Pulsars are rapidly rotating neutron stars, the extremely dense cores left behind when massive stars explode. Astrophysicists have recently observed a pulsar rotating over 300 times per second that appeared to be slowing down unusually quickly.
Studies suggested that this pulsar’s apparent slowdown could be linked to the gravitational pull of another star. Ultimately, its rotation offered clues about a rare system of three stars orbiting together.
Researchers at the National Astronomical Observatories of the Chinese Academy of Sciences and other institutions recently investigated this pulsar, called PSR J0435+3233, and its companion stars. Their paper, published in The Astrophysical Journal Letters, identifies a close pair consisting of the pulsar and a white dwarf, accompanied by a sunlike star in a much wider orbit.
The newly identified system could help astronomers to understand how three stars can remain gravitationally bound through the explosion that transforms one of them into a neutron star.
Examining the pulses originating from PSR J0435+3233
Pulsars emit beams of electromagnetic radiation that sweep through space as they rotate, producing detectable pulses when the beams sweep across Earth. The pulses originating from PSR J0435+3233 arrive approximately 3.2 milliseconds apart, and examining them allows astronomers to track its rotation.
J0435+3233 was found to be in orbit with another type of stellar remnant known as a helium white dwarf. Together, these two remnants complete an almost circular orbit every eight days.
To investigate the system, the researchers reprocessed 192 observing sessions from China’s Five-hundred-meter Aperture Spherical radio Telescope, known as FAST, spanning nearly five years. Changes in both the observed pulse frequency and the inner pair’s orbital frequency pointed to an external acceleration shared by the two stellar remnants.
Motion toward or away from Earth changes the spacing of arriving pulses via the so-called Doppler effect. If the pulsar accelerates away from Earth, the intervals between pulses become longer, making its rotation appear to slow more rapidly than it actually does.
By analyzing archived optical and infrared observations, the team identified the likely source of this gravitational influence: a star that is very close to the pulsar’s position in the sky and moves similarly across the sky. The researchers interpreted its properties as those of a sunlike star beginning to evolve beyond its main hydrogen-burning stage. The sunlike star was estimated to be around 7,000 light-years away, although its orbital motion could affect this estimate.
The team also identified gamma-ray pulses in observations from NASA’s Fermi satellite. By combining these observations with FAST measurements and information about the visible star, they could reconstruct an elongated outer orbit lasting approximately 73.5 years.
As the researchers’ observations cover only part of this orbit, its exact geometry remains to be determined. Moreover, their analysis did not fully account for all effects on pulse timing.
A separate analysis, accepted for publication in Astronomy & Astrophysics and available now as a preprint on arXiv, also supports their triple-star interpretation, indicating a much smaller actual slowdown. This analysis relied on overlapping observations and proposes a different classification of the visible star.
Surviving a stellar explosion
The stellar trio identified by the authors lies outside a dense star cluster, where encounters between stars could prompt the assembly of unusual systems. In their paper, the researchers delineate a possible history that could have led to the concurrent birth of all three stars.
In their proposed sequence of events, the most massive star shed its outer envelope and exploded, leaving behind a neutron star. Later, material transferred from its close companion increased the neutron star’s rotation rate, producing a millisecond pulsar. The donor star then became the helium white dwarf.
Surviving the massive star’s explosion would have been challenging for the three-star system. In fact, a sudden loss of mass and a possible kick to the newborn neutron star could break a triple-star system apart.
In the team’s baseline simulation, only about 35% of simulated systems remained bound, even when the explosion imparted no additional kick. Under these modeled conditions, “the survival probability remains relatively low,” the researchers wrote in their paper.
The surviving sunlike star identified by the authors could offer a window into the system’s evolutionary history. The researchers expect its wide orbit to prevent significant material exchange with the inner pair of stars. It could therefore have evolved largely as an isolated star, eventually becoming a white dwarf and leaving a neutron star and two white dwarfs still bound together.
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Publication details
Z. L. Yang et al., The PSR J0435+3233 Triple System, The Astrophysical Journal Letters (2026). DOI: 10.3847/2041-8213/aeaa29
Paulo C. C. Freire et al., On the triple nature of the PSR J0435+3233 system, arXiv (2026). DOI: 10.48550/arxiv.2607.27462
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A pulsar’s apparent slowdown reveals a rare stellar trio (2026, October 20)
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