The pulsar J0337+1715 has two white dwarf companions, one orbiting it tightly and a second close by, all packed within a space no bigger than Earth's orbit.
When it comes to cosmic exotica, pulsars are right up there. These ultradense degenerate remnants of former stars often spin hundreds of times per second. Although astronomers have spotted a couple hundred such millisecond pulsars across the Milky Way, it wasn't until January 5th that a team announced the discovery of one of these superspinning lighthouses in a triple stellar system.
The pulsar, J0337+1715, has two white dwarf companions — one in a tight orbit only 3 million miles (5 million km) away and the second somewhat farther out — all packed within a space no bigger than Earth's orbit.
Scott Ransom (NRAO) and others reported the result in January 5th's online edition of Nature and at a press conference at the winter meeting of the American Astronomical Society, now under way in Washington, D.C.
Buzz has arisen over the system because astronomers hope it'll be a good place to test one aspect of Einstein's theory of gravity. Per what's called the strong equivalence principle, the outer white dwarf should have the same gravitational effect on both the inner dwarf and the pulsar. If that's not true, physics has a problem.
But such testing hasn't been done yet. For now, the more interesting aspect of the system is how it came to be. The orbits are highly circular and, what's more, in the same plane — an unlikely orientation. The trio probably formed and evolved together, but with each star dying on its own.
The one that created the neutron star went first, as a supernova. Next, roughly a billion years later, the outermost star swelled and sloughed off its outer layers to leave behind the white dwarf. Lastly, the inner star swelled, spun up the neutron star to its whirligig, 366-spins-per-second rate by pouring material onto it, and then also ended up a white dwarf.
Details beyond that basic scheme are still hazy. The team suggests that the outermost star might have engulfed the inner two when it ballooned, creating the close orbital alignment through drag. Alternately, Thomas Tauris (Max Planck Institute for Radio Astronomy and the Argelander Institute for Astronomy, Germany) and Ed van den Heuvel (University of Amsterdam) posit that the pulsar's progenitor swelled up and engulfed (at least partially) the other two stars. In their scenario, published online January 6th in Astrophysical Journal Letters, the outermost star dumps material onto the inner system instead of engulfing them.
The fact that the pulsar J0337+1715 has survived three phases of stellar swelling and a supernova explosion is incredible — "a truly remarkable journey for a triple system," as Tauris and van den Heuvel note in their conclusion. You can find more details in the press releases here and here.
S. M. Ransom et al. "A millisecond pulsar in a stellar triple system." Nature, 5 January 2014.
T. M Tauris and E. P. J. van den Heuvel. "Formation of the galactic millisecond pulsar triple system PSR J0337+1715 — a neutron star with two orbiting white dwarfs." Astrophysical Journal Letters, 6 January 2014.