A Pulsar's Odd Slowdown Is Explained by a Rare Triple-Star System
Chinese astronomers say a sunlike star in a 73.5-year orbit tugs on a pulsar and its white dwarf partner, making the pulsar look like it is spinning down faster than it is.
A pulsar's unusually fast apparent slowdown comes from a rare triple-star system, researchers at the National Astronomical Observatories of the Chinese Academy of Sciences and other institutions report in The Astrophysical Journal Letters. The pulsar, PSR J0435+3233, is a millisecond pulsar that rotates more than 300 times per second, with pulses arriving about 3.2 milliseconds apart.
The paper, led by Z. L. Yang and colleagues, identifies a close inner pair made of the pulsar and a helium white dwarf, plus a sunlike star in a much wider orbit. The pulsar and white dwarf circle each other in a nearly circular path about every eight days.
The team reprocessed 192 observing sessions from China's FAST radio telescope, spanning nearly five years. Changes in the pulse frequency and in the inner pair's orbital frequency pointed to an outside acceleration acting on both remnants.
The Doppler effect explains the apparent slowdown. If the pulsar is accelerating away from Earth, the gaps between pulses lengthen, so the pulsar seems to spin down faster than it really does. Archival optical and infrared data identified a likely culprit: a star close to the pulsar on the sky and moving similarly across it. The researchers interpret it as a sunlike star beginning to evolve beyond core hydrogen burning, about 7,000 light-years away, though its orbital motion could affect that figure.
Gamma-ray pulses detected with NASA's Fermi satellite helped. Combining those with the FAST timing and the optical data, the team reconstructed an elongated outer orbit lasting roughly 73.5 years. The observations cover only part of that orbit, so its exact geometry is still undetermined, and the analysis did not fully account for every effect on pulse timing.
A separate paper by Paulo C. C. Freire and colleagues, accepted in Astronomy & Astrophysics and posted as a preprint, also supports the triple-star interpretation. It finds a much smaller actual slowdown, relies on overlapping observations, and proposes a different classification for the visible star.
The formation history is the hard part. The paper proposes that the most massive star in the system shed its outer envelope and exploded, leaving a neutron star. Material from a close companion later spun the neutron star up into a millisecond pulsar, and that companion became the helium white dwarf.
Surviving the explosion was difficult, since sudden mass loss and a possible kick could have disrupted the system. In the team's baseline simulation, only about 35% of systems stayed bound even with no added kick. The researchers wrote that "the survival probability remains relatively low." The system lies outside a dense star cluster, where stellar encounters could otherwise help form such oddities. The sunlike star's wide orbit should keep it from exchanging significant material with the inner pair, so it will likely evolve like an isolated star and eventually become a white dwarf, leaving a neutron star and two white dwarfs still bound together.
