Astronomers using a fleet of orbiting telescopes, including ESA’s Integral satellite, NASA’s Swift satellite and the Japanese MAXI telescope, have observed a strong outburst of gamma rays and X-rays produced by a low-mass black hole in a binary system called V404 Cygni.
Artist’s impression of a black hole feasting on matter from its companion star in a binary system. Image credit: ESA / ATG medialab.
V404 Cygni lies in the constellation Cygnus at a distance of about 8,000 light-years from Earth. It is a binary system comprising a black hole and a star orbiting one another.
In this type of binary, material flows from the star towards the black hole and gathers in a disc, where it is heated up, shining brightly at optical, UV and X-ray wavelengths before spiraling into the black hole.
V404 Cygni first jumped to prominence during a nova explosion more than 70 years ago, in 1938.
The system also produced eruption in 1989, which was discovered by the Japanese X-ray satellite Ginga and high-energy instruments on board the Mir space station.
The 1989 outburst (Nova Cygni 1989) was crucial in the study of black holes. Until then, astronomers knew only a handful of objects that they thought could be black holes, and V404 Cygni was one of the most convincing candidates.
On 15 June 2015, the first signs of renewed activity in this system were spotted by NASA’s Swift satellite, detecting a sudden burst of gamma rays, and then triggering observations with its X-ray telescope. Soon after, the Japanese MAXI (Monitor of All-sky X-ray Image) observed an X-ray flare from the same patch of the sky.
As part of the worldwide effort, ESA’s Integral gamma-ray observatory started monitoring the out-bursting black hole on 17 June.
“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. In these moments, it becomes the brightest object in the X-ray sky – up to 50 times brighter than the Crab Nebula, normally one of the brightest sources in the high-energy sky,” said Dr Erik Kuulkers of ESA, a scientist for the Integral mission.
Since the first outburst detection on 15 June, the system has remained very active, keeping scientists extremely busy.
There are only a handful of black-hole binary systems for which data have been collected simultaneously at many wavelengths, and the current outburst of V404 Cygni offers the rare chance to gather more observations of this kind.
Over the past week, several teams of astronomers around the world published over twenty Astronomical Telegrams and other official communications, sharing the progress of the observations at different wavelengths.
“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,” said Dr Carlo Ferrigno from the Integral Science Data Center at the University of Geneva, Switzerland.
“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,” Dr Kuulkers said.
Dr Peter Kretschmar of ESA, a manager for the Integral mission, added: “the observations will soon be made available publicly, so that astronomers across the world can exploit them to learn more about this unique object.”
