The motion and variability of the radio components in the low mass X-ray binary system Sco X-1 have been monitored with extensive VLBI imaging at 1.7 and 5.0 GHz over four years , including a 56-hour continuous VLBI observation in 1999 June .
We detect one strong and one weak compact radio component , moving in opposite directions from the radio core .
Their relative motion and flux densities are consistent with relativistic effects , from which we derive an average component speed of v/c= 0.45 \pm 0.03 at an angle of 44 ^ { \circ } \pm 6 ^ { \circ } to the line of sight .
This inclination of the binary orbit suggests a mass of the secondary star that is < 0.9 ~ { } M _ { \odot } , assuming a neutron star mass of 1.4 M _ { \odot } .
We suggest that the two moving radio components consist of ultra-relativistic plasma that is produced at a working surface where the energy in dual-opposing beams disrupt .
The radio lobe advance velocity is constant over many hours , but differs among lobe-pairs : 0.32c , 0.46c , 0.48c , and 0.57c .
A lobe-pair lifetime is less than two days , with a new pair formed near the core within a day .
The lobe flux has flux density that is variable over a time-scale of one hour , a measured minimum size of 1 mas ( 4 \times 10 ^ { 8 } km ) , and is extended perpendicular to its motion .
This time-scale and size are consistent with an electron radiative lifetime of < 1 hr .
Such a short lifetime can be caused by synchrotron losses if the lobe magnetic field is 300 G or by adiabatic expansion of the electrons as soon as they are produced at the working surface .
The lobes also show periods of slow expansion and a steepening radio spectrum .
Two of the core flares are correlated with the lobe flares under the assumption that the flares are produced by a coherent energy burst traveling down the beams with a speed > 0.95 c .

The radio morphology for Sco X-1 differs from most other Galactic jet sources .
Possible reasons for the morphology difference are : Sco X-1 is associated with a neutron star , it is a persistent X-ray source , the source is viewed significantly away from the angle of motion .
However , the lobes in Sco X-1 are similar to the hot-spots found in many extragalactic radio double sources .
Scaling the phenomena observed in Sco X-1 to extragalactic sources implies radio source hot-spot variability time-scales of 10 ^ { 4 } yr and hot-spot lifetimes of 10 ^ { 5 } yr .