Near-infrared observations of Galactic black hole candidates

We report on several European Southern Observatory (ESO) near-infrared (NIR) observational campaigns aimed at understanding the nature of Galactic black hole candidates. Our results, including NIR photometry of the sources GRO J1655-40, GRS 1739-278, GRS 1716-249, GRS 1121-68 and GX 339-4, show that all the sources but GRO J1655-40 are consistent with low-mass stars as the companion star of the binary system. By locating the counterparts on a colour-magnitude diagram (CMD), we better constrain the spectral type of the companion star of three of the systems considered here, and confirm a fourth one. The spectral types are respectively: M0-5 V for GRS 1716-249, F8-G2 III for GX 339-4 and later than F5 V for GRS 1739-278. We confirm the already known spectral type of the companion in GRS 1121-68 (K0-5 V). The location of GRO J1655-40 on the CMD is consistent with the sub-giant luminosity class and with this source crossing the Hertzsprung gap. However, a non-stellar emission seems to contribute to the NIR flux of this source.