Rapidly Evolving Circularly Polarized Emission during the 1994 Outburst of GRO J1665-40

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 2.4GHz, but not at 4.8 and 8.4GHz, 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.