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 performed using data from pointed observations of the Rossi X-ray Timing Explorer coordinated with radio observations. In the neutron star systems the radio luminosity LR is correlated with the characteristic frequency of the Lh Lorentzian component detected contemporaneously in the power spectrum, and anticorrelated with its strength. Similarly, in the black hole system GX 339-4 LR is correlated with the frequency of the Ll component in the power spectrum and anticorrelated with its strength. The index of a power-law fit to the correlation is similar in both cases, LR~ν~1.4 and LR~ (rms)-2.3. At lower timing frequencies, the radio luminosity is further found to be correlated with the characteristic (break) frequency of the Lb 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 millisecond accreting X-ray pulsars, the possible role of the neutron star magnetic field