Özet:
The observations proposed here will provide crucial piece of information in our long-term programme of monitoring selected microlensing events, candidates for black hole lenses. We concentrate on rare bright but also long events (t_E>100 days) for which Gaia will provide astrometric time-series resulting in the measurement of the Einstein radius. However, without the spectroscopic observations carried out close to the maximum magnification, it will be impossible to unambiguously attribute a black hole to the lens. Our team is experienced with microlensing surveys and are active members of the Gaia and OGLE monitoring programmes, which deliver microlensing event candidates on daily basis. The events for spectroscopy will be selected among those found on the rise or near their peak (when the source vs lens contrast is the highest). The analysis of the spectral data will rely on fitting the optical region (550–900 nm) of spectral energy distribution (SED) from templates, Bressan et al. (2012 MNRAS, 427, 127) stellar isochrones and/or Coelho et al. (2014) stellar atmosphere models and extinction. We will also use prominent atomic lines (Halpha, Hbeta, Na, Ca, Mg, etc.) and TiO absorption bands, which classically serve as an important spectral type indicator from Teff, log g and [M/H].