygni using the HEAO 1 satellite. Both dwarf novae were on the declining phase of an optical outburst. The average amplitudes of the pulses are 15% for U Gem and 18% for SS Cyg. The pulsations are quasi-coherent in character. If the authors define P/sub c/ to be the centroid of the period distribution and P/sub 1/ to be the standard deviation of the pulsation period then during the U Gem observation P/sub c/ = 27 s and P/sub 1/ = 6 s, while during the SS Cyg observation P/sub c/ = 10.7 s and P/sub 1/ = 1.8 s. In a previous X-ray observation of SS Cyg, and oscillation with P/sub c/ = 8.8 s and P/sub 1/ = 0.4 s was detected, illustrating that significant changes in period and coherence may occur. A blackbody spectral fit to the U Gem data yields best fit parameters of kT approx. 25 eV, N/sub H/ less than or equal to 5 x 10/sup 18/ cm/sup -2/, and a 0.13-0.48 keV X-ray flux at the Earth of 3 x 10/supmore » -10/ ergs cm/sup -2/ s/sup -1/. From pulse phase X-ray spectroscopic data, an upper limit of 50% may be set on any variation in temperature across U Gem's pulse. There is no evidence for an X-ray variation correlated with the optical eclipse of this 4.25 hr binary system. The average soft X-ray flux of SS Cyg during the second HEAO 1 pointing observation is f/sub x/ approx. 2 x 10/sup -11/ ergs cm/sup -2/ s/sup -1/, one-half that detected in the first HEAO 1 soft X-ray pointing at this star. The authors review all the data on the X-ray and optical pulsations thus far detected in these two dwarf novae and discuss the observational constraints on various current models for the pulsations.« less