Abstract
This paper briefly reviews past, current, and future efforts to image black holes. Black holes seem like mystical objects, but they are an integral part of current astrophysics and are at the center of attempts to unify quantum physics and general relativity. Yet, nobody has ever seen a black hole. What do they look like? Initially, this question seemed more of an academic nature. However, this has changed over the past two decades. Observations and theoretical considerations suggest that the supermassive black hole, Sgr A*, in the center of our Milky Way is surrounded by a compact, foggy emission region radiating at and above 230 GHz. It has been predicted that the event horizon of Sgr A* should cast its shadow onto that emission region, which could be detectable with a global VLBI array of radio telescopes. In contrast to earlier pictures of black holes, that dark feature is not supposed to be due to a hole in the accretion flow, but would represent a true negative image of the event horizon. Currently, the global Event Horizon Telescope consortium is attempting to make such an image. In the future those images could be improved by adding more telescopes to the array, in particular at high sites in Africa. Ultimately, a space array at THz frequencies, the Event Horizon Imager, could produce much more detailed images of black holes. In combination with numerical simulations and precise measurements of the orbits of stars – ideally also of pulsars – these images will allow us to study black holes with unprecedented precision.
Highlights
When the name black hole was coined in the sixties – allegedly a term picked up by science journalist Ann Ewing and later popularized by John A
It is crucial that the emission region extends well inside the ISCO, otherwise we would mainly see a hole in the accretion flow, rather than a black hole shadow
Current general-relativistic magnetohydrodynamic simulations (GRMHD) models with a radiatively inefficient accretion flow and a jet outflow already explain the data for Sgr A* and M87* well
Summary
When the name black hole was coined in the sixties – allegedly a term picked up by science journalist Ann Ewing and later popularized by John A. The compact radio source, Sgr A*, found by Balick & Brown (1974), was often compared to radio cores in AGN, and considered to mark the supermassive black hole in our Milky Way. Near-infrared measurements of stellar orbits (Eckart & Genzel, 1996; Ghez et al, 1998), revealing stars moving with up to 10,000 km/s around this radio source, and very-long baseline interferometry (VLBI) observations (Reid & Brunthaler, 2004), have confirmed that there is a dark mass of about 4.3 million solar masses (Gillessen et al, 2017) within a few Schwarzschild radii of Sgr A* (see Genzel et al, 2010; Falcke & Markoff, 2013, for reviews). In Moscibrodzka & Falcke (2013) and Moscibrodzka et al (2014) we showed that the original simple analytical jet model does hold up in GRMHD simulations, reproduces the observational characteristics, and produces radio mm-wave emission close to the event horizon — just as needed for shadow imaging. ESO’s new near-infrared interferometer, GRAVITY, will make a big impact (Gravity Collaboration et al, 2017)
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