The center of the Milky Way has long puzzled astronomers: around the supermassive black hole Sagittarius A* there are more gamma rays than expected. The source was alternately blamed on dark matter (whose nature was clarified by Vera Rubin and Fritz Zwicky) and on swarms of dead lighthouse stars — pulsars. A new model suggests a third explanation: the glow is an echo of an ancient jet ejected by the black hole.
Millions of years ago, the black hole launched a powerful jet of particles. It not only inflated two giant bubbles of gas above and below the Galaxy, but also scattered heavy protons around. Colliding with gas atoms, they still generate gamma-ray emission — a fading remnant of that long-ago event. The mechanism for extracting energy from the black hole's rotation, proposed by Roger Penrose in 1969, explains how the jet could have formed.
Calculations show: even if other sources make the main contribution, the proton trail of the jet is inevitable. For a black hole spun up to 90% of its maximum speed, it accounts for up to 10% of the observed gamma-ray excess.
Sagittarius A* packs 4 million Suns into a region smaller than Mercury's orbit. Its spin is so rapid that the edge of the black hole moves at nearly the speed of light.
In Isaac Asimov's 'Foundation,' the capital of the Galaxy lies at its very heart — the turbulent and mysterious center of the Milky Way.