An exceptionally bright outburst of high-energy light, produced by a black hole chowing down on material from its stellar companion, has been watched very closely by an ESA satellite over the past week, signalling renewed activity after a relatively silent 26 years.
The black hole-star pairing known as V404 Cygni, located in our Milky Way galaxy at a distance of almost 8,000 light years away in the constellation of Cygnus, was found to be extremely active in 1989 by the Japanese X-ray satellite Ginga as well as the high instruments aboard the now decommissioned Mir space station.
In a binary system like V404 Cygni, the black hole pulls material from the star to form a disc around itself. The stellar material is heated to extremely high temperatures, which causes it to shine brightly in optical, ultraviolet and X-ray wavelengths before it’s swallowed by the black hole. However, astronomers believe that in the case of this exotic duo, there seems to be a case of material slowly piling up in the disc. When a tipping point is reached, the black hole’s feeding routine is dramatically altered for a short while, causing peaks of activity every two to three decades.
“The behaviour 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,” explains Erik Kuulkers, a project scientist of the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite, which observed the source in gamma rays. “In these moments, it becomes the brightest object in the X-ray sky – up to fifty times brighter than the Crab Nebula, which is normally one of the brightest sources in the high-energy sky.”
The very first murmurs of activity in V404 Cygni were picked up by the Burst Alert Telescope on NASA’s Swift satellite, which detected a sudden burst of gamma rays that prompted it to observe the source with its X-ray telescope. It wasn’t too long after that Japan’s MAXI (Monitor of All-Sky X-ray Image) on the International Space Station noticed an X-ray flare in the same area of sky.
“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,” says Kuulkers. “It is definitely a ‘once in a professional lifetime’ opportunity.”
V404 Cygni’s outburst back in 1989 was an important event for our understanding of not only black holes, but also the star in the system, which astronomers turned their attention to when the pairing had returned to a quieter state. They found it to be half the mass of our Sun and, by analysing the behaviour of the binary system, were able to determine that its companion must be a black hole that tipped the scales with a mass 12 times that of our star.
“Now that this extreme object has woken up again, we are all eager to learn more about the engine that powers the outburst we are observing,” says Carlo Ferrigno from the INTEGRAL Science Data Centre at the University of Geneva, Switzerland. “I received a text message at 1:30am on 18 June from our burst alert system, which is designed to detect gamma ray bursts in INTEGRAL’s data. In this case, it turned out to be an exceptional flare – definitely a good reason to be woken up in the middle of the night!”
The exciting source, which has remained very active, has caused astronomers all over the world to turn telescopes – such as Tenerife’s Gran Telescopio Canarias that holds the largest telescope mirror available in optical astronomy – to V404 Cygni’s location to ensure that not one moment of the binary system’s frenzied behaviour is missed.
“We are looking forward to testing our current understanding of black holes and their feeding habits with this data,” says Teo Muñoz-Darias from the Instituto de Astrofísica de Canarias in Tenerife. “This is definitely material for the astrophysics textbooks for the coming years.”