Radio detections of the neutron star X-ray binaries 4U 1820-30 and Ser   X-1 in soft X-ray states
  We 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 GHz 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 pulsar's radio emission falls rapidly with frequency (proportional to \nu^(-3)) and we argue that the X-ray binary's radio emission 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 unambiguously identify its optical counterpart. 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. 
S. Migliari (Univ. of Amsterdam), R.P. Fender (Univ. of Amsterdam), M.   Rupen (NRAO), S. Wachter (SIRTF Science Center/Calt...