Migliari, SimoneFender, R. P.Rupen, M. P.Wachter, S.Jonker, P. G.Homan, J.Van der Klis, M.2016-06-092016-06-092004-060035-8711https://hdl.handle.net/2445/99393We present the analysis of simultaneous X-ray (RXTE) and radio (VLA) observations of two atoll-type neutron star X-ray binaries: 4U 1820 - 30 and Ser X-1. Both sources were steadily in the soft (`banana') X-ray state during the observations. We have detected the radio counterpart of 4U 1820 - 30 at 4.86 and 8.46 GHz at a flux density of ~0.1 mJy. This radio source is positionally coincident with the radio pulsar PSR 1820 - 30A. However, the radio emission of the pulsar falls rapidly with frequency (~ν-3), and we argue that the radio emission of the X-ray binary is dominant above ~2 GHz. Supporting this interpretation, comparison with previous observations reveals variability at the higher radio frequencies that is likely to be due to the X-ray binary. We have detected for the first time the radio counterpart of Ser X-1 at 8.46 GHz, also at a flux density of ~0.1 mJy. The position of the radio counterpart has allowed us to identify its optical counterpart unambiguously. We briefly discuss similarities and differences between the disc-jet coupling in neutron star and black hole X-ray binaries. In particular, we draw attention to the fact that, contrary to other states, neutron star X-ray binaries seem to be more radio-loud than persistent black hole candidates when the emission is `quenched' in the soft state.7 p.application/pdfeng(c) Migliari, Simone et al., 2004Estels binaris de raigs XEstels de neutronsAstronomia de raigs XX-ray binariesNeutron starsX-ray astronomyinfo:eu-repo/semantics/article6340352016-06-09info:eu-repo/semantics/openAccess