AU-Scale Synchrotron Jets and Superluminal Ejecta in GRS 1915+105

Abstract

Radio imaging of the microquasar GRS 1915+105 with the Very Long Baseline Array (VLBA) during various flaring, quiescent, or oscillatory states of the black hole binary, always resolves the nucleus as a compact jet of length about 10.lambda_cm AU. On scales of 2.5-7AU, the nucleus has brightness temperature T_B > 10^9 K, and its properties are better fit by a synchrotron jet, rather than a thermal jet. The nuclear jet varies in 10-60min during minor X-ray/radio outbursts, and re-establishes within 18hrs of a major outburst, indicating the robustness of the disk/jet system to disruption. At lower resolution (80-240AU), time-lapse images reveal discrete superluminal ejecta at 500-600 AU separation from the nucleus. The measured velocity is 1.5c+-0.1c(D/12 kpc) for the approaching component, consistent with ballistic motion of the ejecta from 500AU outwards, perhaps even since birth. The axis of the ejecta differs by <12deg clockwise from the axis of the AU-scale jet, measured in the same observation. Both axes are stable for years. The black hole is astrometrically located to +-1.5mas, after accounting for its secular parallax from Galactic rotation of 5.8+-1.5mas/yr at the distance of 12kpc, and a limit of <100km/s is placed on its proper-motion with respect to its neighbourhood. Our observations suggest that the unresolved flat-spectrum radio cores of accreting black holes are compact quasi-continuous synchrotron jets.