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

Abstract

Observationally, for neutron star low-mass X-ray binaries, so far, the correlation between the radio luminosity $L_{\mathrm{R}}$ and the X-ray luminosity $L_{\mathrm{X}}$, i.e., $L_{\mathrm{R}}\propto L_{\mathrm{X}}^{\beta }$, has been reasonably well-established only in three sources 4U 1728-34, Aql X-1 and EXO 1745-248 in their hard state. The slope $\beta$ of the radio/X-ray correlation of the three sources 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 the first time we explain the different radio/X-ray correlation of 4U 1728-34, Aql X-1 and EXO 1745-248 with the coupled advection-dominated accretion (ADAF)-jet model respectively. We calculate the emergent spectrum of the ADAF-jet model for $L_{\mathrm{X}}$ and $L_{\mathrm{R}}$ at different $\dot{m}$ ($\dot{m}=\dot{M}/{\dot{M}}_{\mathrm{Edd}}$), adjusting $\eta$ ($\eta \equiv {\dot{M}}_{\mathrm{jet}}/\dot{M}$, describing the fraction of the accreted matter in the ADAF transfered vertically forming the jet) to fit the observed radio/X-ray correlations. Then we 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 $\dot{m}$ can be extrapolated down to a lower value of $\dot{m}$, we find that in a wide range of $\dot{m}$, the value of $\eta$ in Aql X-1 is greater than that of in 4U 1728-34 and EXO 1745-248, implying that Aql X-1 may have a relatively stronger large-scale magnetic field, which is supported by the discovery of the coherent millisecond X-ray pulsation in Aql X-1.

arXiv comment8 pages, 5 figures; Accepted for publication by MNRAS

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