Scorpius X-1: Energy Transfer from the Core to the Radio Lobes

The evolution of the radio emission from Sco X-1 is determined from a 56-hourcontinuous VLBI observation and from shorter observations over a four-yearperiod. The radio source consists of a variable core near the binary, and twovariable compact radio lobes which form near the core, move diametricallyoutward, then fade away. Subsequently, a new lobe-pair form near the core andthe behavior repeats. The differences in the radio properties of the two lobesare consistent with the delay and Doppler-boosting associated with an averagespace velocity of 0.45c at 44 deg to the line of sight. Four lobe speeds,between 0.32c and 0.57c, were measured for several lobe-pairs on differentdays. The speed during each epoch remained constant over many hours. Thedirection of motion of the lobes over all epochs remained constant to a fewdegrees. Two core flares are contemporaneous with two lobe flares after removal of thedelay associated with an energy burst moving with speed >0.95c in a twin-beamfrom the core to each lobe. This is the first direct measurement of the speedof energy flow within an astrophysical jet. The similarity of the core and lobeflares suggests that the twin-beam flow is symmetric and that the core islocated near the base of the beam.Comment: 9 pages total, four figures. ApJ Letters, in pres