2011A&A...529A...3C

Microquasars are accreting Galactic sources that are also observed to launch relativistic jets. A key signature of the ejection is non-thermal radio emission. The level of this jet component at high frequencies is still poorly constrained. The X-ray binary and microquasar black hole candidate XTE J1550-564 exhibited a faint X-ray outburst in April 2003 during which it stayed in the X-ray low/hard state. We took optical and near-infrared (NIR) observations with the ESO/NTT telescope during this outburst to distinguish the various contributions to the spectral energy distribution (SED) and investigate the presence of a jet component. Photometric and spectroscopic observations allowed us to construct an SED and also to produce a high time-resolution lightcurve. The SED shows an abrupt change of slope from the NIR domain to the optical. The NIR emission is attributed to non-thermal synchrotron emission from the compact, self-absorbed jet that is known to be present in the low/hard state. This is corroborated by the fast variability, colours, lack of prominent spectral features and evidence for intrinsic polarisation. The SED suggests the jet break from the optically thick to the thin regime occurs in the NIR. The simultaneous optical-NIR data allow an independent confirmation of jet emission in the NIR. The transition to optically thin synchrotron occurs at NIR frequencies or below, which leads to an estimated characteristic size >2x10⁸cm and magnetic field ≲5T for the jet base, assuming a homogeneous one-zone synchrotron model.