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

• Published in: Monthly notices of the Royal Astronomical Society 2017-04, Vol.466 (1), p.1111-1117
• Main Authors: Wang, D. H., Zhang, C. M., Lei, Y. J., Chen, L., Qu, J. L., Zhi, Q. J.
• Format: Article
• Language: English
• Subjects: Accretion; Accretion disks; Astronomy; Binary stars; Clusters; Double stars; Emission; Emissions; Magnetic fields; Neutron stars; Symbols
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw3134

We analyse 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 co-rotation 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 per cent, 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 per cent 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 the 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 (