To gain a better understanding of high-energy Galactic sources, we observed
the Galactic X-ray binary and black hole candidate XTE J1720-318 in the optical
and near-infrared, just after the onset of its X-ray outburst in January, 2003.
These observations were obtained with the ESO/NTT as a Target of Opportunity,
in February and April 2003. We performed an accurate astrometry and analysed
photometrical observations. We then produced a colour-magnitude diagram, looked
at the overall evolution of the multi-wavelength light curve, and analysed the
spectral energy distribution. We discovered the optical counterpart in the
R-band (R ~ 21.5) and confirmed the near-infrared counterpart. We show that,
for an absorption between 6 and 8 magnitudes, XTE J1720-318 is likely to be an
intermediate mass X-ray binary located at a distance between 3 and 10 kpc,
hosting a main sequence star of spectral type between late B and early G. Our
second set of observations took place simultaneously with the third secondary
outburst present in X-ray and near-infrared light curves. The evolution of its
spectral energy distribution shows that XTE J1720-318 entered a transition from
a high-soft to a low-hard state in-between the two observations. We finally
discuss the different phases of the outburst of this source in the general
scheme of outbursts from microquasars.Comment: 12 pages, 5 figures; contributed talk at the VIth Microquasar
Workshop: "Microquasars and beyond" (Como, Italy), eds: T. Belloni et al.
September 200
