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.
