Optical / near - infrared observations of the black hole candidate XTE J 1720 - 318 : from high - soft to low - hard state ⋆

Abstract

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 colourmagnitude 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 (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.

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Cite this paper

@inproceedings{Chaty2006OpticalN, title={Optical / near - infrared observations of the black hole candidate XTE J 1720 - 318 : from high - soft to low - hard state ⋆}, author={S . Chaty and N . Bessolaz}, year={2006} }