For a while, PSR J0435+3233 looked like it was breaking the rules. The millisecond pulsar, picked up by China’s giant FAST radio dish, spins about 312 times a second, yet its rotation appeared to be winding down around a hundred times faster than is normal for such objects in our galaxy. The solution, astronomers now say, is that the pulsar has company. Two companions, in fact.
Researchers around Zonglin Yang, an astronomer in Beijing, merged nearly five years of FAST pulse timing with Fermi gamma-ray data reaching back to 2008, and filled the gaps with optical and infrared pictures. What emerged is a nested trio. Close in, a helium white dwarf whips around the pulsar every eight days on an almost perfectly circular path. Far beyond, a subgiant with roughly the Sun’s mass loops around the pair on a stretched orbit lasting about 73.5 years, tilted sharply against the inner one by either around 85 or about 54 degrees.
That distant star explains the apparent braking. Its gravity tugs the inner binary back and forth along our line of sight, distorting the timing of the pulses. A separate analysis concluded that the pulsar’s true spin-down rate is at least a hundred times lower than it seemed, entirely ordinary for its class.
Only a handful of millisecond pulsars are known to sit in triple systems, and this one is unusual because it lies on its own rather than in a crowded star cluster. That suggests the three stars were born together instead of being thrown together later. Surviving the supernova that created the pulsar was a gamble: even without a kick from the explosion, the odds of the trio holding together were estimated at roughly one in three.
Located some 2.1 kiloparsecs, or close to 7,000 light-years, away, the system should eventually end up as a neutron star with two white dwarfs. Until then, it serves as an unusual testbed for watching stars grow old side by side.
