Building an event horizon telescope: (sub)mm VLBI in the ALMA era

Abstract

Recent technical and scientific progress in (sub)mm VLBI now makes it very likely that we will be able to observe and resolve a black hole event horizon within this decade. Over the next few years, new facilities, including ALMA, will join current (sub)mm VLBI arrays, creating a high sensitivity ’Event Horizon Telescope’ (EHT) capable of angular resolutions approaching 20μas. For SgrA*, the super massive black hole candidate at the Galactic Center, and M87, the black-hole powered core of a giant elliptical galaxy, these angular resolutions correspond to just a few Schwarzschild radii. Recent detections of both these sources confirm the existence of structure on event horizon scales. Enhancing the current (sub)mm VLBI array to enable imaging of these objects and time resolution of flaring structures will require technical development, but the needed improvements are relatively straightforward, making this a very tractable project. The work includes continued improvement in the bandwidth of VLBI backend systems, development of phased array systems to combine the collecting area of connected element arrays, deployment of modern ALMA-quality receivers, development of new (sub)mm VLBI sites, and migration to wideband software correlators.