Jet quenching in the neutron star low-mass X-ray binary 1RXS J180408.9‑342058

We present quasi-simultaneous radio (VLA) and X-ray ($Swift$)observations of the neutron star low-mass X-ray binary (NS-LMXB) 1RXSJ180408.9$-$342058 (J1804) during its 2015 outburst. We found that theradio jet of J1804 was bright ($232 \pm 4 \mu$Jy at $10$ GHz) during theinitial hard X-ray state, before being quenched by more than an order ofmagnitude during the soft X-ray state ($19 \pm 4 \mu$Jy). The sourcethen was undetected in radio (<$13 \mu$Jy) as it faded toquiescence. In NS-LMXBs, possible jet quenching has been observed inonly three sources and the J1804 jet quenching we show here is thedeepest and clearest example to date. Radio observations when the sourcewas fading towards quiescence ($L_X = 10^{34-35}$ erg s$^{-1}$) showthat J1804 must follow a steep track in the radio/X-ray luminosity planewith $\beta > 0.7$ (where $L_R \propto L_X^{\beta}$). Few othersources have been studied in this faint regime, but a steep track isinconsistent with the suggested behaviour for the recently identifiedclass of transitional millisecond pulsars. J1804 also shows fainterradio emission at $L_X <10^{35}$ erg s$^{-1}$ than what is typicallyobserved for accreting millisecond pulsars. This suggests that J1804 islikely not an accreting X-ray or transitional millisecond pulsar.