X-ray timing behaviour of Cygnus X-2 at low intensities

Abstract

It is known that the overall (mean) intensity of the low-mass X-ray binary Cyg X-2 varies on time scales from a day to months, independently from the variations on time scales of hours to a day by which the source moves between the horizontal, normal and flaring branches. We present RXTE PCA observations of Cyg X-2, taken when its overall intensity was near its lowest values, in October 1996 and September 1997. For the first time we perform a study of the fast timing behaviour at such low intensities. During the October 1996 observations the source was in the left part of the horizontal branch, and during the September 1997 observations most likely in the lower parts of the normal branch and flaring branch. We find that the properties of the very low frequency noise during the September 1997 observations are consistent with a monotonic decrease in its strength and power-law index as a function of overall intensity. In contrast, the strength of the ~6 Hz normal branch quasi-periodic oscillations do not vary monotonically with overall intensity. They are strongest at medium overall intensity and weaker both when the overall intensity is low and when the overall intensity is high.