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 binary 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 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 ± 0.03 at an angle of 44° ± 6° to the line of sight. This inclination of the binary orbit suggests a mass of the secondary star that is less than 0.9 M_☉, assuming a neutron star mass of 1.4 M_☉. 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 minimum size of 1 mas (4 × 10⁸ km), and is extended perpendicular to its motion. This timescale and size are consistent with an electron radiative lifetime of less than 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 an energy burst traveling down the beams with a speed greater than 0.95. The radio morphology for Sco X-1 differs from most other Galactic jet sources. Possible reasons for the morphology difference 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 10⁴ yr and hot-spot lifetimes of 10⁵ yr.