Something is stirring out there in the inky blackness of space. After 26 years of keeping quiet, a monster black hole has finally woken up, and its high-energy bursts can be detected all the way from Earth. But there's nothing to fear, this black hole is an old acquaintance. Since awakening from its stellar slumber, the scientific community has been scrabbling around, trying to detect its latest energetic burst in different frequencies all across the globe.

On June 15, 2015, astronomers spotted an X-ray burst from a familiar location. V404 Cygni, a system containing a black hole and a star locked in orbit around each other, is located within the Milky Way, some 8,000 light-years away from Earth in the swan constellation, Cygnus. The last time that V404 Cygni sent us over a sweet "hello" in X-ray form was in 1989. Thus, the moment that X-ray bursts were detected from V404 Cygni was an exciting occassion for many astronomers, and also highlighted how far astrophysics has progressed in the last 26 years.

"The community couldn't be more thrilled: many of us weren't yet professional astronomers back then, and the instruments and facilities available at the time can't compare with the fleet of space telescopes and the vast network of ground-based observatories we can use today. It is definitely a 'once in a professional lifetime' opportunity," said Erik Kuulkers, an Integral project scientist at the European Space Agency (ESA).

Photograph of V404 Cygni suddenly becoming very bright in the sky as it emits X-rays. ESA/Integral/IBIS/ISDC.

Spotting X-rays from the sky is not an everyday occurrence. Only the most energetic bodies in the universe spit out this form of radiation: supernovae, neutron stars or black holes, to name a few candidates. So, how does V404 Cygni – a black hole-star binary system – emit a burst of X-rays?

Occasionally, the star and black hole will get too close to one another and the black hole's powerful gravitational attraction will suck up some of the star's outer layers of matter. The accreted star matter swirls in a disk around the black hole, where it gains energy and heats up. As the temperature rises, the stardust starts to release more and more energetic bursts of light – from optical to ultraviolet, all the way to X-rays (and sometimes even gamma rays if it gets unimaginably hot).

"The behavior of this source is extraordinary at the moment, with repeated bright flashes of light on time scales shorter than an hour, something rarely seen in other black hole systems," said Kuulkers. "In these moments, it becomes the brightest object in the X-ray sky – up to fifty times brighter than the Crab Nebula, normally one of the brightest sources in the high-energy sky."

[Via ESA]