Plot of X-ray binary radio and X-ray luminosities from Bahramian & Rushton (2022), with our radio nondetection of 4U 1556–60 indicated with a red star and a line of constant L_R/L_X plotted for reference at other source distances. Three correlation lines are plotted for visual reference: solid blue for BHXBs; dash-dot magenta for NSXBs with β = 0.7; and dashed orange for NSXBs with β = 1.4. AMXPs are accreting millisecond pulsars with stronger magnetic fields than general NSXBs, and tMSPs are transitional millisecond pulsars whose radio emission during their XRB-like state is thought to not arise from a jet alone. The very low upper limit of 4U 1556–60’s radio emission for its X-ray luminosity is strongly indicative of a neutron star accretor, as it is about 1000 times fainter than expected for a black hole accretor. The previous deepest upper limit in radio luminosity for a NSXB, Cen X–4 in quiescence at ∼1.2 kpc (van den Eijnden et al. 2022), is also shown as a blue plus sign.