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

Radio imaging of the microquasar GRS 1915+105 with the Very Long Baseline Array (VLBA) over a range of wavelengths (13, 3.6, 2.0, and 0.7 cm), in different states of the black hole binary, always resolves the nucleus as a compact jet of length ~10λ_cm AU. The nucleus is best imaged at the shorter wavelengths, on scales of 2.5-7 AU (0.2-0.6 mas resolution). The brightness temperature of the core is T_B ≥ 10⁹ K, and its properties are better fitted by a conically expanding synchrotron jet model rather than a thermal jet. The nuclear jet varies in ~30 minutes during minor X-ray/radio outbursts and reestablishes within ~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 ~500 AU separation from the stationary core. Time-lapse images clearly detect the superluminal motion 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 the ejecta differs by ≤12° clockwise from the axis of the AU-scale jet, measured in the same observation. Both axes are stable in time (±5°), the AU scale for 2 yr and the large scale for over 4 yr. Astrometry over 2 yr relative to an extragalactic reference locates the black hole to ±1.5 mas, and its secular parallax due to Galactic rotation is 5.8 ± 1.5 mas yr^-1, consistent with a distance of 12 kpc. Finally, a limit of ≤100 km s^-1 is placed on its proper motion with respect to its neighborhood.

Some accreting black holes of stellar mass (e.g., Cyg X-1, 1E 1740-2942, GRS 1758-258, GX 339-4) and supermassive black holes at the center of galaxies (e.g., Sgr A*) lack evidence of large flares and discrete transient ejecta but have compact radio cores with steady, flat-spectrum "plateau" states, like GRS 1915+105. To the present day GRS 1915+105 is the only system where both AU-scale steady jets and large-scale superluminal ejections have been unambiguously observed. Our observations suggest that the unresolved flat-spectrum radio cores of accreting black holes are compact quasi-continuous synchrotron jets.