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 Wide-field Infrared Survey Explorer (WISE) during a very bright, hard accretion state in 2010. Combined with a rich contemporaneous multiwavelength data set, clear spectral curvature is found in the infrared, associated with the peak flux density expected from the compact jet. An optically thin slope of ∼-0.7 and a jet radiative power of >6 × 10³⁵ergs^-1 (d/8 kpc)² are measured. A ∼24hr WISE light curve shows dramatic variations in mid-infrared spectral slope on timescales at least as short as the satellite orbital period ∼95 minutes. There is also significant change during one pair of observations spaced by only 11s. 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 four-band simultaneous mid-infrared detections, the break is constrained to frequencies of ≈4.6^+3.5 _{- 2.0} × 10¹³Hz in at least two epochs of observation, consistent with a magnetic field B ≈1.5(± 0.8) × 10⁴ G assuming a single-zone synchrotron emission region. The observed variability implies that either B or the size of the acceleration zone above the jet base is being modulated by factors of 10 on relatively short timescales.