The culprit is a staggering flare-up of Markarian 421, a "blazar" that hosts a supermassive black hole.
BBC: Blazar Markarian 421's flare-up is cosmic coincidence
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Using an array of 10 powerful radio telescopes, aimed at the galaxy BL Lacertae, the researchers studied a black hole just as it was sending forth a blazar jet.
BBC: Black holes reveal more secrets
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Active galaxies emit jets of light - up to trillions of times more energetic than the light we see - and a blazar is one with a jet pointing toward the Earth.
BBC: Blazar Markarian 421's flare-up is cosmic coincidence