SIMBAD references

Parallel tracks in the relation between the lower kilohertz quasi-periodic oscillation (kHz QPO) frequency ν₁ and the X-ray intensity I have been observed in the neutron star low-mass X-ray binary (NS-LMXB) 4U 1728-34, and we try to ascribe these phenomena to the bimodal accretion modes, i.e. the disc accretion and radial accretion. For the source with one constant radial accretion rate {dot}M_ r_, as the increase of the disc accretion rate {dot}M_ d_, the NS magnetosphere-disc radius can shrink from ∼18 to ∼16 km, while ν₁ increases from ∼670 to ∼870 Hz. Then, due to the increase of the kHz QPO frequency and total X-ray intensity I_tot, the source will trace out an oblique track in ν₁-I relation. While, if the source enters another constant radial accretion rate due to the variation of the accretion environment, the increase of the disc accretion rate can arise another parallel track in ν₁-I relation. Finally, the reliability of the theoretical assumptions, the variability of the accretion radius and accretion rate, as well as the dependence of the two accretion modes, are also investigated and discussed.