# Links between quasi-periodic oscillations and accretion states in neutron star low mass X-ray binaries

## TL;DR

This study systematically analyzes RXTE data to explore the relationship between quasi-periodic oscillations and accretion states in neutron star and black hole low-mass X-ray binaries, revealing strong similarities and some differences in their variability properties.

## Contribution

It extends the classification of QPOs from Z-sources to Atoll sources and compares NS and BH systems, highlighting the weak sensitivity of QPOs to the nature of the central object.

## Key findings

- HBOs, NBOs, and FBOs are also found in Atoll sources.
- NS and BH LMXBs show similar QPO properties.
- The association between kHz QPOs and high-frequency QPOs is not straightforward.

## Abstract

We analysed the Rossi X-ray Timing Explorer (RXTE) data from a sample of bright accreting neutron star (NS) low-mass X-ray binaries (LMXBs). With the aim of studying the quasi-periodic variability as a function of the accretion regime, we carried out a systematic search of the quasi-periodic oscillations (QPOs) in the X-ray time series of these systems, using the integrated fractional variability as a tracker for the accretion states. We found that the three QPO types originally identified in the '80s for the brightest LMXBs, the so-called Z-sources, i.e., horizontal, normal and flaring branch oscillations (HBOs, NBOs and FBOs, respectively), are also identified in the slightly less bright NS LMXBs, the so-called Atoll sources, where we see QPOs with a behaviour consistent with the HBOs and FBOs. We compared the quasi-periodic variability properties of our NS sample with those of a sample of black hole (BH) LMXBs. We confirm the association between HBOs, NBOs and FBOs observed in Z-sources, with the type-C, type-B and type-A QPOs, respectively, observed in BH systems, and we extended the comparison to the HBO-like and FBO-like QPOs seen in Atoll sources. We conclude that the variability properties of BH and weakly-magnetized NS LMXBs show strong similarities, with QPOs only weakly sensitive to the nature of the central compact object in both classes of systems. We find that the historical association between kHz QPOs and high-frequency QPOs, seen around NSs and BHs, respectively, is not obvious when comparing similar accretion states in the two kinds of systems.