Near-Infrared Synchrotron Emission from the Compact Jet of GX 339−4

We have compiled contemporaneous broadband observations of the black holecandidate X-ray binary GX 339-4 when in the low/hard X-ray state in 1981 and1997. The data clearly reveal the presence of two spectral components, withthermal and non-thermal spectra, overlapping in the optical -- near-infraredbands. The non-thermal component lies on an extrapolation of the radio spectrumof the source, and we interpret it as optically thin synchrotron emission fromthe powerful, compact jet in the system. Detection of this break fromself-absorbed to optically thin synchrotron emission from the jet allows us toplace a firm lower limit on the ratio of jet (synchrotron) to X-rayluminosities of $\geq 5$%. We further note that extrapolation of the opticallythin synchrotron component from the near-infrared to higher frequenciescoincides with the observed X-ray spectrum, supporting models in which theX-rays could originate via optically thin synchrotron emission from the jet(possibly instead of Comptonisation).Comment: Accepted for publication in ApJ Lette