By analyzing the data from NASA’s Fermi gamma-ray space telescope, Chinese astronomers have investigated a recently identified millisecond pulsar known as PSR J0435+3233. As a result, they found that the pulsar exhibits gamma-ray pulsations. The discovery was detailed in a paper published July 17 on the pre-print server arXiv.
In general, pulsars are highly magnetized, rotating neutron stars emitting a beam of electromagnetic radiation, and the most rapidly rotating ones (with rotation periods below 30 milliseconds) are known as millisecond pulsars (MSPs). It is assumed that MSPs are formed in binary systems when the initially more massive component turns into a neutron star that is then spun up due to accretion of matter from its companion.
A good place to see gamma-rays
PSR J0435+3233 was discovered in 2026 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). It is a binary MSP at a distance of some 3,900 light years, with a spin period of approximately 3.2 milliseconds and an exceptionally large period derivative, which is at least two orders of magnitude larger than those of other known MSPs. The pulsar is also known for its extremely high spin-down luminosity of 58.9 undecillion erg/s, comparable to those of young, energetic pulsars.
Shortly after the discovery of PSR J0435+3233, a team of astronomers led by Mengqing Zhang of Yunnan University in China, decided to further inspect this pulsar as its properties make it a promising candidate for detectable gamma-ray emission.
“Motivated by these properties, we analyzed about 17.7 years of Fermi-LAT (Fermi-Large Area Telescope) observations in the 0.1–500 GeV energy range for this pulsar,” the researchers wrote in the paper.
Gamma-ray pulsations detected
The team identified a gamma-ray source, which received the designation 4FGL J0435.5+3232, lying only 0.01 degrees from the radio timing position of PSR J0435+3233. By analyzing the data, they detected gamma-ray pulsations from this position.
The gamma-ray emission was found to be concentrated predominantly within the rotational-phase interval of 0.44-0.69, which spans 0.25 in phase, while no significant emission is detected in the remaining off-pulse interval.
According to the authors of the study, the positional coincidence, the detection of pulsations at the pulsar’s spin period, and the phase-dependent emission indicate that 4FGL J0435.5+3232 is a gamma-ray counterpart of PSR J0435+3233.
Based on the collected data, the gamma-ray luminosity of PSR J0435+3233 was measured to be 0.626 decillion erg/s. It turned out that despite the remarkably high spin-down luminosity of PSR J0435+3233, comparable to those of young, energetic pulsars, it has an inferred apparent gamma-ray efficiency of only 0.00001. Moreover, PSR J0435+3233 has an unusually strong surface dipole magnetic field of around 12.6 billion Gauss.
Unusual gamma-ray millisecond pulsar
The authors of the paper conclude that their findings make PSR J0435+3233 a gamma-ray MSP with unusual properties.
“Our detection establishes PSR J0435+3233 as a gamma-ray MSP, and the striking combination of its high spin-down power and low apparent gamma-ray efficiency provides a new probe of particle acceleration, radiation beaming, and viewing geometry in the magnetospheres of millisecond pulsars with extreme rotational properties,” the scientists wrote.
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Publication details
Mengqing Zhang et al, Discovery of γ-Ray Pulsations from the Extreme-Spin-Down Millisecond Pulsar PSR J0435+3233, arXiv (2026). DOI: 10.48550/arxiv.2607.16119
Journal information:
arXiv
Key concepts
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Gamma-ray pulsations reveal extreme 3.2-millisecond pulsar 3,900 light-years away (2026, July 26)
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