Near-infrared jet emission in the microquasar XTE J1550-564
  Context: 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. Aims: 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 disentangle the
various contributions to the spectral energy distribution (SED) and investigate
the presence of a jet component. Methods: Photometric and spectroscopic
observations allowed us to construct an SED and also to produce a high
time-resolution lightcurve. Results: 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. Conclusions: 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 greater than 2x10^8cm and
magnetic field less than 5T for the jet base, assuming a homogeneous one-zone
synchrotron model.
