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Optical/near‐infrared (optical/NIR, OIR) light from low‐mass neutron star X‐ray binaries (NSXBs) in outburst is traditionally thought to be thermal emission from the accretion disc. Here we present a comprehensive collection of quasi‐simultaneous OIR and X‐ray data from 19 low magnetic field NSXBs, including new observations of three sources: 4U 0614+09, LMC X−2 and GX 349+2. The average radio–OIR spectrum for NSXBs is  α≈+ 0.2  (where   L  ν ∝ν α  ) at least at high luminosities when the radio jet is detected. This is comparable to, but slightly more inverted than the  α≈ 0.0  found for black hole X‐ray binaries. The OIR spectra and relations between OIR and X‐ray fluxes are compared to those expected if the OIR emission is dominated by thermal emission from an X‐ray or viscously heated disc, or synchrotron emission from the inner regions of the jets. We find that thermal emission due to X‐ray reprocessing can explain all the data except at high luminosities for some NSXBs, namely, the atolls and millisecond X‐ray pulsars. Optically thin synchrotron emission from the jets (with an observed OIR spectral index of  α thin  &lt; 0 ) dominate the NIR light above  and the optical above  in these systems. For NSXB Z‐sources, the OIR observations can be explained by X‐ray reprocessing alone, although synchrotron emission may make a low‐level contribution to the NIR, and could dominate the OIR in one or two cases.

Evidence for a jet contribution to the optical/infrared light of neutron star X‐ray binaries