Deep in space, about 3,900 light-years from Earth, orbits one of the most unusual objects known to astronomers. This is the millisecond pulsar PSR J0435+3233 — a neutron star that completes a full rotation in just 3.2 milliseconds. Recently, an international team of scientists, analyzing data from the Fermi telescope, discovered that this pulsar emits gamma rays in a strictly periodic manner, making it a rare example among similar objects.
The discovery began with radio observations. In 2026, China's FAST (Five-hundred-meter Aperture Spherical Telescope) radio telescope first registered signals from PSR J0435+3233. The pulsar turned out to be part of a binary system and immediately attracted attention with its extreme characteristics. Its spin-down rate — the speed at which its rotation slows — was two orders of magnitude higher than that of typical millisecond pulsars. The spin-down luminosity reaches a huge value of about 5.89 × 10^37 erg/s, comparable to young, energetic pulsars, even though the object itself belongs to the class of "recycled" neutron stars.
Millisecond pulsars usually form in binary systems: the neutron star is "spun up" by accreting matter from its companion. This process smooths out the magnetic field, making them relatively "quiet." However, PSR J0435+3233 stands out with a strong surface dipole magnetic field — about 12.6 billion Gauss — and a high rate of energy loss. Such properties made it an excellent candidate for searching for gamma-ray emission.
A team led by Mengxin Zhang from Yunnan University took on the task. They studied almost 17.7 years of observations from the Fermi-LAT telescope in the energy range of 0.1 to 500 GeV. The Fermi catalog already noted a gamma-ray source, 4FGL J0435.5+3232, located just 0.01 degrees from the pulsar's position. Phase-folding analysis revealed clear pulsations with a significance level of about 6.8σ. The gamma-ray emission is concentrated in a narrow phase interval — approximately from 0.44 to 0.69, which is only a quarter of the star's full rotation. During the rest of the time, the pulsar is "silent" in the gamma-ray range.
The gamma-ray luminosity was about 6.26 × 10^32 erg/s. With such colossal rotational energy, the efficiency of gamma-ray emission turned out to be unexpectedly low — on the order of 10^{-5}. This contrasts with expectations for objects with similar power and opens new questions about the mechanisms of particle acceleration, the geometry of radiation beams, and the magnetosphere structure of extreme millisecond pulsars.
The detection of gamma-ray pulsations from PSR J0435+3233 turns it into a valuable laboratory for studying neutron star physics. Scientists note that the combination of high spin-down energy and low gamma-ray efficiency will allow for a better understanding of how the magnetospheres of such fast and powerful objects work. Each new observation of this distant "lighthouse" brings us closer to understanding the most extreme conditions in the Universe — where the density of matter and field strengths reach limits unattainable by Earth-based laboratories.
