Evidence for a jet contribution to the optical/infrared light of neutron star X-ray binaries |
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Authors: | D. M. Russell R. P. Fender P. G. Jonker |
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Affiliation: | School of Physics &Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ;SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands;Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, MS 83, Cambridge, MA 02138, USA;Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA, Utrecht, the Netherlands |
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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 α≈+ 0.2 (where L ν∝να ) 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 αthin < 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. |
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Keywords: | accretion, accretion discs ISM: jets and outflows stars: neutron X-rays: binaries |
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