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Super-dense pulsing star ducks behind invisibility cloak

For the first time, a distant, pulsing neutron star was there for astronomers to study -- until it disappeared. Crave's Eric Mack explains the extreme forces hiding it from view.

The Terzan globular cluster, home to the now-disappeared pulsar. NASA

It's like a cosmic magic act, but more astonishing than any old-school sleight of hand. A distant binary pulsar system was observed, then disappeared. An international team of astronomers managed to beat the clock in their scramble to measure it before it created its own home-spun cloaking system.

The findings were published last week in the Astrophysical Journal. They mark the first time such a vanishing act has been witnessed and also serve as a real-world demonstration of some of the wackier aspects of gravity.

The binary pulsar system that vanished is named J1906+0746, and it's basically a pair of two crazy-dense neutron stars. Neutron stars are like the leftovers from a massive star that went supernova and collapsed -- they might have a radius that's only as long as Manhattan but have a mass greater than our sun's. So, J1906 is made up of two of these things orbiting each other closely and quickly, but one of them has an axis that wobbles like a top and emits a pulsing lighthouse-like beam of radio waves every 144 milliseconds.

"By precisely tracking the motion of the pulsar, we were able to measure the gravitational interaction between the two highly compact stars with extreme precision," said Ingrid Stairs, professor of physics and astronomy at the University British Columbia and a member of the team, in a release.

To bring it a little closer to home, imagine that these stars are two of the leanest, meanest, quickest boxers ever, and they're circling each other in the ring. Their mutual desire to land a big blow and simultaneous fear of getting hit with one from the other fighter keeps them circling each other in a similar way. Those forces in the ring are kind of like gravity, simultaneously pushing and pulling the two fighters away from each other.

And the difference between the super-intense gravity happening in J1906 and the gravity we experience in our solar system is like the difference between this bout between two fierce champion boxers and a fourth-grade pillow fight.

But there is something unique about one of our neutron star boxers -- he shoots a constant laser beam out of his eyes that reaches deep into space. Don't worry about why or what it does, just picture it.

And this is where you come in, because you are trying to watch this wild boxing match from 5 miles away on a mountaintop. (Standing in for the astronomers who are observing their pulsar from thousands of light-years away.) You're tracking the match by using specialized binoculars that pick up that laser beam.

As this match wears on, the laser-eyed boxer gets weary and wobbly and eventually needs to take a breather and lean over, head down and hands on knees, for an extended period. At this point you can no longer observe the match because the laser beam from his eyes that you've been using to locate and watch it is pointed completely away from your vantage point.

Just as the intense competition caused our metaphorical boxer to wobble until he and his opponent were no longer visible, the intense gravity between the two neutron stars actually warps the space-time around them, and this caused the pulsar to wobble until its axis is tilted at such an angle that we can no longer pick up its rapid radio pulses from where we sit here on Earth.

Astronomers scrambled to measure the warp in space-time caused by the immense gravitational interactions within the distant binary pulsar system before it disappeared from our view. Only a handful of such double neutron stars have ever been measured, say the researchers, and J1906 is the youngest so far. It's located over 25,000 light-years from Earth.

"The pulsar is now all but invisible to even the largest telescopes on Earth," explained Joeri van Leeuwen, an astrophysicist at the Netherlands Institute for Radio Astronomy who led the study. "This is the first time such a young pulsar has disappeared through precession [the wobbling effect]."

The curve of space-time won't hide J1906 from us forever, though. The pulsar will eventually wobble back into view; it's just we might have to wait another 160 years or so before that happens.

If you're still confused by my boxing image, watch a video of how the warp actually hid the pulsar below.