On the distance of GRO J1655-40

¹ European Southern Observatory, 3107 Alonso de Cordova, Casilla 19001, Vitacura, Santiago, Chile e-mail: cfoellmi@eso.org
² Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
³ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Campus San Joaquín, Vicuña Mackenna 4860 Casilla 306, Santiago 22, Chile
⁴ On leave from Service d'astrophysique-CEA-Saclay, 91191 Gif-sur-Yvette, France

Received: 13 December 2005
Accepted: 12 June 2006

Abstract

Aims.We challenge the accepted distance of 3.2 kpc of GRO J1655-40. We present VLT-UVES spectroscopic observations to estimate the absorption toward the source, and determine a maximum distance of GRO J1655-40.

Methods.We show that the accepted value of 3.2 kpc is taken for granted by many authors. We retrieved in the ESO archive UVES spectra taken in April 2004 when GRO J1655-40 was in quiescence to determine the spectral type of the secondary star. For the first time we build a flux-calibrated mean (UVES) spectrum of GRO J1655-40 and compare its observed flux to that of five nearby stars of similar spectral types. We strengthen our results with the traditional pair method, using published photometric data of the comparison stars.

Results.We show that the distance of 3.2 kpc is questionable. We determine a spectral type F6IV for the secondary star. We demonstrate in details that the distance of GRO J1655-40 must be smaller than 1.7 kpc.

Conclusions.The runaway black hole GRO J1655-40 could be associated with the open cluster NGC 6242 which is located at 1.0 ± 0.1 kpc from the Sun. At 1.7 kpc the jets are not a superluminal, and GRO J1655-40 becomes one of the closest known black holes to the Sun.