The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation 1 . Sagittarius A*, the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4 million times that of the Sun 2,3 . A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A* where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering 4-7 . Here we report observations at a wavelength of 1.3 mm that set a size

Black Hole Close-Up M87 is a giant elliptical galaxy about 55 million light-years away. Accretion of matter onto its central massive black hole is thought to power its relativistic jet. To probe structures on scales similar to that of the black hole's event horizon, Doeleman et al. (p. 355 , published online 27 September) observed the relativistic jet in M87 at a wavelength of 1.3 mm using the Event Horizon Telescope, a special purpose, very-long-baseline interferometry array consisting of four radio telescopes located in Arizona, California, and Hawaii. The analysis suggests that the accretion disk that powers the jet orbits in the same direction as the spin of the black hole.

Sagittarius A*, the ∼ 4 × 10 6 M ⊙ black hole candidate at the Galactic Center, can be studied on Schwarzschild radius scales with (sub)millimeter wavelength Very Long Baseline Interferometry (VLBI). We report on 1.3 mm wavelength observations of Sgr A* using a VLBI array consisting of the JCMT on Mauna Kea, the ARO/SMT on Mt. Graham in Arizona, and two telescopes of the CARMA array at Cedar Flat in California. Both Sgr A* and the quasar calibrator 1924−292 were observed over three consecutive nights, and both sources were clearly detected on all baselines. For the first time, we are able to extract 1.3 mm VLBI interferometer phase information on Sgr A* through measurement of closure phase on the triangle of baselines. On the third night of observing, the correlated flux density of Sgr A* on all VLBI baselines increased relative to the first two nights, providing strong evidence for time-variable change on scales of a few Schwarzschild radii. These results suggest that future VLBI observations with greater sensitivity and additional baselines will play a valuable role in determining the structure of emission near the event horizon of Sgr A*.

The Galactic Center black hole SagittariusA * (Sgr A * ) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on SgrA * obtained with the EHT on a total of 13 observing nights over four years. Closure phases, which are the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180°rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of SgrA * that is not obscured by refraction due to interstellar electrons along the line of sight.

The Mark 4 VLBI correlator is a station‐based system designed to process data from up to 16 stations at an aggregate data rate of 1 Gbps/station. It is compatible with data recorded on Mark 3A, Mark 4, and VLBA tape‐based data‐acquisition systems, as well as the new disc‐based Mark 5 system which also supports both real‐time and quasi‐real‐time data‐transmission over high‐speed networks. The system incorporates an XF algorithm implemented in a custom‐VLSI chip that can be flexibly configured to cross‐correlate VLBI data from one to four baselines of a single 2‐bits/sample channel at 64 Mbps/channel/station. Each chip also incorporates integral phase generators, rotators and vernier‐delay management circuitry. Each of the sixteen Correlator Boards in the system contains 32 correlator chips and can process up to 8192 complex lags that can be flexibly traded off between baselines, lags and channels. Innovative algorithms allow station‐model information to be injected periodically into the data stream from each channel of each station to drive the correlation process. The correlator control software allows up to four independent multistation scans to be processed simultaneously to significantly improve processing efficiency.