Nothing can emerge from a black hole. Yet, in nature, we find ultra-powerful jets of
energy that shoot out from the immediate vicinities of growing black holes. How these
jets form remains a puzzle.
In a new study appearing in the journal Nature Astronomy, astronomers
announce that they have new clues to this mystery. They studied V404 Cygni – one of
the famous black hole binaries in our Galaxy – using the ULTRACAM fast imager mounted on the William Herschel Telescope (WHT),
combined with NASA's NuSTAR telescope in Earth orbit, when it was undergoing a bright episode
of growth activity during June 2015. They found a fleeting time delay of just
100 milli-seconds (0.1 seconds) between X-rays and optical flashes of light.
"The time delay between the X-rays and the optical thus tells us the size of the inner jet
where the plasma undergoes strong acceleration," said Dr. Poshak Gandhi, lead author
from the University of Southampton, UK. "The nature of these optical flashes has been
questioned. But V404 Cygni showed these delayed flashes while the jet emission was
observed to be strengthening. This means that the fastest flashes originate in the jet."
The time delay can be translated to a maximum distance that the plasma could have
travelled in this duration. This distance is approximately 30,000 kilometers – a tiny
distance in cosmic terms – representing the inner acceleration zone in the jet. Beyond
this region, the jet plasma glows brightly, possibly because of shocks created by chunks
of plasma colliding at very high speed.
"Probing these inner zones in jets is exciting because it allows us to constrain theories
of extreme particle acceleration in nature," said Dr. Gandhi. "Strong magnetic fields
have been invoked to explain the launching of jet plasma but there remain many
uncertainties in matching up theory with observations. Our new observations will
certainly help in this regard."
The scientists are further excited because of evidence that jets in supermassive black
holes weighing millions or times more than V404 Cygni could behave similarly.
