Abstract
Atoll-type neutron star (NS) X-ray binaries share lots of properties with low/hard state black hole candidates (BHC) and are therefore the key sources for our understanding of the differences and similarities in the disc-jet connection of BHC and NS systems and the physical processes involved. We present the analysis of simultaneous radio (VLA) and X-ray (RXTE) observations of three atoll sources: 4U 1728–34, 4U 1820–30 and Ser X-1 (first radio detection). In 4U 1728–34 we find a significant correlation between radio and X-ray flux and, for the first time in an X-ray binary, a significant quantitative correlation between radio flux and X-ray timing features. As in BHCs it seems that also in atolls (i.e. in 4U 1820-30, Ser X-1 and in one observation of 4U 1728-34) the radio emission is 'quenched' above a certain luminosity, when the sources are in a softer state.
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