The first glimpses of a pulsar’s surface hint at complex magnetism

A pulsar in the Milky Way is ready for its close-up.

Two teams of astronomers independently have gotten the first glimpses of the surface of a pulsar, a rapidly spinning neutron star. Newly created maps of that surface reveal a smattering of bright blemishes in the star’s southern hemisphere, hinting at the presence of complex magnetic fields.

These new data, along with precise measurements of the star’s mass and size, could help researchers zero in on how matter behaves under extreme pressure.

Neutron stars, the cores of massive stars left behind after a supernova explosion (SN: 9/25/18), pack roughly the mass of the sun into an orb not much wider than a major city. Researchers don’t really know what happens to matter when it’s squeezed that tightly. But “neutron stars themselves can give us hints and contribute to fundamental physics,” says Cole Miller, an astrophysicist at the University of Maryland in College Park and author of one of the studies, both of which appear in the Dec. 10 Astrophysical Journal Letters.

To search for those hints, the teams sized up a pulsar dubbed PSR J0030+0451 with NICER, an X-ray telescope attached to the International Space Station. They monitored how the pulsar’s X-ray brightness fluctuated in sync with its rotation. Then, with the help of supercomputers, the researchers reverse engineered what the star might look like.

Both teams found that the pulsar, located just over 1,000 light-years away from Earth, is about 1.4 times as massive the sun and nearly 26 kilometers wide. “It sits right in the middle of what we expect,” says Anna Watts, an astrophysicist at the University of Amsterdam and an author of the other study.

The surface of the star, however, had a surprise in store. Bright spots on a pulsar mark where atomic particles, guided by magnetic fields, slam into the star. The locations of those spots reveal the architecture of the magnetic field. In textbooks, a pulsar’s magnetic field resembles a bar magnet, with clearly defined north and south poles. If that picture is correct, then the researchers would have seen two bright spots, one in each hemisphere. But that’s not what either team saw.

“The classic picture of a pulsar as a beautiful symmetric thing is nonsense,” Watts says. The two groups came up with slightly different spot patterns, but the same overarching message: Rather than single bright spots near each of the star’s poles, one hemisphere is littered with a few bright smudges while the other appears clean.

“This is exciting, but not a breakthrough yet,” says Feryal Özel, an astrophysicist at the University of Arizona in Tucson, who was not involved in either study. While Özel agrees that the data point to magnetic complexity — a complexity she says is hinted at in contemporary computer simulations — she would like to see future maps go a step further. Rather than just placing spots anywhere to best match the X-ray fluctuations, she says, the next round of maps might instead figure out specifically what magnetic architecture would get the job done.

“Definitely more understanding will come out of this,” she says. “This is a good place to start.”