A Variable Mid-Infrared Synchrotron Break Associated with the Compact Jet in GX 339-4

Many X-ray binaries remain undetected in the mid-infrared, a regime where emission from their compact jets is likely to dominate. Here, we report the detection of the black hole binary GX 339-4 with the Widefield Infrared Survey Explorer (WISE) during a very bright, hard accretion state in 2010. Combined with a rich contemporaneous multiwavelength dataset, clear spectral curvature is found in the infrared, associatedwith the peak flux density expected from the compact jet. An optically-thin slope of -0.7 and a jet radiative power of andgt;6×1035 erg s−1 (d/8 kpc)2 are measured. A 24 h WISE light curve shows dramatic variations in mid-infrared spectral slope on timescales at least as short as the satellite orbital period 95 mins. There is also significant change during one pair of observations spaced by only 11 s. These variations imply that the spectral break associated with the transition from self-absorbed to optically-thin jet synchrotron radiation must be varying across the full wavelength range of 3-22 μm that WISE is sensitive to, and more. Based on fourband simultaneous mid-infrared detections, the break is constrained to frequencies of 4.6+3.5 −2.0×1013 Hz in at least two epochs of observation, consistent with a magnetic field B1.5(±0.8)×104 G assuming a single-zonesynchrotron emission region. The observed variability implies that either B, or the size of the acceleration zone above the jet base, are being modulated by factors of 10 on relatively-short timescales.