Optical/near-infrared observations of the black hole candidate XTE J1720-318 : from high-soft to low-hard state
AIM – Astrophysique Interactions Multi-échelles (UMR 7158 CEA/CNRS/Université Paris 7 Denis Diderot), CEA Saclay, DSM/DAPNIA/Service d'Astrophysique, Bât. 709, L'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France e-mail: email@example.com
2 Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
Accepted: 6 April 2006
Aims.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 the Target of Opportunity, in February and April 2003.
Methods.We performed an accurate astrometry and analysed photometrical and spectroscopic 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.
Results.We discovered the optical counterpart in the R-band () and confirmed the near-infrared counterpart. We show that, for an absorption between 6 and 8 mag, 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.
Key words: stars: binaries: close / stars: Hertzsprung-Russell (HR) and C-M diagrams / infrared: stars / X-rays: binaries / X-rays: individuals: XTE J1720-318
© ESO, 2006