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

TL;DR

This study analyzes optical/infrared and X-ray data from neutron star X-ray binaries, revealing jet-related synchrotron emission significantly contributes to their optical/NIR light, especially at high luminosities, challenging the traditional thermal emission model.

Contribution

It provides the first comprehensive evidence that jet synchrotron emission plays a major role in the optical/NIR light of neutron star X-ray binaries, especially at high luminosities.

Findings

Jet synchrotron emission dominates NIR light above L_x ~ 10^36 erg/s.

Optical/NIR spectra are more inverted than black hole binaries, indicating different jet properties.

X-ray reprocessing explains most data, but jets are significant at high luminosities.

Abstract

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 ~ +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 ~ 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 ~ 10^36 erg/s and the optical above L_x ~ 10^37 erg/s 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.