Repeated relativistic ejections in GRS 1915+105

In 1994 February-August we observed with the VLA four ejection events of radio-emitting clouds from the high-energy source GRS 1915+105. These events are all consistent with antiparallel ejections of twin pairs of clouds moving away from the compact source at similar to 0.92 of the speed of light and angles of similar to 70 degrees with respect to the line of sight. The flux ratios and time evolution of the expelled clouds are consistent with actual motions of the radiating matter rather than with the simple propagation of pulses in a medium moving at slower velocities. The large kinetic power of the sudden, short, and rather discontinuous ejections exceeds by more than an order of magnitude the maximum steady photon luminosity of the source: suggesting that in GRS 1915+105 a radiation acceleration mechanism of the ejecta is unlikely. As in other galactic and extragalactic radio sources, the decrease in flux density as a function of angular separation from the central source shows a steepening with distance. Additional ejection events have been observed in 1995 and 1997 and we compare them with the 1994 events.