A Flare for Acceleration

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Abstract
As black holes swallow matter, they often expel jets of plasma that can reach speeds very close to the speed of light. In the giant elliptical galaxy M87, 50 million light-years away, radio observations have traced such jets down to the immediate surroundings of the black hole, to less than 100 event horizon radii (1, 2). However, radio observations alone cannot provide a complete picture of the power carried by these jets, their speed, or their composition. Gamma-rays, first detected from jets more than a decade ago, provide crucial evidence that particles are being accelerated to very high energies. But the relatively poor angular resolution of gamma-ray telescopes has prevented direct determination of where these particles are coming from. That uncertainty has now been resolved, at least for M87. On page 444 of this issue, Acciari et al. (3) report the first radio imaging of the site of a gamma-ray flare—and it is very close to the black hole.