The first black hole found in our galaxy flashed violent red blasts 1,000 times as intense as the Sun as it gorged itself on a star during a two-week feeding frenzy last year.
In the journal Monthly Notices of the Royal Astronomical Society, astronomers led by Poshak Gandhi from the University of Southampton in the UK report that the black hole V404 Cygni blasted dazzling red flashes as it expelled material it could not swallow.
Black holes such as V404 Cygni, which is around 7,800 light-years form Earth, undergo periods of relative calm punctuated by furious feeding. But their unpredictable nature, and rarity of these outbursts, means astronomers have very little time to swing their telescopes in their direction.
Luckily, in June 2015, V404 Cygni was feeling particularly greedy as it devoured material stripped off an orbiting companion star for two weeks, flashing red all the while.
The red colour, astronomers think, is the result of fast-moving jets of matter, scraps ejected from near the black hole.
"The very high speed tells us that the region where this red light is being emitted must be very compact," Gandhi says.
"Piecing together clues about the colour, speed and the power of these flashes, we conclude that this light is being emitted from the base of the black hole jet."
And while the origin of these jets is still unknown, astronomers suspect strong magnetic fields play a role.
"The duration of these flashing episodes could be related to the switching on and off of the jet, seen for the first time in detail," Gandhi says.