Neutron stars are as dense as the nucleus of an atom. They contain a star’s worth of matter in a sphere only a dozen kilometers wide. And they are light-years away. So how can we possibly understand their interior structure? One way would be to simply spin it. Just spin it faster and faster until it reaches a maximum limit. That limit can tell us about how neutron stars hold together and even how they might form. Obviously, we can’t actually spin up a neutron star, but it can happen naturally, which is one of the reasons astronomers are interested in these maximally spinning stars. And recently a team has discovered a new one.
All neutron stars rotate on their axes. They form from the collapse of a massive star’s core, and just as an ice skater spins faster as they pull in their arms, a neutron star spins up as it forms. Young neutron stars can rotate hundreds of times a second, though they generally slow down as they age. Interactions between their magnetic fields and interstellar space cause their rate of rotation to decay. This is why, for example, we can observe pulsars gradually slow down over time.
But many neutron stars have a binary companion. If their companion happens to be a closely orbiting regular star, the neutron star can pull off some of the companion’s outer layer and capture it. The slow exchange of matter can cause the neutron star to speed up as it essentially steals some of the orbital angular momentum of the companion. They are known as millisecond pulsars because they emit a radio pulse every few milliseconds. They are the fastest-rotating stars in the cosmos.
So, just how fast can these neutron stars spin? The record for the fastest spinning pulsar is held by PSR J1748–2446ad. Observations in 2004 and 2005 confirmed it rotates 716 times per second. That’s a bit faster than number two, which rotates at 707 times a second. This new study has found another neutron star rotating at 716 times a second, and it’s interesting because it isn’t a pulsar.
Known as 4U 1820-30, it is part of a binary X-ray system. As the neutron star captures material from its companion, part of its surface will heat up to such a degree that it emits X-rays. As the neutron star rotates, the hot-spot swings in and out of view, and we observe a periodic pulsation of X-rays. Using NASA’s NICER X-ray telescope, the team observed the binary from 2017 to 2021 and captured data on 15 powerful X-ray bursts. One of these bursts had a clear periodicity of 716 Hz. This strongly suggests the neutron star rotates at that rate.
While it could just be a statistical fluke, the fact that we now have two 716 Hz neutron stars found in two different ways suggests they may be near the maximal rotation limit for a neutron star.
Reference: Jaisawal, Gaurava K., et al. “A Comprehensive Study of Thermonuclear X-Ray Bursts from 4U 1820–30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation.” The Astrophysical Journal 975.1 (2024): 67.