MERLIN observations of relativistic ejections from GRS 1915+105

We present high-resolution MERLIN radio images of multiple relativistic ejections from GRS 1915+105 in 1997 October/November. The observations were made at a time of complex radio behaviour, corresponding to multiple optically thin outbursts and several days of rapid radio flux oscillations. This activity followed ∼20 d of a plateau state of inverted-spectrum radio emission and hard, quasi-stable X-ray emission. The radio imaging resolved four major ejection events from the system. As previously reported from earlier VLA observations of the source, we observe apparent superluminal motions resulting from intrinsically relativistic motions of the ejecta. However, our measured proper motions are significantly greater than those observed on larger angular scales with the VLA. Under the assumption of an intrinsically symmetric ejection, we can place an upper limit on the distance to GRS 1915+105 of 11.2 ± 0.8 kpc. Solutions for the velocities unambiguously require a higher intrinsic speed by about 0.1c than that derived from the earlier VLA observations, whilst the angle to the line of sight is not found to be significantly different. At a distance of 11 kpc, we obtain solutions of υ = 0.98+0.02-0.05c and θ = 66° ± 2°. The jet also appears to be curved on a scale that corresponds to a period of around 7 d. We observe significant evolution of the linear polarization of the approaching component, with large rotations in position angle and a general decrease in fractional polarization. This may be the result of increasing randomization of the magnetic field within the ejected component. We do not at any time detect significant linear polarization from the core, including periods when the flux density from this region is dominated by radio oscillations. The power input into the formation of the jet is very large, ≥1038 erg s-1 at 11 kpc for a pair plasma. If the plasma contains a cold proton for each electron, then the mass outflow rate, ≥1018 g s-1. is comparable to inflow rates previously derived from X-ray spectral fits.