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. (arXiv:1905.05996v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Qiao_E/0/1/0/all/0/1">Erlin Qiao</a> (NAOC), <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_B/0/1/0/all/0/1">B.F. Liu</a> (NAOC)
Observationally, for neutron star low-mass X-ray binaries, so far, the
correlation 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 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 $L_{rm X}$ and $L_{rm R}$ at different $dot m$ ($dot m=dot M/dot
M_{rm Edd}$), adjusting $eta$ ($eta equiv dot M_{rm jet}/dot M$,
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.
Observationally, for neutron star low-mass X-ray binaries, so far, the
correlation 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 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 $L_{rm X}$ and $L_{rm R}$ at different $dot m$ ($dot m=dot M/dot
M_{rm Edd}$), adjusting $eta$ ($eta equiv dot M_{rm jet}/dot M$,
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.
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