Variable circular polarization associated with relativistic ejections from GRS 1915+105

We report the discovery of variable circularly polarized radio emission associated with relativistic ejections from GRS 1915 + 105, based on observations with the Australia Telescope Compact Array (ATCA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN). Following a radio flare in 2001 January, significant and variable circular polarization, at a fractional level of 0.2-0.4 per cent, was measured with ATCA at four frequencies between 1 and 9 GHz. Following an additional outburst 65 d later in 2001 March, further ATCA observations measured a comparable sign and level of circular polarization at two frequencies. At this second epoch, contemporaneous MERLIN observations directly imaged a relativistic ejection event and allowed us confidently to associate both the circularly and the linearly polarized emission with the relativistic ejecta, allowing a detailed measurement of the full polarization properties in the optically thin phase. The fractional circular polarization spectrum appears to flatten at higher frequencies in 2001 January, when there is strong evidence for multiple components at different optical depths. While we cannot conclusively distinguish between synchrotron or propagation-induced conversion as the origin of the circularly polarized component, we do not consider that coherent or birefringent scintillation mechanisms are likely. The implication is therefore that the ejections from GRS 1915 + 105 are associated with a significant population of low-energy electrons, with associated consequences for the energetics of relativistic ejection events. We briefly compare the data for SS 433 and GRS 1915 + 105 with those for active galactic nuclei, and note that linear-to-circular polarization ratios <1 are observed in both binary sources at the lowest frequency, and ratios >1 at the higher frequencies, illustrating the role of Faraday depolarization. In addition, the 2001 January ATCA observations reveal a linear polarization 'rotator' event, probing the (variable) orientation of the magnetic field structure in the outflow.