It was the smallest bullet you could possibly imagine, a subatomic particle weighing barely more than a thought. It had been fired out of a gravitational gun barrel by a cosmic blunderbuss, a supermassive black hole.
On Sept. 22, 2017, a particle known as a neutrino zinged down from the sky and through the ice of Antarctica at nearly the speed of light, setting off a cascade of alarms in an array of detectors called IceCube.
Within seconds IceCube had alerted an armada of astronomical satellites, including the Fermi Gamma-ray Space Telescope. That spacecraft traced the neutrino back to an obscure dot in the sky, a distant galaxy known as TXS 0506+056, just off the left shoulder of the constellation Orion, which was having a high-energy outburst of X-rays and gamma-rays.
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