Are the distances of galactic microquasars reliable?

Abstract

We review some specific and quantitative issues in popular methods used to determine the distance of galactic objects, in the context of the microquasars GRO J1655-40 and 1A 0620-00. In particular, we discuss the domain of validity of the relationship between the Sodium lines and the color excess, and the supposedly systematic overestimation of optical absorption from X-ray data. Both problems imply a possible underestimation of the absorption toward GRO J1655-40. We also discuss the main issue concerning the absolute magnitude of the secondary star in our own maximum-distance method, that has been used on GRO J1655-40. We show in this respect that the dynamical parameters of this microquasar are not definitive, and that the mean properties of the secondary star is not so much different from that of a normal single star of same spectral type. Following the possibility that GRO J1655-40 is located at about 1.0 kpc, associated with the open cluster NGC 6242, we rise the question: which black-hole is the closest to the Sun? In that context, we quickly discuss the situation of the microquasar 1A 0620-00, by addressing the specific point of its 30-years-old extinction value. Finally, we present some details about a possible new distance method, using the surface properties of the stars, based on the cross-correlation function bisector, and that is now accessible thanks to the advent of a high-resolution echelle spectrograph called CRIRES at the VLT. We quickly discuss the strengths and weaknesses of this method, and how it possibly address the caveats of the other methods.