Scientists are pushing the boundaries of astrophysics by studying the fastest-spinning objects known in the universe: millisecond pulsars. These incredibly dense stellar remnants, the collapsed cores of massive stars, can rotate hundreds of times per second, approaching the theoretical limits of matter. By identifying and analyzing these cosmic spinners, researchers are gaining unprecedented insights into the fundamental properties of neutron stars, the exotic state of matter within them, and the extreme physics of gravity and magnetism. The quest to find the absolute fastest pulsar is a search for the universe’s ultimate rotational speed limit, a boundary dictated by the very fabric of spacetime and the cohesion of nuclear matter.
A neutron star is the crushed core left over after a massive star explodes as a supernova. It packs the mass of half a million Earths into a sphere no larger than a city. While young neutron stars are born spinning rapidly, they typically slow down over cosmic timescales. However, some acquire a second life. If a neutron star is in a binary system with a companion star, it can pull material from its partner. This process, known as accretion, not only adds mass but also transfers angular momentum, spinning the neutron star up to incredible speeds, sometimes over 42,000 revolutions per minute. Studying these rejuvenated pulsars allows scientists to probe how matter behaves under conditions of density and rotation impossible to replicate on Earth, providing a natural laboratory for testing the limits of physics.
Probing the Limits of Matter
The extreme rotational velocity of millisecond pulsars places immense stress on the stellar remnant, and current theories predict a hard physical limit to how fast they can spin. Scientists estimate that if a pulsar were to rotate at a rate of approximately 1,500 times per second, the centrifugal force at its equator would overcome its powerful gravity, causing it to break apart. Even before reaching this catastrophic point, at speeds above 1,000 rotations per second, a pulsar is predicted to lose energy through gravitational waves more rapidly than the accretion from a companion star could speed it up. This theoretical ceiling makes the discovery of pulsars spinning near this limit a critical test for models of neutron star structure.
The internal composition of a neutron star dictates how well it can resist deformation and disintegration from rapid spinning. By finding the fastest possible pulsars, astronomers can constrain these models. A star made of more rigid, exotic matter might be able to withstand higher spin rates than one with a softer core. The current record-holders are already providing valuable data. Their observed speeds help scientists refine equations of state, which describe the relationship between pressure and density within a neutron star. This, in turn, offers clues about the nature of the strong nuclear force and whether the core of a neutron star consists of simple neutrons or more exotic particles like quarks and gluons.
Record-Holding Cosmic Gyroscopes
The reigning champion of spin is PSR J1748-2446ad, the fastest-spinning pulsar known to science. Discovered in 2004, this object rotates 716 times every second, which translates to an equatorial speed of over 70,000 kilometers per second, or about 24% the speed of light. It is located approximately 18,000 light-years from Earth in the dense globular star cluster Terzan 5, within the constellation Sagittarius. PSR J1748-2446ad exists in a binary system, undergoing regular eclipses as its companion star passes in front of it from our point of view. Its incredible velocity pushes right up against the theoretical limits predicted by astrophysicists.
Intriguingly, a second object has been found that appears to match this record speed. Known as 4U 1820-30, this neutron star was not detected as a traditional radio pulsar but as part of an X-ray binary system. As it accretes matter from its companion, hotspots form on its surface, emitting powerful X-rays. Using NASA’s NICER X-ray telescope, astronomers observed a periodic pulsation of X-rays at a frequency of 716 Hz, strongly suggesting a rotation rate identical to the record-holder. The fact that two different types of neutron star systems, observed in different ways, have yielded the same maximum spin rate suggests that this may indeed be a common physical limit for neutron star rotation.
Mechanisms of Extreme Acceleration
Neutron stars are not born spinning at millisecond periods. They are forged in the collapse of a massive star’s core, and while they start fast, their powerful magnetic fields act as a brake over millions of years, causing their rotation to slow down. The path to becoming a millisecond pulsar requires a partner. When a neutron star is part of a close binary system, its immense gravity can siphon gas from the outer layers of its companion star. This material forms a swirling disk, known as an accretion disk, around the neutron star.
As matter from this disk falls onto the neutron star’s surface, it transfers its orbital angular momentum to the star, effectively hitting the accelerator. This process can spin up an old, slowly rotating neutron star to hundreds of rotations per second. The constant stream of matter from the companion provides the energy needed to achieve and sustain these extreme velocities, turning the stellar remnant into one of the fastest-rotating objects in the cosmos. These are often called “recycled” pulsars because they have been spun up to a new life of rapid rotation long after their initial formation.
A Violent Partnership
The “Black Widow” System
The process of rejuvenation can be destructive for the companion star. In some of the most extreme cases, known as “black widow” or “redback” pulsars, the newly energized pulsar begins to destroy its partner. The second-fastest known pulsar, PSR J0952-0607, is a prime example of this phenomenon. Spinning at a blistering 707 times per second, this pulsar is the heaviest neutron star observed to date and has consumed nearly the entire mass of its stellar companion.
The pulsar unleashes a torrent of high-energy particles and radiation, including gamma rays and X-rays, in a powerful stellar wind. This wind blasts the nearby companion star, stripping away its material and causing it to evaporate over time. In the case of PSR J0952-0607, the companion has been whittled down to less than 20 times the mass of the planet Jupiter. This violent, one-sided relationship gives these systems their arachnid-inspired name, as the pulsar effectively consumes its mate.
Hunting for Hidden Spinners
Discovering these faint, rapidly spinning objects requires sophisticated instruments and clever search techniques. Many millisecond pulsars are found by first identifying mysterious high-energy sources mapped out by space telescopes like NASA’s Fermi Gamma-ray Space Telescope, which scans the entire sky for gamma-ray emitters. Once a promising source is located, radio telescopes on the ground are used to search for the characteristic regular pulses that signal the presence of a pulsar.
Telescopes like the Netherlands-based Low Frequency Array (LOFAR) are particularly effective at this follow-up work. LOFAR detected the pulses from PSR J0952-0607 at very low radio frequencies, which are often missed by conventional searches. In some binary systems, the material being blown off the companion star can obscure the pulsar’s radio beams, making them difficult or impossible to detect. In these cases, astronomers must use higher-frequency radio observations that can penetrate the obscuring material or, as with 4U 1820-30, look for pulsations in other wavelengths like X-rays to uncover the neutron star’s hidden spin.
