Probing the accretion disc structure by the twin kHz QPOs and spins of neutron stars in LMXBs

Abstract

We analyze the relation between the emission radii of twin kilohertz quasi-periodic oscillations (kHz QPOs) and the co-rotation radii of the 12 neutron star low mass X-ray binaries (NS-LMXBs) which are simultaneously detected with the twin kHz QPOs and NS spins. We find that the average co-rotation radius of these sources is r co ∼ 32 km, and all the emission positions of twin kHz QPOs lie inside the corotation radii, indicating that the twin kHz QPOs are formed in the spin-up process. It is noticed that the upper frequency of twin kHz QPOs is higher than NS spin frequency by 10%, which may account for a critical velocity difference between the Keplerian motion of accretion matter and NS spin that is corresponding to the production of twin kHz QPOs. In addition, we also find that ∼ 83% of twin kHz QPOs cluster around the radius range of 15 − 20 km, which may be affected by the hard surface or the local strong magnetic field of NS. As a special case, SAX J1808.4-3658 shows the larger emission radii of twin kHz QPOs of r ∼ 21 − 24 km, which may be due to its low accretion rate or small measured NS mass (< 1.4 M ⊙ ).

Exploring the Accretion Disk Structure and X-ray Radiation of GX 17+2 Based on kHz QPOs and Cross-correlations

“…It is noticed that NS is in the spin-up state when R m < R co (or ν K > ν s ), while it is in the spin-down state when R m > R co (or ν K < ν s ). To explore the magnetosphere-disk structures of NS-LMXBs and kHz QPOs, Wang et al (2017Wang et al ( , 2020 investigated the link of the twin kHz QPOs with NS spin-up state. Moreover, the authors also showed that the magnetosphere-disk radii of HB/NB in Z sources cluster around the range of ∼20-29 km/∼16-20 km, respectively.…”