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 ≥10 9 K, and its properties are better fit by a conically expanding synchrotron jet model, rather than a thermal jet. The nuclear jet varies in ∼30 min during minor X-ray/radio outbursts, and re-establishes within ∼18 hours 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 1.5±0.1 c (D/12 kpc) for the approaching component, and is consistent with ballistic motion of the ejecta from 500 AU outwards, 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 two years, and the large scale for over four years. Astrometry over two years 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 neighbourhood.Some accreting black holes of stellar mass (e.g. Cyg X-1, 1E 1740-2942, GRS 1758 and supermassive black holes at the centre of galaxies (e.g. Sgr A * ) lack evidence of large flares and discrete transient ejecta, -3but have compact radio cores with steady, flat-spectrum 'plateau' states, like GRS 1915+105. Until now GRS 1915+105 is the only system where both AUscale 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.

A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

Using the Very Long Baseline Array, we have measured a trigonometric parallax for the microquasar GRS 1915+105, which contains a black hole and a K-giant companion. This yields a direct distance estimate of 8.6 +2.0 −1.6 kpc and a revised estimate for the mass of the black hole of 12.4 +2.0 −1.8 M ⊙ . GRS 1915+105 is at about the same distance as some H II regions and water masers associated with high-mass star formation in the Sagittarius spiral arm of the Galaxy. The absolute proper motion of GRS 1915+105 is −3.19 ± 0.03 mas y −1 and −6.24 ± 0.05 mas y −1 toward the east and north, respectively, which corresponds to a modest peculiar speed of 22 ± 24 km s −1 at the parallax distance, suggesting that the binary did not receive a large velocity kick when the black hole formed. On one observational epoch, GRS 1915+105 displayed superluminal motion along the direction of its approaching jet. Considering previous observations of jet motions, the jet in GRS 1915+105 can be modeled with a jet inclination to the line of sight of 60 • ± 5 • and a variable flow speed between 0.65c and 0.81c, which possibly indicates deceleration of the jet at distances from the black hole 2000 AU. Finally, using our measurements of distance and estimates of black hole mass and inclination, we provisionally confirm our earlier result that the black hole is spinning very rapidly.Unlike the black hole binaries V404 Cyg (Miller-Jones et al. 2009) and Cyg X-1 (Reid et al. 2011), for which measurements of trigonometric parallax were relatively straightforward, GRS 1915+105 is a difficult target for several reasons. Firstly, compact radio emission from the base of the jets, associated with the X-ray hard state, occurs only about 25% of the time. Secondly, when in the hard state, the source is typically weak, < 10 mJy at cm-wavelengths, and often falls below astrometrically useful flux densities of < ∼ 1 mJy. Owing to these difficulties, a long series of observations were required, as documented in Section 2. Thirdly, on some occasions, bright knots of radio emission propagate down the jets, leading to outlying data points when fitting for parallax and proper motion. Such behavior was clearly observed