Scorpius X-1: The evolution and nature of the twin compact radio lobes

The motion and variability of the radio components in the low-mass X-ray bi nary system Sco X-1 have been monitored with extensive VLBI imaging at 1.7 and 5.0 GHz over 4 yr, including a 56 hr continuous VLBI observation in 199 9 June. We detect one strong and one weak compact radio component, moving i n opposite directions from the radio core. Their relative motion and flux d ensities are consistent with relativistic effects, from which we derive an average component speed of v/c = 0.45 +/- 0.03 at an angle of 44 degrees +/ - 6 degrees to the line of sight. This inclination of the binary orbit sugg ests a mass of the secondary star that is less than 0.9 M-circle dot, assum ing a neutron star mass of 1.4 M-circle dot. We suggest that the two moving radio components consist of ultrarelativistic 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 2 days, with a new pair formed near the core within a day. The lobe flux has flux density that is variable over a timescale of 1 hr, has a measured mini mum size of 1 mas (4 x 10(8) km), and is extended perpendicular to its moti on. This timescale and size are consistent with an electron radiative lifet ime of less than 1 hr. Such a short lifetime can be caused by synchrotron l osses 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 al so show periods of slow expansion and a steepening radio spectrum. Two of t he core flares are correlated with the lobe flares under the assumption tha t the flares are produced by an energy burst traveling down the beams with a speed greater than 0.95. The radio morphology for Sco X-1 differs from mo st other Galactic jet sources. Possible reasons for the morphology differen ce are that Sco X-1 is associated with a neutron star, it is a persistent X -ray source, and 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 timescales of 10(4) yr and hot-spot lifetimes of 10(5) yr.