Quasi-periodic oscillations in the Z source GX 5-1

Abstract

We report on a detailed study of the time variability in the Z source GX 5-1 using Ginga. The source was observed in the horizontal branch (HB) and in the normal branch (NB). Our five-day observations provide the most extensive continuous data set obtained to date. Intensity-dependent high-frequency quasi-periodic oscillations (QPO) (∼ 13–48 Hz) were observed in the HB in which the source flux varied by a factor of 1.9. At the low-intensity end of the hardness–intensity diagram, the HB shows an upward bend which is very strong in the colour–colour diagram (CCD). This bend forms essentially a new spectral branch, giving the CCD the shape of an ‘S’ since there is no flaring branch in GX 5-1. The QPO frequency in this portion of the HB ranges from ∼ 13 to 17 Hz which is lower than previously observed in any Z source. It is possible that other Z sources will also exhibit the sharp upward turn of the HB in the CCD if their intensities (and thus the QPO frequencies) become sufficiently low. The upward turn of the HB indicates that our understanding of the spectral dependence on the mass accretion rate is rather poor. High-frequency ( ∼ 48 Hz) and low-frequency ( ∼ 6 Hz) QPO were simultaneously observed in the NB. The strength of both forms of QPO and the strength of the low-frequency noise increase rapidly with increasing photon energy. We discuss our results in connection with existing models.