Letter to the Editor

A microquasar shot out from its birth place

¹ Service d'Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette, France e-mail: fmirabel@cea.fr
² Instituto de Astronomía y Física del Espacio/Conicet. Bs As, Argentina
³ Instituto de Física, Universidade Federal do Rio Grande do Sul, CP 15001, 91501-970 Porto Alegre, RS, Brazil
⁴ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China

Received: 31 March 2004
Accepted: 25 May 2004

Abstract

We show that the microquasar LS I +61° 303 is running away from its birth place in a young complex of massive stars. The supernova explosion that formed the compact object shot out the X-ray binary with a linear momentum of  km s^-1, which is comparable to the linear momenta found in solitary runaway neutron stars and millisecond pulsars. The properties of the binary system and its runaway motion of  km s^-1 imply that the natal supernova was asymmetric and that the upper limit for the mass that could have been suddenly ejected in the explosion is ~2 . The initial mass of the progenitor star of the compact object that is inferred depends on whether the formation of massive stars in the parent stellar cluster was coeval or a sequential process.