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

Radio imaging of the microquasar GRS 1915+105 with the Very Long BaselineArray (VLBA) during various flaring, quiescent, or oscillatory states of theblack hole binary, always resolves the nucleus as a compact jet of length about10.lambda_cm AU. On scales of 2.5-7AU, the nucleus has brightness temperatureT_B > 10^9 K, and its properties are better fit by a synchrotron jet, ratherthan a thermal jet. The nuclear jet varies in 10-60min during minor X-ray/radiooutbursts, and re-establishes within 18hrs of a major outburst, indicating therobustness of the disk/jet system to disruption. At lower resolution(80-240AU), time-lapse images reveal discrete superluminal ejecta at 500-600 AUseparation from the nucleus. The measured velocity is 1.5c+-0.1c(D/12 kpc) forthe approaching component, consistent with ballistic motion of the ejecta from500AU 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 sameobservation. Both axes are stable for years. The black hole is astrometricallylocated to +-1.5mas, after accounting for its secular parallax from Galacticrotation of 5.8+-1.5mas/yr at the distance of 12kpc, and a limit of <100km/s isplaced on its proper-motion with respect to its neighbourhood. Our observationssuggest that the unresolved flat-spectrum radio cores of accreting black holesare compact quasi-continuous synchrotron jets.Comment: 32 pages + 10 figures; accepted Ap.J. 543, 1Nov2000. Replaced to fix merged figures. no changes in conten