First glimpses of a pulsar’s surface suggest complex magnetism

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

Two teams of astronomers independently obtained the first glimpses of the surface of a pulsar, a rapidly rotating neutron star. Newly created maps of this surface reveal a handful of bright spots 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 focus on the behavior of matter under extreme pressure.

Neutron stars, the massive star nuclei left behind after a supernova explosion (NS: 09/25/18), roughly pack the mass of the sun into an orb not much larger than a large city. Researchers aren’t quite sure what matters when it’s that tight. But “the neutron stars themselves can give us clues and contribute to fundamental physics,” says Cole Miller, an astrophysicist at the University of Maryland at College Park and author of one of the studies, both of which appear in December 10. Letters from astrophysical journals.

To find these clues, the teams sized a pulsar called 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 reversed the star’s appearance.

The two teams found that the pulsar, located just over 1,000 light years from Earth, is about 1.4 times more massive than the sun and nearly 26 kilometers wide. “This is right in the middle of what we would expect,” says Anna Watts, an astrophysicist at the University of Amsterdam and author of the other study.

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

pulsar
This computer simulation shows a possible magnetic field architecture that could create hot spots only in the southern hemisphere of a rapidly rotating neutron star called a pulsar. The hot spots would be where the blue lines hit the star’s surface. NASA Goddard Space Flight Center

“The classic image of a pulsar as a beautiful, symmetrical thing is nonsense,” says Watts. The two groups proposed slightly different stain patterns, but the same overall message: rather than just bright spots near each of the star’s poles, one hemisphere is littered with a few bright spots while the other looks clean.

“It’s exciting, but it’s not yet a breakthrough,” says Feryal Özel, an astrophysicist at the University of Arizona at Tucson, who was not involved in either study. While Özel agrees the data points to magnetic complexity – a complexity she says is evoked in contemporary computer simulations – she would like future cards to go further. Rather than just placing dots anywhere to best match x-ray fluctuations, she says, the next set of maps might instead determine precisely which magnetic architecture would do the job.

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


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