Evolving jets in X-ray binaries: a mid-infrared view

Black hole X-ray binaries are the brightest X-ray sources in the sky. During their outbursts, they can produce relativistic outflows in the form of collimated jets. The jets produce synchrotron emission from radio to infrared/optical wavelengths. These jets are notoriously difficult to detect in the mid-IR to mm regime due to their faintness, highly variable nature, and lack of sensitive instrumentation. This has been greatly improved with detections of black hole X-ray binaries with VISIR on the VLT at 5-12 microns. For several years we have been detecting X-ray binaries at mid-IR wavelengths with VISIR, and some at mm with ALMA. We have discovered that the break in the jet spectrum lies in the mid-IR range, and shifts to longer (mm) wavelengths when these jets weaken during state transitions. In some cases, a dramatic evolution of the jet spectrum when the accretion state is changing is observed, which provides an insight into the physical processes responsible for suppressing and reactivating the jet, and has implications for the conditions required for jets to be produced, and hence their launching process. In this talk, we report on the first mid-IR variability study of an X-ray binary on short (minute) timescales: the case of MAXI J1535-571. On some dates the mid-IR flux of MAXI J1535-571 varied by a factor of two in less than 15 minutes. The mid-IR fractional rms variability on minute timescales is ~ 15%. The spectrum and variability amplitude are consistent with a jet internal shock model. We then show some new results obtained from mid-infrared observations of the black hole X-ray binary MAXI J1820+020. These two systems, observed with the VISIR instrument on the VLT, are among the brightest mid-infrared black hole X-ray binaries known so far. These results demonstrate how rich, multi-wavelength time-resolved data of X-ray binaries can help in refining models of the disc-jet connection and jet launching in these systems.