Correlation between radio luminosity and X-ray timing frequencies in neutron star and black hole X-ray binaries

We report on correlations between radio luminosity and X-ray timing features in X-ray binary systems containing low magnetic field neutron stars and black holes. The sample of neutron star systems consists of 4U 1728-34, 4U 1820-34, Ser X-1, MXB 1730-335, GX 13+1, the millisecond X-ray pulsars SAX J1808.4-3658 and IGR J00291+5934, and these are compared with the black hole system GX 339-4. The analysis has been done using data from pointed observations of the Rossi X-ray Timing Explorer coordinated with radio observations. In the neutron star systems the radio luminosity L_{R} is correlated with the characteristic frequency of the L_{h} Lorentzian component detected contemporaneously in the power spectrum, and anticorrelated with its strength. Similarly, in the black hole system GX 339-4 L_{R} is correlated with the frequency of the L_{\ell} component in the power spectrum, and anti-correlated with its strength. The index of a power-law fit to the correlation is similar in both cases, L_{R} \propto \nu^{~1.4} and L_{R} \propto (rms)^{-2.3}. At lower timing frequencies, the radio luminosity is further found to be correlated with the characteristic (break) frequency of the L_{b} component of the power spectra in the neutron stars and, marginally, with the equivalent break frequency in GX 339-4. We briefly discuss the coupling between the innermost regions of the accretion disc and the production of the jet and, from the behaviour of the ms accreting X-ray pulsars, the possible role of the NS magnetic field.