On the nature of the microquasar GRS 1915+105: clues from radio polarization imaging

MIRABEL IF

Washington

Congreso; AAS Meeting 218, 230.11; 2011

American Astronomical Society

We present a sequence of images of the polarized radio emission from the Galactic superluminal source GRS 1915+105 made from archival VLA data taken in 1994. Between February and April there are 10 observations made in the A array, mostly at 8 GHz, and four outbursts can be seen. The images reveal a wealth of information which cannot be obtained from the total intensity images. The second and third outbursts are well observed in polarization and they exhibit very different behaviors. In the second outburst the magnetic field direction is aligned along the jets throughout the burst. We show that the evolution of the total intensity and fractional polarization can be fit with a simple shock-in-jet model. The third outburst (in which Mirabel and Rodriguez discovered superluminal motion) behaves quite differently. It is a much more powerful outburst and it decays more slowly than the second outburst. Its polarization behavior is complex. The polarization electric vector position angles in both jets rotate rapidly, but in opposite directions and at different rates. The fractional polarization also changes in a complex way indicating internal polarization structure. In the last three epochs the VLA resolves this polarization structure in the south-east (approaching) jet, which is seen at later times in its evolution than the north-west (receding) jet, due to light travel time effects. The third outburst evolves too rapidly and in too complex a manner to fit a detailed model, but a qualitative description in terms of the shock-in-jet model can be given. This work was supported by the NSF.