Open AccessRadio detections of the neutron star X‐ray binaries 4U 1820 − 30 and Ser X‐1 in soft X‐ray states
Author(s) -
Migliari S.,
Fender R. P.,
Rupen M.,
Wachter S.,
Jonker P. G.,
Homan J.,
Klis M. van der
Publication year - 2004
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2004.07768.x
Subject(s) - physics , neutron star , astrophysics , pulsar , x ray binary , black hole (networking) , be star , x ray pulsar , astronomy , binary number , x ray , stars , computer network , routing protocol , routing (electronic design automation) , arithmetic , mathematics , quantum mechanics , computer science , link state routing protocol
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 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.