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

TL;DR

This study investigates how the radio jet in the neutron star binary 4U 1820-30 varies with X-ray states, revealing jet quenching during high X-ray luminosity and a connection to accretion processes.

Contribution

First detailed radio observations of 4U 1820-30 across different X-ray modes, linking jet behavior to accretion states and luminosity.

Findings

Jet is compact and flat-spectrum during low X-ray mode.

Jet is quenched with steep spectrum during high X-ray mode.

Jet transition occurs at X-ray luminosity of ~3.5 x 10^{37} erg s^{-1}.

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 $L_{\rm X (3-10 keV)} \sim 3.5 \times 10^{37} (D/\rm{7.6 kpc})^{2}$ 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 disk or boundary layer.