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The optical counterpart to IGR J06074+2205: a Be/X-ray binary showing disc loss and V/R variability

Present X-ray missions are regularly discovering new X/gamma-ray sources. The identification of the counterparts of these high-energy sources at other wavelengths is important to determine their nature. In particular, optical observations are an essential tool in the study of X-ray binary populations in our Galaxy. The main goal of this work is to determine the properties of the optical counterpart to the INTEGRAL source IGR J06074+2205, and study its long-term optical variability. Although its nature as a high-mass X-ray binary has been suggested, little is known about its physical parameters. We have been monitoring IGR J06074+2205 since 2006 in the optical band. We present optical photometric BVRI and spectroscopic observations covering the wavelength band 4000-7000 A. The blue spectra allow us to determine the spectral type and luminosity class of the optical companion; the red spectra, together with the photometric magnitudes, were used to derive the colour excess E(B-V) and estimate the distance. We have carried out the first detailed optical study of the massive component in the high-mass X-ray binary IGR J06074+2205. We find that the optical counterpart is a V=12.3 B0.5Ve star located at a distance of ~4.5 kpc. The monitoring of the Halpha line reveals V/R variability and an overall decline of its equivalent width. The Halpha line has been seen to revert from an emission to an absorption profile. We attribute this variability to global changes in the structure of the Be star's circumstellar disc which eventually led to the complete loss of the disc. The density perturbation that gives rise to the V/R variability vanishes when the disc becomes too small.