Identification of the optical counterparts of high-mass X-ray binaries through optical photometry and spectroscopy

We present the results of our search for optical counterparts to high-mass X-ray transient sources discovered by various X-ray missions. We obtained CCD images of the X-ray fields through BVR and Halpha filters in order to identify early-type stars in the R-Halpha versus B-V colour-colour diagram. We also obtained medium-resolution spectroscopy of the candidates in order to confirm the presence of H\alpha emission and perform spectral classification. We report on the discovery of the optical counterparts to two X-ray sources: XTE J1858+034 and IGR J01363+6610, and the follow-up observations of another two, newly identified by our group: SAX J2103.5+4545 and GRO J2058+42. For another source, IGR J00370+6122, we present the first detailed optical spectral analysis. The optical photometry and spectroscopy reveal B-type companions in all five sources; GRO J2058+42, SAX J2103.5+4545 and IGR J01363+6610 are positively identified with Be/X-ray binaries, IGR J00370+6122 with a supergiant X-ray binary, while the nature of XTE J1858+034 is uncertain. We also study the relationship between the optical and X-ray emission during quiescent states.