A possible black hole in the γ-ray microquasar LS 5039

The population of high-energy and very high-energy gamma-ray sources, detected with EGRET and the new generation of ground-based Cherenkov telescopes, forms a reduced but physically important sample. Most of these sources are extragalactic (e.g. blazars), while among the galactic ones there are pulsars and supernova remnants. The microquasar LS 5039, previously proposed to be associated with an EGRET source by Paredes et al., has recently been detected at TeV energies, confirming that microquasars should be regarded as a class of high-energy gamma-ray sources. To model and understand how the energetic photons are produced and escape from LS 5039, it is crucial to unveil the nature of the compact object, which remains unknown. Here, we present new intermediate-dispersion spectroscopy of this source, which, combined with values reported in the literature, provides an orbital period of P-orb= 3.906 03 +/- 0.000 17 d, a mass function f(M) = 0.0053 +/- 0.0009 M-circle dot and an eccentricity