Jets in neutron star X-ray binaries: a comparison with black holes

We present a comprehensive study of the relation between radio and X-ray emission in neutron star X-ray binaries, use this to infer the general properties of the disc–jet coupling in such systems, and compare the results quantitatively with those already established for black hole systems. There are clear qualitative similarities between the two classes of object: hard states below about 1% of the Eddington luminosity produce steady jets, while transient jets are associated with outbursting and variable sources at the highest luminosities. However, there are important quantitative differences: the neutron stars are less radio-loud for a given X-ray luminosity (regardless of mass corrections), and they do not appear to show the strong suppression of radio emission in steady soft states which we observe in black hole systems. Furthermore, in the hard states the correlation between radio and X-ray luminosities of the neutron star systems is steeper than the relation observed in black holes by about a factor of two. This result strongly suggests that the X-ray emission in the black hole systems is radiatively inefficient, with an approximate relation of the form LX ∝ ṁ, consistent with both advection-dominated models and jet-dominated scenario. On the contrary the jet power in both classes of object scales linearly with accretion rate. This constitutes some of the first observational evidence for the radiatively inefficient scaling of X-ray luminosity with accretion rate in accreting black hole systems. Moreover, based on simultaneous radio/X-ray observations of Z-type neutron stars (the brightest of our galaxy, always near or at the Eddington accretion rate), we draw a model that can describe the disc-jet coupling in such sources, finding a possible association between a particular X-ray state transition (horizontal brach-to-normal branch) and the emission of transient jets.