The source of the cosmic light show is a massive flare-up of Markarian 421, a "blazar" that hosts a supermassive black hole, they said.
Markarian 421 is a hot topic of discussion at the American Physical Society meeting in Denver.
"It's really quite exciting because we can exchange ideas about it while we're here at the meeting in the same place," Greg Madejski of the Kavli Institute of Particle Astrophysics and Cosmology said.
Blazars are a unique example of "active galaxies" hosting supermassive black holes emitting huge amounts of light across the whole electromagnetic spectrum as they feed on surrounding matter.
The galaxies emit jets of light, trillions of times more energetic than visible light, and a blazar is such a galaxy whose jet happens to be aimed directly at the Earth.
Markarian 421 was already in the known catalogue of blazars, but its flare-up beginning last week was the strongest ever observed, astronomers said.
"I'm in shock and awe at how bright it is," Julie McEnery, project scientist for the Fermi gamma-ray telescope, told the BBC.
"This thing is blowing us away."