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

Abstract

The evolution of the radio emission from Sco X-1 is determined from a 56-hour continuous VLBI observation and from shorter observations over a four-year period. The radio source consists of a variable core near the binary, and two variable compact radio lobes which form near the core, move diametrically outward, then fade away. Subsequently, a new lobe-pair form near the core and the behavior repeats. The differences in the radio properties of the two lobes are consistent with the delay and Doppler-boosting associated with an average space 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 different days. The speed during each epoch remained constant over many hours. The direction of motion of the lobes over all epochs remained constant to a few degrees. Two core flares are contemporaneous with two lobe flares after removal of the delay associated with an energy burst moving with speed >0.95c in a twin-beam from the core to each lobe. This is the first direct measurement of the speed of energy flow within an astrophysical jet. The similarity of the core and lobe flares suggests that the twin-beam flow is symmetric and that the core is located near the base of the beam.