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

Abstract

The persistently bright ultra-compact neutron star low-mass X-ray binary 4U 1820$-$30 displays a $\sim$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 $\approx 8$. 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 $L_{\mathrm{X}(3-10keV)}\sim 3.5\times 10^{37}(D/7.6kpc{)}^2$ 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 disk or boundary layer.

6 pages, 2 Figures. Online supplementary information supplied in the appendix. Accepted for publication by MNRAS Letters

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• Radio monitoring of transient Be/X-ray binaries and the inflow-outflow coupling of strongly-magnetized accreting neutron stars
• The MAVERIC survey: A catalogue of radio sources in southern globular clusters from the Australia Telescope Compact Array
• Long term radio monitoring of the neutron star X-ray binary Swift J1858.6-0814
• Tracking the Enigmatic Globular Cluster Ultracompact X-ray Binary X1850--087: Extreme Radio Variability in the Hard State
• Evaluating the jet/accretion coupling of Aql X-1: probing the contribution of accretion flow spectral components
• A Multi-wavelength Characterization of the 2023 Outburst of MAXI J1807+132: Manifestations of Disk Instability and Jet Emission