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We have taken the kHz QPOs as a tool to probe the correlation between thetracks of X-ray color-color diagram (CCD) and magnetosphere-disk positions forthe atoll source 4U 1728-34, based on the assumptions that the upper kHz QPO isascribed to the Keplerian orbital motion and the neutron star (NS)magnetosphere is defined by the dipole magnetic field. We find that from theisland to the banana state, the inner accretion disk gradually approaches theNS surface with the radius decreasing from r ~33.0km to ~15.9 km, correspondingto the magnetic field from B(r) ~4.8*10^6 G to ~4.3*10^7 G. In addition, wenote the characteristics of some particular radii of magnetosphere-disk -r are:firstly, the whole atoll shape of the CCD links the disk radius range of ~15.9- 33.0 km, which is just located inside the corotation radius of 4U 1728-34-r_co ( ~34.4 km), implying that the CCD shape is involved in the NS spin-upstate. Secondly, the island and banana states of CCD correspond to the twoparticular boundaries: (I)-near the corotation radius at r ~27.2 - 33.0 km,where the source lies in the island state; (II)-near the NS surface at r ~15.9- 22.3 km, where the source lies in both the island and banana states. Thirdly,the vertex of the atoll shape in CCD, where the radiation transition from thehard to soft photons occurs, is found to be near the NS surface at r ~16.4 km.The above results suggest that both the magnetic field and accretionenvironment are related to the CCD structure of atoll track, where thecorotation radius and NS hard surface play the significant roles in theradiation distribution of atoll source.

The kHz QPOs as a probe of the X-ray color-color diagram and accretion-disk structure for the atoll source 4U 1728-34