The evolving radio jet from the neutron star X-ray binary 4U 1820−30

• Published in: Monthly notices of the Royal Astronomical Society. Letters Vol. 508; no. 1; pp. L6 - L11
• Main Authors: Russell, T D, Degenaar, N, den Eijnden, J van, Del Santo, M, Segreto, A, Altamirano, D, Beri, A, Díaz Trigo, M, Miller-Jones, J C A
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01-11-2021
• Subjects: radio continuum: transients; X-rays: binaries; accretion, accretion discs; stars: neutron
• ISSN: 1745-3925; 1745-3933
• DOI: 10.1093/mnrasl/slab087

ABSTRACT
The persistently bright ultracompact neutron star low-mass X-ray binary 4U 1820−30 displays an ∼170 d accretion cycle, evolving between phases of high and low X-ray modes, where the 3–10 keV X-ray flux changes by a factor of up to ≈8. The source is generally in a soft X-ray spectral state, but may transition to a harder state in the low X-ray mode. Here, we present new and archival radio observations of 4U 1820−30 during its high and low X-ray modes. For radio observations taken within a low mode, we observed a flat radio spectrum consistent with 4U 1820−30 launching a compact radio jet. However, during the high X-ray modes the compact jet was quenched and the radio spectrum was steep, consistent with optically thin synchrotron emission. The jet emission appeared to transition at an X-ray luminosity of $L_{\rm X (3-10\, keV)} \sim 3.5 \times 10^{37} (D/\rm {7.6\, kpc})^{2}$ erg s−1. We also find that the low-state radio spectrum appeared consistent regardless of X-ray hardness, implying a connection between jet quenching and mass accretion rate in 4U 1820−30, possibly related to the properties of the inner accretion disc or boundary layer.