An ALMA beamformer for VLBI and phased array science

Phasing all of the 12m ALMA dishes together will enable the array to function as a single telescope with an effective aperture of ~ 85m diameter. In conjunction with other (sub)mm wavelength facilities, a phased ALMA will serve as the high sensitivity anchor for (sub)mm VLBI arrays capable of resolving super massive black holes on Schwarzschild radius scales. Current (sub)mm VLBI arrays have already detected time variable Schwarzschild radius scale structure in Sgr A*, the presumed ~ 4×10⁶ M_sun black hole at the center of the Milky Way [1,2]. Harnessing the full collecting area of ALMA will transform short wavelength VLBI arrays by doubling angular resolutions and improving sensitivity by an order of magnitude. At 1.3mm and 0.8mm wavelength, VLBI arrays including a phased ALMA will be able to time-resolve changing structures at the event horizon of Sgr A*, search for periodic signatures of orbiting hot-spots in the innermost accretion flow, and study the jet launching region of the M87 jet with Schwarzschild radius resolution. A phased ALMA will also be a sensitive pulsar/transient observatory with the ability to search for shallow spectrum pulsars towards the Galactic Center and study known high frequency magnetars with sub-ns time resolution. This presentation will focus on the technical considerations of constructing and integrating a phased-array processor into the ALMA system. A detailed plan and design that conforms to all ALMA requirements and the construction schedule will be described. This design will enable initial VLBI and phased array science projects to be carried out with ALMA within 3 years.