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 \alpha \approx + 0.2 ( where L _ { \nu } \propto \nu ^ { \alpha } ) at least at high luminosities when the radio jet is detected .
This is comparable to , but slightly more inverted than the \alpha \approx 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 ( MSXPs ) .
Optically thin synchrotron emission from the jets ( with an observed OIR spectral index of \alpha _ { thin } < 0 ) dominate the NIR light above L _ { X } \approx 10 ^ { 36 } erg s ^ { -1 } and the optical above L _ { X } \approx 10 ^ { 37 } erg s ^ { -1 } 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 .