Disc–jet coupling in low-luminosity accreting neutron stars

In outburst, neutron star X-ray binaries produce less powerful jets thanblack holes at a given X-ray luminosity. This has made them moredifficult to study as they fade towards quiescence. To explore whetherneutron stars power jets at low accretion rates (LX ≲1036 erg s-1), we investigate the radio and X-rayproperties of three accreting millisecond X-ray pulsars (IGRJ17511-3057, SAX J1808.4-3658 and IGR J00291+5934) during theiroutbursts in 2015, and of the non-pulsing neutron star Cen X-4 inquiescence (2015) and in outburst (1979). We did not detect the radiocounterpart of IGR J17511-3057 in outburst or of Cen X-4 in quiescence,but did detect IGR J00291+5934 and SAX J1808.4-3658, showing that atleast some neutron stars launch jets at low accretion rates. While theradio and X-ray emission in IGR J00291+5934 seem to be tightlycorrelated, the relationship in SAX J1808.4-3658 is more complicated. Wefind that SAX J1808.4-3658 produces jets during the reflaring tail, andwe explore a toy model to ascertain whether the radio emission could beattributed to the onset of a strong propeller. The lack of a universalradio/X-ray correlation, with different behaviours in different neutronstar systems (with various radio/X-ray correlations; some being radiofaint and others not), points at distinct disc-jet interactions inindividual sources, while always being fainter in the radio band thanblack holes at the same X-ray luminosity.