Au-scale synchrotron jets and superluminal ejecta in GRS 1915+105

Radio imaging of the microquasar GRS 1915+105 with the Very Long Baseline A rray (VLBA) over a range of wavelengths (13, 3.6, 2.0, and 0.7 cm), in diff erent states of the black hole binary, always resolves the nucleus as a com pact jet of length similar to 10 lambda (cm),, AU. The nucleus is best imag ed at the shorter wavelengths, on scales of 2.5-7 AU (0.2-0.6 mas resolutio n). The brightness temperature of the core is T-B greater than or equal to 10(9) K, and its properties are better fitted by a conically expanding sync hrotron jet model rather than a thermal jet. The nuclear jet varies in simi lar to 30 minutes during minor X-ray/radio outbursts and reestablishes with in similar to 18 hr of a major outburst, indicating the robustness of the X -ray/radio (or disk/jet) system to disruption. At lower resolution (80-240 AU), more extended ejecta are imaged at similar to 500 AU separation from t he stationary core. Time-lapse images clearly detect the superluminal motio n of the ejecta in a few hours. The measured velocity is 1.5c + 0.1c (D/12 kpc) for the approaching component and is consistent with ballistic motion of the ejecta from 500 AU outward, perhaps even since birth. The axis of th e ejecta differs by less than or equal to 12 degrees clockwise from the axi s of the AU-scale jet, measured in the same observation. Both axes are stab le in time (+/- 5 degrees), the AU scale for 2 yr and the large scale for o ver 4 yr. Astrometry over 2 yr relative to an extragalactic reference locat es the black hole to +/-1.5 mas, and its secular parallax due to Galactic r otation is 5.8 +/- 1.5 mas yr(-1), consistent with a distance of 12 kpc. Fi nally, a limit of less than or equal to 100 km s(-1) is placed on its prope r motion with respect to its neighborhood. Some accreting black holes of stellar mass (e.g., Cyg X-1, 1E 1740-2942, GR S 1758-258, GX 339-4) and supermassive black holes at the center of galaxie s (e.g., Sgr A*) lack evidence of large flares and discrete transient eject a but have compact radio cores with steady, hat-spectrum "plateau" states, like GRS 1915 + 105. To the present day GRS 1915 + 105 is the only system w here both AU-scale steady jets and large-scale superluminal ejections have been unambiguously observed. Our observations suggest that the unresolved f lat-spectrum radio cores of accreting black holes are compact quasi-continu ous synchrotron jets.