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.
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.
Astrophysicists can determine how the blazar grew brighter in different parts of the spectrum at different times and refine their models of how fast-moving particles within the jets give rise to the high-energy light.