From Harvard Astronomy: “Shep Doelman: Imaging Black Holes with The Event Horizon Telescope”

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Harvard Astronomy

November 19, 2013

Shep Doelman,

Recent technical advances and observations have now demonstrated that the goal of making an image of a black hole is within reach. Using the technique of Very Long Baseline Interferometry (VLBI), in which widely separated radio dishes are linked together to form an Earth-sized array, our group has succeeded in confirming event horizon scale structures in two super massive black holes: Sagittarius A*, the 4 million solar mass black hole at the center of the Milky Way (Nature, 455, 78, ’08), and M87, a 6 billion solar mass black hole in the giant elliptical galaxy Virgo A (Science, 338, 355, ’12). This has been accomplished by extending the VLBI technique to the highest observing frequencies and bandwidths, which has provided the required angular resolution and sensitivity.


To achieve true imaging capability, an international collaboration is developing next-generation VLBI instrumentation for deployment on a Global array of mm and submm wavelength facilities. This will extend the current 1.3mm VLBI array to Earth-diameter baselines for which the angular resolution obtained is well matched to the SgrA* and M87 event horizons. Efforts are also aimed at shorter wavelength observations at 0.87mm, where Global baselines can achieve <20 micro arcsecond resolution. This new array is called the Event Horizon Telescope (EHT).

EHT observations will target modeling and imaging of strong General Relativistic signatures that should become evident hear the black hole. Foremost among these is the black hole ‘shadow’, a consequence of light bending in the black hole’s strong gravity, leading to an annular brightening of the last photon orbit. The size and shape of this shadow is a prediction of Einstein’s GR. Non-imaging analyses of EHT data will be very sensitive to asymmetries caused by orbiting ‘hot-spots’ or Magnetohydrodynamic turbulence in the accretion flow. Observations of M87 will lead to direct imaging of emission at the base of a relativistic AGN jet. The overall goal is to spatially resolve a region of space-time where gravity is dominant, with an aim to test GR and models of black hole accretion and jet formation on Schwarzschild radius scales.

See the full article here.

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