Optical polarimetric observations of the microquasar LS 5039^*

¹ Departamento de Física, Escuela Politécnica Superior, Universidad de Jaén, Virgen de la Cabeza 2, 23071 Jaén, Spain e-mail: [jcombi;jmarti]@ujaen.es
² Instituto Argentino de Radioastronomía, C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina
³ Facultad de Ciencias Astronómicas y Geofísicas UNLP, Paseo del Bosque, B1900FWA La Plata, Argentina e-mail: scellone@fcaglp.fcaglp.unlp.edu.ar
⁴ Service d'Astrophysique, CEA Saclay, Bât. 709, L'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France e-mail: [mribo;mirabel]@discovery.saclay.cea.fr
⁵ Instituto de Astronomía y Física del Espacio, CONICET, C.C.67, Suc. 28, 1428 Buenos Aires, Argentina
⁶ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain e-mail: jcv@ll.iac.es

Received: 8 June 2004
Accepted: 4 August 2004

Abstract

We present the first optical polarimetric observations of the runaway microquasar LS 5039. Our results reveal the presence of a large amount (~5%) of polarized emission towards this binary system. By combining polarimetric and spectroscopic observations of some stars in the field together with available statistical information on the galactic interstellar polarization of the region, we have estimated and subtracted the contribution of the interstellar polarization in this direction. As a result, we obtain an intrinsic polarization of ~3% for the object, much higher than what would be expected from jet emission in the optical domain. We suggest that the polarized light originates by electron Thomson scattering in the stellar envelope of the companion star. This allows us to constrain the size of the region where the polarized emission originates, as well as estimating the scattering electronic density and the wind velocity at such distance.