New evidence on the origin of the microquasar GRO J1655-40

Abstract

Aims. Motivated by the new determination of the distance to the microquasar GRO J1655- 40 by Foellmi et al. ( 2006, A&A, 457, 249), we conduct a detailed study of the distribution of the atomic ( HI) gas, molecular ( CO) gas, and dust around the open cluster NGC 6242, the possible birth place of the microquasar (Mirabel et al. 2002, A&A, 395, 595). The proximity and relative height of the cluster on the galactic disk provides a unique opportunity to study SNR evolution and its possible physical link with microquasar formation.

Methods. We search in the interstellar atomic and molecular gas around NGC 6242 for traces that may have been left from a supernova explosion associated to the formation of the black hole in GRO J1655- 40. Furthermore, the 60/100 mu m IR color is used as a tracer of shocked-heated dust.

Results. At the kinematical distance of the cluster the observations have revealed the existence of a HI hole of 1.degrees 5 x 1.degrees 5 in diameter and compressed CO material acumulated along the south-eastern internal border of the HI cavity. In this same area, we found extended infrared emission with characteristics of shocked-heated dust. Based on the morphology and physical characteristics of the HI, CO and FIR emissions, we suggest that the cavity in the ISM was produced by a supernova explosion occured within NGC 6242. The lower limit to the kinematic energy transferred by the supernova shock to the surrounding interstellar medium is similar to 10(49) erg and the atomic and molecular mass displaced to form the cavity of similar to 16.500 M-circle dot. The lower limit to the time elapsed since the SN explosion is similar to 2.2 x 10(5) yr, which is consistent with the time required by GRO J1655- 40 to move from the cluster up to its present position. The observations suggest that GRO J1655- 40 could have been born inside NGC 6242, being one of the nearest microquasars known so far.