Very Long Baseline Interferometry Imaging

Radio interferometry is an astronomical observing technique to obtain high-resolution images of radio sources. In particular, VLBI (very long baseline interferometry) uses a global network of radio telescopes spread across different continents to form a virtual Earth-sized telescope. The radio wave signals emitted by e.g. the hot plasma surrounding the black hole are recorded separately at the individual telescopes. Afterwards, the signals are cross-correlated for all pairs of antennas at a central location, e.g. at the Max Planck Institute for Radio Astronomy in Bonn, Germany or MIT Haystack Observatory in Westford, MA, USA. With the correlated signals measured on each baseline (the imaginary line connecting the two telescopes as seen from the observed source), an image of the emission surrounding the black hole can eventually be reconstructed.

The short baselines between telescopes located closely together give information about the large-scale structure of the source, and the long baselines between telescopes located far away from each other sample the detailed small-scale structure of the source. The larger the separation between the telescopes (or the longer the baseline), the higher is the resolving power. In order to reconstruct a high-fidelity image of the source, a VLBI array should include as many stations as possible and the resulting baselines should include both east-west and north-south orientations across different continents (as shown in the image). A VLBI observation is always performed for several hours so that the Earth rotates during the observations, changing the baselines as seen from the source, so that as many components of the source structure as possible can be sampled. The angular resolution of the VLBI array not only increases with the distance between the telescopes, but also with the frequency of the observed radiation. At frequencies of about 230 GHz, corresponding of a wavelength of 1.3 mm, Earth-size baselines correspond to an angular resolution of tens of microarcseconds, which is comparable to the size of an apple on the Moon. This is the highest angular resolution ever achieved in ground-based astronomy, and it allows us to observe supermassive black holes such as Sgr A* and M87 on scales comparable to the Event Horizon.

For this purpose, an international collaboration, including BlackHoleCam, has assembled the Event Horizon Telescope (EHT), a mm-VLBI network of existing (and up-coming) mm-wavelength telescopes spread across several continents to form a global interferometer. The EHT includes mm-telescopes in Europe (IRAM Pico Veleta, and the up-coming phased-NOEMA), USA (JCMT/SMA, SMT, KPNO), Mexico (LMT), South America (APEX, ALMA), and South Pole. The first observations are scheduled for April 2017. For more details, please visit the EHT website.