Quasi-periodic oscillations and noise in neutron star and black-hole X-ray binaries

Before the launch of the Rossi X-ray Timing Explorer (RXTE) satellite, the differences in the rapid X-ray variability between the two main types of neutron star binaries (i.e., the Z and atoll sources) could be explained by invoking different mass accretion rates and magnetic field strengths. However, the results obtained with RXTE now show that these systems are more similar than previously thought and although differences in mass accretion rate are still likely, the differences in the magnetic field strength have become questionable. The great similarities between the neutron star systems and the black-hole candidates at low mass accretion rates also point towards a similar origin of their timing phenomena indicating that the presence or absence of a solid surface, a magnetic field, or an event horizon do not play a significant role in the production mechanisms for the rapid X-ray variability. NEUTRON STAR SYSTEMS: THE PRE-RXTE VIEW Before the start of the RXTE mission, the neutron star (NS) low-mass X-ray binaries (LMXBs) had been intensively studied with previous X-ray instruments (i.e., EXOSAT and Ginga). The introduction of X-ray color-color diagrams (CDs; e.g., Hasinger & van der Klis 1989) proved to be extremely useful for the study of the correlations between the changes in X-ray spectrum and X-ray timing behavior of these systems. On the basis of this correlated behavior, they were classified into the Z sources and the atoll sources (Hasinger & van der Klis 1989). The Z sources trace out a Z shaped track in the CD (see Fig. 1) with the branches labeled, from top to bottom, the horizontal branch (HB), the normal branch (NB), and the flaring branch (FB). The power spectra show (Fig. 1) on the horizontal branch, strong band-limited noise (called low frequency noise or LFN) which cuts off below several Hertz, simultaneous with 15–60 Hz quasi-periodic oscillations (QPOs), which are called horizontal branch oscillations (HBOs). On the normal branch these QPOs can still be seen, often simultaneous with other 5–7 Hz QPOs, which are called normal branch oscillations (NBOs). The NBOs smoothly merge with the 7–20 Hz QPOs seen on the flaring branch, the flaring branch oscillations (FBOs). On all branches also two other noise components are found, one at very low frequencies (the very low frequency noise or VLFN), following a power law, and one at frequencies above 10 Hz (the high frequency noise or HFN), which cuts off between 50 and 100 Hz. Motion of the source along the Z track is thought (e.g., Hasinger & van der Klis 1989) to be due to variations in the mass accretion rate (Ṁ ), which is lowest on the horizontal branch, increasing on the normal branch onto the flaring branch, where it reaches the Eddington limit Ṁ (ṀEdd). The atoll sources trace out a curved branch in the CD (Fig. 1), which can be divided in the island state (IS) and the banana branch (sub-divided into the lower banana [LB] and upper banana [UB] branch). The power spectrum in the island state (Fig. 1) is dominated by very strong (sometimes more than 20% rms amplitude) band-limited noise, superimposed on which are broad bumps (sometimes called QPOs). The band-limited noise is also called HFN but it is at much lower frequencies then the HFN observed in the Z sources and most likely they are not related. In the power spectrum on the banana branch only a weak (several percent rms amplitude) power law noise