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Observationally, for neutron star low-mass X-ray binaries, so far, thecorrelation between the radio luminosity $L_{\rm R}$ and the X-ray luminosity$L_{\rm X}$, i.e., $L_{\rm R}\propto L_{\rm X}^{\beta}$, has been reasonablywell-established only in three sources 4U 1728-34, Aql X-1 and EXO 1745-248 intheir hard state. The slope $\beta$ of the radio/X-ray correlation of the threesources is different, i.e., $\beta \sim 1.4$ for 4U 1728-34, $\beta \sim 0.4$for Aql X-1, and $\beta \sim 1.6$ for EXO 1745-248. In this paper, for thefirst time we explain the different radio/X-ray correlation of 4U 1728-34, AqlX-1 and EXO 1745-248 with the coupled advection-dominated accretion (ADAF)-jetmodel respectively. We calculate the emergent spectrum of the ADAF-jet modelfor $L_{\rm X}$ and $L_{\rm R}$ at different $\dot m$ ($\dot m=\dot M/\dotM_{\rm Edd}$), adjusting $\eta$ ($\eta \equiv \dot M_{\rm jet}/\dot M$,describing the fraction of the accreted matter in the ADAF transferedvertically forming the jet) to fit the observed radio/X-ray correlations. Thenwe derive a fitting formula of $\eta$ as a function of $\dot m$ for 4U 1728-34,Aql X-1 and EXO 1745-248 respectively. If the relation between $\eta$ and $\dotm$ can be extrapolated down to a lower value of $\dot m$, we find that in awide range of $\dot m$, the value of $\eta$ in Aql X-1 is greater than that ofin 4U 1728-34 and EXO 1745-248, implying that Aql X-1 may have a relativelystronger large-scale magnetic field, which is supported by the discovery of thecoherent millisecond X-ray pulsation in Aql X-1.

A model for the radio/X-ray correlation in three neutron star low-mass X-ray binaries 4U 1728-34, Aql X-1 and EXO 1745-248