Pulsars are rotating neutron stars aligned with Earth in just such a way that the energy radiated from their magnetic poles sweeps across us with each rotation. From this, we see a regular pulse of radio light, like a cosmic lighthouse. The fastest pulsars can rotate very quickly, pulsing hundreds of times per second. These are known as millisecond pulsars.
It’s thought that the fastest of pulsars have been sped up thanks to a close-orbiting companion star. This leads to an interesting evolution of millisecond pulsars. Early on, the pulsar captures mass from its binary sibling, reeling it in like a spider capturing its prey. The rotation of the pulsar increases and the orbital period of the two stars gets ever shorter. But eventually, the companion star has lost so much mass that the pulsar can’t continue to consume it. The pulsar gradually pushes away its companion and its orbital period slows down.
This spider-like astrophysical process has led astronomers to refer to millisecond pulsars with a close binary as spider pulsars. If the companion star is less than a tenth of the Sun’s mass, then they are known as black widows, and they are known as redbacks if the companion is greater than a tenth of a solar mass. About 20 black widow pulsars and 10 redback pulsars have been discovered, and it’s generally thought that millisecond pulsars evolve from redback to black widow.
But if that’s the case, there should be some spider pulsars that are between the two varieties. These intermediary pulsars should have a very close orbiting companion, while not actively consuming it. The idea is that such a pulsar would have consumed all it can from the companion, but hasn’t yet driven it farther away. Now a team with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China thinks they have found one.1
As published in Nature, the team has found a millisecond pulsar that orbits its companion in just 53 minutes, which is the closest spider binary ever observed. The companion star is just 0.07 solar masses, which suggests it has been stripped of mass by the pulsar. When compared with other known spider pulsars, the new pulsar seems to lie in between the range of redback and black widow pulsars.
The orbit of this new pulsar is almost face-on, meaning its pulses don’t Doppler shift as it orbits the companion. It was discovered thanks to the high-resolution position capabilities of FAST. And while it seems to be a good example of the middle ground between redback and black widow pulsars, it is just one example. Astronomers will need to find more pulsars like it to prove the evolutionary model of spider pulsars.
So, the goal is to capture more spider pulsars in our observational web.
Pan, Z., et al. “A Binary Pulsar in a 53-minute Orbit.” Nature (2023): 1-3. ↩︎