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

Abstract

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