Astronomers Clock a Black Hole Spinning at Half the Speed of Light

By Amber Jorgenson | January 9, 2019 4:15 pm
Black hole event horizon

This artist’s impression shows a disk of hot gas orbiting close to a black hole. (Credit: NASA/CXC/M. Weiss)

Black holes are massive beasts that annihilate anything that dares to cross them. We don’t know a whole lot about these invisible, terrifying bodies, but astronomers have found a new way to study their mysterious behavior.

By observing the X-rays blasting from a star torn apart by a black hole, a team of researchers were able to calculate how fast the black hole spins — clocking it at nearly 50 percent the speed of light. This marks the first time that astronomers used X-rays, which orbit the black hole every 131 seconds, to calculate its incredible speed. The research, which could help correlate a black hole’s age with its speed, was published today in the journal Science.

Stellar Shredding

The discovery dates back to November 2014, when astronomers were observing a galaxy 300 million light years from Earth. They saw the galaxy’s central, supermassive black hole lure in and rip apart a passing star. Known as a tidal disruption flare, this event created a blast of X-ray radiation that was strong enough to be seen from Earth. Since black holes don’t emit many X-rays on their own, a group of researchers decided to home in on the event.

And luckily for them, various space telescopes started measuring the black hole’s X-ray emissions after the flare was spotted. After combing through their data, the MIT-led team noticed a peculiar trend. They found that bursts of X-rays were appearing once every 131 seconds near the black hole’s event horizon — the point where it starts to swallow up material. These periodic emissions, which persisted for over 450 days, boosted the black hole’s total X-rays emissions by 40 percent.

Daring Duo

The team thinks that these powerful radiation bursts are actually caused by two stars instead of one. The original sighting in 2014 still holds up: a black hole lured in a passing star and tore it to pieces. Some of these stellar shreds, which emit massive amounts of X-ray radiation, were sucked into the back hole. Others, though, remained in the innermost stable circular orbit (ISCO) — the closest spot where objects can orbit a black hole without being devoured by it.

In that dangerously close orbit lies another star, thought to be a tiny, dense white dwarf. Researchers think that the dwarf’s gravity pulled in the bright stellar remnants, creating a halo of X-rays around it. These X-rays can be seen every time the star orbits the black hole, which is once every 131 seconds.

They combined the orbital speed of the star with the black hole’s mass, which is thought to be one million time more massive than the Sun, to figure out how quickly the black hole is spinning. According to their calculations, the black hole is rotating at nearly half the speed of light.

“That’s not super fast — there are other black holes with spins estimated to be near 99 percent the speed of light,” said Dheeraj Pasham, a postdoctoral fellow at MIT and lead author of the paper, in a media release. “But this is the first time we’re able to use tidal disruption flares to constrain the spins of supermassive black holes.”

These events only emit X-rays for a few hundred years, so the odds of witnessing one is incredibly rare. Even so, the researchers plan to search for more flare events around both younger and older black holes. By comparing the speeds of black holes at various ages, researchers can determine if they get faster and grow more powerful over time. These stats could shed light on their mysterious evolutions, plus reveal how they feed on a galaxy’s stars.

CATEGORIZED UNDER: Space & Physics, top posts
MORE ABOUT: black holes, stars
  • Oni-Sendai X

    does time dilation effect this measurement?
    have fast does the neutron star orbit if measured from it’s surface?
    I’m confused????

  • Nate

    Describing rotational speed in terms of c is somewhat ambiguous, since mixing linear and rotational units is generally a no-no. I assume they are measuring the tangential speed of matter at the ISCO? Or at some more exotic location such as the event horizon?

    Also, saying X-rays are orbiting doesn’t sound correct either. I assume photons of x-ray level energy do in fact orbit black holes, but we wouldn’t see those since they are stuck in the gravity well. If we do see them, they are obviously not in orbit but hitting our instruments. The stellar remnants emitting those x-rays is what orbits the black hole

  • OWilson

    Perhaps the term Black Hole is misleading?

    The original classical theory (1873) was that a black hole was a collapsed massive star, that who’s gravity was so strong that even light did not have sufficient escape velocity to escape, thus we could never directly observe one. It was assumed that the star was still there, consuming everything that came within reach.

    So perhaps this super massive star still exists, happily feeding on it’s neighbors. If so, it is hardly a ‘hole’, rather it is a very dense massive physical object. Which would leave open the possibility that the center of these massive stars would, like the center of the Earth, has no directional gravity, and would certainly not be some kind of infintesimal singularity.

    If so, and if they are relatively common in the universe, they would be great candidates for the Dark Matter. Invisible, perhaps for ever?

    May brain is starting to hurt. :)

    Anybody? Links, cites?

  • Uncle Al

    “The Lense-Thirring effect causes orbital precession of X-ray emitting gas near a rotating black hole. X-ray emission peaks at periods matching the frame-dragging.”

    This Kerr black hole (complex geometry!) brings with it a big box of footnotes. Included are extreme relativistic effects (SR and GR) distorting the image. One presumes the research exceeds its sound bites.

  • Deplorablewinner

    “…once every 131 secs.”… sounds more like an electromagnetic pulse (pulsar). Hard to believe anything spinning at that speed could hold together, let alone emitting a regularly timed pulse.

    • GORT

      My guess is that the black hole’s gravity is strong enough to overcome any centrifugal force acting on the star.

      Wouldn’t that also be why black holes don’t fling themselves into oblivion?


      • Deplorablewinner

        Oh! I thought Black Holes were a mathematical construct. You mean, they’re real?

        • GORT

          *Something* with a massive gravitational field sits at the center of most, if not all, galaxies, and it’s commonly referred to as a Black Hole.

          As for mathematical constructs, I’d argue that, just as dividing by zero is forbidden, if a construct produces either a singularity or an infinite value, the construct is flawed and must be modified or abandoned.


          P.S. Whoever named the Milky Way’s black hole Sagittarius A* (where “Sagittarius” is sometimes condensed to “Sag” and the asterisk morphs into “star”, so it comes out “Sag a star”), should be stripped of all future naming responsibilities. That’s absolutely horrible. I suggest “Milky Way Prime” (MWP). Say it out loud: “Boy, that MWP sure is an awesome mathematical construct, eh? See, it rolls right off the tongue. We could even pronounce it “M-WhiP”. 😉

          • Deplorablewinner

            Better you go easy on the weed.


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