Quasi-Periodic Oscillations in Galactic Bulge Sources Observed by Hakucho

Abstract

X-ray sources referred to as galactic bulge sources generally show no eclipses and no periodic pulsations. However, binary periods in some of them have been discovered. Sco X-1 is the first source of this class with a binary period of 0.787 day (Gottlieb et al, 1975; Cowley and Crampton,1975). The short binary periods of this class of sources are consistent with a model of Joss and Rappaport (1979) wherein the source is a neutron star in a binary system with a very low-mass companion (≲ 0.5 M_⊙). A much longer binary period has been discovered for Cyg X-2 (9.843 days) by optical spectroscopic observations (Cowley et al,1979). Such a long binary period is not consistent with the low-mass close binary model of Joss and Rappaport (1979); rather, it suggests that the primary is a somewhat evolved low-mass star (Cowley et at,1979).