Radio detections of the neutron star X-ray binaries 4U 1820-30 and Ser X-1 in soft X-ray states

Details Radio detections of the neutron star X-ray binaries 4U 1820-30 and Ser X-1 in soft X-ray states Radio detections of the neutron star X-ray binaries 4U 1820-30 and Ser X-1 in soft X-ray states

2004-02-25

arxiv.org/abs/astro-ph/0402600v1

Mon.Not.Roy.Astron.Soc. 351 (2004) 186

Astronomy and Astrophysics

Astrophysics

accepted for publication in MNRAS

Scientific paper

10.1111/j.1365-2966.2004.07768.x

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.

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