What Really is the Relativistic Radio-Jet Distance of GRO J1655--40?

Abstract

GRO J1655--40 is a galactic microquasar, i.e. a short-period binary with relativistic radio-jets and where the companion is a black-hole. A little after its discovery in 1994 by $BATSE$, a distance of 3.2$\pm$0.2 kpc has been estimated, and at which the radio jets appeared superluminal. Since that time, this distance of GRO J1655--40 has been discussed in many studies, often strengthening the value of 3.2 kpc, and used in numerous models. However, recently, \citet{Foellmi-etal-2006b-astroph} used new VLT-UVES and {\it published} photometric data to show that GRO J1655--40 must be closer than 1.7 kpc and that the accuracy of the main distance estimators for this source can be questioned. Still, the details on how really the distance of 3.2 kpc has been build was not fully clear. It is the purpose of this article to show that while the {\it upper} limit to the distance is rather firm, the lower limit is not. We draw some conclusions about the new understanding we have of GRO J1655--40, and finally present a new and promising method that can be used to determine the distance of GRO J1655--40, and maybe many faint and embedded stars.