  Neutron-star and black-hole X-ray binaries (XRBs) exhibit radio jets, whose
properties depend on the X-ray spectral state and history of the source. In
particular, black-hole XRBs emit compact, steady radio jets when they are in
the so-called hard state, the jets become eruptive as the sources move toward
the soft state, disappear in the soft state, and re-appear when the sources
return to the hard state. On the other hand, jets from neutron-star X-ray
binaries are typically weaker radio emitters than the black-hole ones at the
same X-ray luminosity and in some cases radio emission is detected in the soft
state. Significant phenomenology has been accumulated so far regarding the
spectral states of neutron-star and black-hole XRBs, and there is general
agreement about the type of the accretion disk around the compact object in the
various spectral states. Our aim is to investigate whether the phenomenology
regarding the X-ray emission on one hand and the jet appearance and
disappearance on the other can be put together in a consistent physical
picture. It has been shown that the so-called Poynting-Robertson Cosmic Battery
(PRCB) explains in a natural way the formation of magnetic fields in the disks
of AGN and the ejection of jets. We investigate whether the PRCB can also
explain the formation, destruction, and variability of jets in XRBs. We find
excellent agreement between the conditions under which the PRCB is efficient
(i.e., the type of the accretion disk) and the emission or destruction of the
radio jet. The disk-jet connection in XRBs is explained in a natural way using
the PRCB.
