2018MNRAS.473.4862W

We investigate the correlation between the frequencies of twin kilohertz quasi-periodic oscillations (kHz QPOs) and neutron star (NS) spins in low-mass X-ray binaries (LMXBs), based on the data sets of 12 sources with simultaneously detected twin kHz QPOs and NS spins, and find that the histogram of the ratio between the frequency difference of twin kHz QPOs (Δν ≡ ν₂ - ν₁) and NS spin ν_s shows a non-uniform distribution with a gap at Δν/ν_s ∼ 0.65. We try to classify the 12 sources into two categories according to this gap: (I) slow rotators with <ν_s> ∼ 311 Hz, XTE J1807.4-294, 4U 1915-05, IGR J17191-2821, 4U 1702-43, 4U 1728-34 and 4U 0614+09 follow a relation Δν/ν_s > 0.65; (II) fast rotators with <ν_s> ∼ 546 Hz, SAX J1808.4-3658, KS 1731-260, Aql X-1, 4U 1636-53, SAX J1750.8-2900 and 4U 1608-52 satisfy the relation Δν/ν_s < 0.65. However, the linear fittings of Δν versus ν_s relations of groups (I) and (II) are unsatisfactory to ensure any certain correlations. We suggest that this phenomenon may arise from the fact that most measured kHz QPOs and spins satisfy the conditions of 1.1 ν_s <= ν₂ < 1300 Hz and Δν decreasing with ν₂. Apparently, the diversified distribution of Δν/ν_s refutes the simple beat-frequency model, and the statistical correlations between the twin kHz QPOs and NS spins may arise from the magnetosphere-disc boundary environments, e.g. co-rotation radius and NS radius, that modulate the occurrences of X-ray signals. Furthermore, we also find a distribution of the ratio of ν₂ to ν₁ clusters around the value of <ν₂/ν₁> ∼ 3: 2, which shows no obvious correlation with NS spins.