Pulsars are rotating neutron stars that are aligned with Earth and radiate energy from their magnetic poles around us with each rotation. From this, we see a regular pulse of radio light, like a cosmic beacon. Fast pulsars spin very quickly, beating hundreds of times per second. These are called millisecond pulsars.
The fastest pulsars are thought to be accelerated by a closely orbiting companion star. This leads to the interesting evolution of millisecond pulsars. Initially, the pulsar grabs mass from its binary sibling, like a spider grabbing its prey. The rotation of the pulsar increases and the orbital period of the two stars decreases forever. But eventually, the companion star lost too much mass for the pulsar to continue consuming it. The pulsar gradually pushes away its companion and its orbital period decreases.
This spider-like astrophysical process has led astronomers to refer to millisecond pulsars as close binary spider pulsars. If the companion star is less than one-tenth the mass of the Sun, they are called black widows, and if the companion star is more than one-tenth the mass of the Sun, they are called red dwarfs. About 20 black widow pulsars and 10 redback pulsars have been discovered, and millisecond pulsars are generally thought to evolve from redbacks to black widows.
But in that case, there must be some spider pulsars between the two types. These intermediate pulsars should have a very close orbital companion, while not actively consuming it. The idea is that such a pulsar would have consumed everything it could from the companion, but not yet driven it far. Now a team in China with a five-hundred meter aperture spherical radio telescope (FAST) thinks they have found one.
Published as Nature, the team discovered a millisecond pulsar orbiting its companion in just 53 minutes, the closest spider binary ever observed. The companion star is just 0.07 solar masses, suggesting that it has been stripped of mass by a pulsar. Compared to other known spider pulsars, the new pulsar lies between the redback and black widow pulsars.
The orbit of this new pulsar is almost face-on, meaning that the Doppler will not shift as its pulses orbit the companion. This was discovered thanks to FAST’s high-resolution positioning capabilities. While this may seem like a good example of the middle ground between redback and black widow pulsars, it is only an example. Astronomers need to find many more such pulsars to prove the evolution model of spider pulsars.
Therefore, the goal is to catch more spider pulsars in our observational net.
Note: Pan, Z, et al. „A binary pulsar with a 53 minute orbit.” Nature (2023): 1-3.
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