30 October 2017
An international team of scientists, led by the UK, are a step closer to understanding how black holes shoot out ultra-powerful jets of energy, like nature’s very own Death Star beams.
With funding from the STFC, the research team has studied these mysterious cosmic phenomena which appear like deadly rays from the Star Wars super-weapon – known as relativistic jets – to try to understand how they form.
One theory suggests that they develop within the ‘accretion disc’ – the matter sucked into the orbit of a growing black hole. Extreme gravity within the disc twists and stretches magnetic fields, squeezing hot, magnetised disc material called plasma until it erupts in the form of oppositely directed magnetic pillars along the black hole’s rotational axis.
Plasma travels along these focused jets and gains tremendous speed, shooting across vast stretches of space. At some point, the plasma begins to shine brightly, but how and where this occurs in the jet has been debated by scientists.
The team studied one of the brightest bursts of light ever seen from a black hole, in a binary system called V404 Cygni – consisting of a star and a black hole closely orbiting each other, with the black hole feeding off matter from the star that falls through the disc – to throw light on this hotly debated phenomenon.
V404 Cygni is located about 7,800 light years away in the constellation of Cygnus, and weighs as much as about nine of our Suns put together. The energy emitted from the X-ray burst was powerful enough to tear apart an Earth-like planet if properly focused.
Using telescopes on Earth and up in space, the team captured a 0.1-second delay between X-ray flares emitted from near the black hole, where the jet forms, and the appearance of visible light flashes 19,000 miles away.
In Star Wars terms, this can roughly be likened to measuring the distance between the surface of the Death Star, where multiple rays of light shoot out, and the point where they converge into a single bright beam.
Lead author Dr Poshak Gandhi, of the University of Southampton, said: “Scientists have been observing jets for decades, but are far from understanding how nature creates these mind-bogglingly vast and energetic structures.
“Now, for the first time, we have captured the time delay between the appearance of X-rays and the appearance of optical light in a stellar-mass black hole at the moment jet plasma is activated. This lays to rest the controversy regarding the origin of the optical flashes, and also gives us a critical distance over which jet plasma must have been strongly accelerated to speeds approaching that of light.
“But the physics of black hole jets has nothing to do with lasers or the fictional Kyber crystals that power the Death Star. Nature has found other ways to power jets. Gravity and magnetic fields play the key roles here, and this is the mechanism we are trying to unravel.”