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31 October, 2017

WHT Helps to Measure the Size of a Stellar-Mass Black Hole Jet

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.

Artist's impression of V404 Cygni, showing the black hole accreting material from a companion star through an accretion disc. Some plasma is accelerated through the jet, and begins to glow brightly. The location of these glowing flashes was measured by WHT/ULTRACAM. Credit: Gabriel Pérez Díaz (Instituto de Astrofísica de Canarias). Large format: JPEG.

Jets consist of a hot plasma soup, which is somehow sped up to relativistic speeds approaching that of light. Along the way, this plasma begins to glow with radiation, forming two bright columns along the black hole's axis of rotation. Scientists have long debated where and how this happens in the jet.

V404 Cygni comprises a black hole weighing about 9 times our Sun, orbited by a companion star which supplies material to feed the black hole. The X-ray light detected by NuSTAR is emitted by this infalling material as it spirals inwards in the form of a disc. The optical flashes emerge from the outflowing plasma in the jet.

Phil Charles, former Head of the Astronomy Group at the ING and Professor Emeritus at Southampton and Oxford placed these findings in context: "Black holes in our Galaxy were only definitively confirmed with the measurement of the compact object mass in V404 Cygni almost 25 years ago - an absolute minimum of 6 times that of our Sun meant that it could not possibly be a neutron star. Thus began this field of "stellar-mass black holes", and almost all the early ground-based observations were made with the WHT on La Palma.

It is then entirely fitting that this new discovery was also made with the WHT, equipped with ULTRACAM, a superfast imager par excellence. And it also involved two of the original V404 Cygni observers from the early 90s, Phil Charles and Jorge Casares, who have exploited the superb La Palma atmospheric conditions for black-hole hunting throughout the history of the WHT. The fact that we can now potentially unite the physics of jets in stellar and supermassive black holes is immensely satisfying."

Super-fast visible light flickering from a bright outbursting black hole V404 Cygni in 2015. Images from WHT/ULTRACAM. Credit: Poshak Gandhi (University of Southampton, UK) [ Animated GIF ]

Carrying out these measurements required several telescopes working together across the entire electromagnetic spectrum. Prof. Jorge Casares of the IAC said: "Simultaneous multiwavelength astronomy from telescopes around the Earth is not easy. Therefore, there are only a handful of observations like these which can study the behaviour of black holes at high time resolution across the electromagnetic spectrum. But this is a glimpse of the other studies which will surely come in the future."

More information:

P.Gandhi, et al., 2017, "An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system", Nature Astronomy, DOI: 10.1038/s41550-017-0273-3 [ Astro-ph | Nature Astronomy ]

"The William Herschel Telescope in La Palma measures the size of a stellar-mass black hole jet", IAC press release, 30 October 2017.

"Scientists penetrate mystery of raging black hole beams", University of Southampton press release, 30 October 2017.

"NuSTAR Probes Black Hole Jet Mystery", JPL press release, 30 October 2017.

"Scientists Discover New Insight into Nature's Own Death Stars Beams", University of Sheffield press release, 30 October 2017.

  • "A black hole's jets light up in less than a second", Nature Research Highlights, 6th November 2017.

    ING's ULTRACAM web page.

    From scientisits: Animation made using some representative data sets. "A Yardstick for Measuring Astrophysical Jets".

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    Last modified: 14 November 2017