Rapidly evolving circularly polarized emission during the 1994 outburst of GRO J1655–40

¹ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
² Mullard Space Science Laboratory, University College London, Holmbury St Mary, Surrey RH5 6NT, UK, and School of Physics A28, University of Sydney, NSW 2006, Australia e-mail: kw@mssl.ucl.ac.uk
³ Australia Telescope National Facility, Narrabri, NSW 2390, Australia e-mail: rsault@atnf.csiro.au
⁴ Department of Physics and Astronomy, Southampton University, Southampton, SO17 1BJ, UK
⁵ Observatory, PO Box 14, 00014 University of Helsinki, Finland e-mail: diana@astro.helsinki.fi


Corresponding author: J.-P. Macquart, jpm@astro.rug.nl

Received: 11 April 2002
Accepted: 24 September 2002

Abstract

We report the detection of circular polarization during the 1994 outburst of the Galactic microquasar GRO J1655-40. The circular polarization is clearly detected at 1.4 and 2.4 GHz, but not at 4.8 and 8.4 GHz, where its magnitude never exceeds 5 mJy. Both the sign and magnitude of the circular polarization evolve during the outburst. The time dependence and magnitude of the polarized emission can be qualitatively explained by a model based on synchrotron emission from the outbursts, but is most consistent with circular polarization arising from propagation effects through the relativistic plasma surrounding the object.