In a remarkable breakthrough in the study of the cosmos, an international team of astronomers has uncovered a new “spider” millisecond pulsar, designated PSR J1544−2555. This discovery, detailed in a research paper published on September 11 on the arXiv pre-print server, adds another intriguing member to the mysterious family of rapidly spinning neutron stars. Millisecond pulsars (MSPs), known for their astonishingly short rotation periods of less than 30 milliseconds, are among the fastest-spinning objects in the universe. They are thought to form in binary systems, where a neutron star—born from the collapse of a massive star—is accelerated to incredible rotational speeds by accreting matter from its companion.
The Unique World of “Spider” Pulsars
Spider pulsars are an unusual subclass of millisecond pulsars. They are characterized by eclipses in the radio spectrum and semi-degenerate companion stars, earning their evocative name due to the “predatory” relationship between the neutron star and its companion. Within this classification, astronomers distinguish between “black widows”—where the companion is extremely lightweight, less than 0.1 solar masses—and “redbacks,” where the secondary star is comparatively heavier. These systems are fascinating laboratories for understanding the extremes of stellar evolution, gravitational interactions, and high-energy astrophysics.
PSR J1544−2555 belongs to the black-widow category, meaning its companion star is extraordinarily small and gradually losing mass under the influence of the pulsar’s intense radiation. These systems are as enigmatic as they are extreme, often producing exotic phenomena that challenge our understanding of binary star dynamics.
Observing the Invisible: How the Discovery Was Made
The discovery of PSR J1544−2555 was the result of a careful, collaborative effort led by Sergio Belmonte Diaz from the University of Manchester, UK. The team scoured the Fermi-LAT catalog for periodic optical sources, using the highly sensitive ULTRACAM imaging photometer mounted on the 3.5-meter New Technology Telescope at the European Southern Observatory (ESO) in Chile. This optical survey allowed the astronomers to identify subtle variations in light caused by the orbital dance of the neutron star and its companion, ultimately leading them to the new black-widow pulsar.
According to the researchers, PSR J1544−2555 is associated with the Fermi-LAT source 4FGL J1544.2−2554. The neutron star at the heart of this system rotates once every 2.39 milliseconds, while its companion, a tiny stellar object with a radius of just 0.17 times that of the Sun, orbits it in a rapid 2.7-hour cycle. The companion’s mass is estimated at a mere 0.095 solar masses, emphasizing just how extreme the dynamics of this system are.
A System of Extreme Forces
PSR J1544−2555 is located approximately 6,500 light-years from Earth, a relatively modest distance in cosmic terms. Its dispersion measure, which quantifies the effect of interstellar plasma on radio signals, is measured at 25.8 parsecs per cubic centimeter. The pulsar’s surface magnetic field is an extraordinary 150 million Gauss, millions of times stronger than any magnetic field naturally occurring on Earth.
The system exhibits classic black-widow behavior, including variations in its orbital period and non-thermal emissions—energetic radiation not generated by heat but by the pulsar’s powerful electromagnetic activity. These traits not only confirm its classification but also provide researchers with a natural laboratory for studying the physics of matter under extreme conditions, from intense gravity to ultrafast rotation and magnetic fields far beyond anything experienced in our solar system.
Illuminating the Universe Through Gamma Rays
Beyond radio and optical observations, the team detected gamma-ray pulsations from PSR J1544−2555. These high-energy emissions reveal processes occurring near the pulsar’s magnetic poles and in the surrounding magnetosphere, shedding light on how such systems convert rotational energy into radiation. The gamma-ray luminosity of PSR J1544−2555 was calculated to be approximately 4 decillion erg per second, a staggering output that highlights the extraordinary power contained in even the smallest neutron star systems.
The Significance of This Discovery
The discovery of PSR J1544−2555 underscores the value of combining optical surveys with high-energy gamma-ray catalogs like Fermi-LAT. It demonstrates that variable optical sources can serve as reliable signposts for millisecond pulsar candidates, which can then be followed up with targeted radio observations. In doing so, astronomers can uncover some of the most exotic and energetic objects in our galaxy, advancing our understanding of both stellar evolution and the extreme physics of neutron stars.
As the researchers note, this discovery reinforces the importance of coordinated, multi-wavelength observations in modern astrophysics. Each new pulsar discovered is more than just a celestial curiosity—it is a window into the fundamental workings of matter, energy, and gravity under conditions far beyond those found on Earth.
A Gateway to Future Exploration
PSR J1544−2555 represents another step forward in the quest to understand the universe at its most extreme. Black-widow pulsars like this one challenge our understanding of how stars interact, evolve, and survive in close binary systems. They push the limits of physics and provide an unparalleled opportunity to study phenomena like rapid rotation, strong magnetic fields, and high-energy radiation in detail.
Discoveries like this remind us of the vastness and complexity of the cosmos and of our relentless human drive to explore it. Each pulsar we uncover is not just a distant star—it is a testament to the ingenuity, curiosity, and persistence of astronomers who peer into the depths of space to uncover the hidden rhythms of the universe. PSR J1544−2555 is more than a new entry in a catalog; it is a story of cosmic extremes and an invitation to continue exploring the remarkable tapestry of our galaxy.
More information: Sergio Belmonte Diaz et al, Multiwavelength observations of a new black-widow millisecond pulsar PSR J1544-2555, arXiv (2025). DOI: 10.48550/arxiv.2509.09605
