Probing strong gravity in a binary neutron star system

Duncan Lorimer, professor in the Department of Physics and Astronomy, played a key role in the discovery of the binary pulsar system known as PSR J1906+0746, or J1906 for short, in 2004. The pulsar spins and emits a lighthouse-like beam of radio waves every 144 milliseconds. It orbits another neutron star -- or maybe a white dwarf -- in a little under four hours.

The results are published in The Astrophysical Journal, and was presented at the 225th meeting of the American Astronomical Society in Seattle, on January 8.

The pulsar is currently orbiting a companion star, causing it to "wobble." This is due to the neighboring star's immense gravity, causing space-time to warp around the pulsar and affect its visibility to us on Earth. This effect is called geodetic precession.

"What the companion is doing is causing the pulsar's beam to constantly change the direction of its orientation," Lorimer said. The beam's orientation has changed so much due to the common star that it won't be visible to Earth for as many as 160 years.

Pulsars are born in the supernova explosions of massive stars. These catastrophic events happen about once a century in our Galaxy, the Milky Way, and most of the time disrupt any pre-existing binary system leaving the pulsar as an isolated object. Seeing a pulsar interact with another star in such a way was especially rare.

The team kept track of the movement of the pulsar over five years using telescopes from around the world, including the Robert C. Byrd Green Bank Telescope in West Virginia, the Arecibo Telescope (USA), Nançay Telescope (France), the Lovell Telescope (UK) and the Westerbork Synthesis Radio Telescope (The Netherlands). The campaign kept track of all 1 billion rotations of the neutron star over the time period.

Only a handful of other double neutron stars have masses been measured, and J1906 is by far the youngest. As the supernova explosion that formed J1906 occurred only 100,000 years ago, the binary is in a remarkably pristine and un-evolved state. Normal pulsars live to be a few 10s of million years old; they can then be spun-up or "recycled" by the accretion of matter from a binary companion to live for yet another 1 billion years. If the companion to J1906 is a neutron star, it is likely recycled, although it appears to not be shining our way.

"Through the effects of the immense mutual gravitational pull, the spin axis of the pulsar has now wobbled so much that the beams no longer hit Earth," said Joeri van Leeuwen, an astrophysicist at ASTRON, The Netherlands Institute for Radio Astronomy, and University of Amsterdam, The Netherlands, who led the study. "The pulsar is now all but invisible to even the largest telescopes on Earth. This is the first time such a young pulsar has disappeared through precession."

While this particular pulsar is hiding from our view, more may soon be visible to us.

"We can expect other pulsars to precess into our line of sight," Lorimer said. "What we're doing here at WVU and elsewhere is continually searching the sky for these types of systems. Learning more about the demographics of these systems will teach us about how massive stars end their lives."

Video: https://www.youtube.com/watch?v=1FFPYklQs10