Astronomers Map a Neutron Star’s Surface for the First Time

NASA’s NICER instrument reveals that neutron stars are not as simple as we thought.

Pulsars are the lighthouses of the universe. These tiny, compact objects are neutron stars — the remnants of once-massive stars — that spin rapidly, beaming radiation into space. Now, for the first time, astronomers have mapped the surface of a 16-mile-wide pulsar in exquisite detail. The discovery calls into question astronomers’ textbook depiction of pulsar appearance and opens the door to learning more about these extreme objects.

The Neutron star Interior Composition Explorer, or NICER, searches for X-rays from extreme astronomical objects such as pulsars from its perch on the exterior of the International Space Station. Researchers used NICER to observe the pulsar J0030+0451, or J0030 for short, which is located 1,100 light-years away in the constellation Pisces, in a series of papers published in The Astrophysical Journal Letters. Two teams, one led by researchers at the University of Amsterdam and the other by researchers at the University of Maryland, used X-ray light from J0030 to map the pulsar’s surface and calculate its mass. Both teams arrived at a conclusion that was unexpected.

Making a map

Pulsars are extremely dense but extremely small objects, similar to black holes. Their immense gravity bends space-time around them, allowing us to see the pulsar’s far side even as it rotates out of view. The effect also causes the pulsar to appear slightly larger than it is. Because NICER can precisely time the arrival of X-rays from the pulsar (better than 100 nanoseconds), the researchers were able to create a map of the star’s surface and measure its size.

The teams determined that the neutron star is between 1.3 and 1.4 times the mass of the Sun. It’s also about 16 miles (26 kilometers) wide. (By comparison, our Sun is just over 864,000 miles [1.3 million kilometers] across.)

Those figures aren’t surprising. The astronomers then attempted to map the location of hotspots on J0030’s surface. Pulsars are depicted in the textbook image as having two hotspots, one at each of their magnetic poles. As the star spins, the hotspots emit radiation in thin beams into space, much like a lighthouse. Astronomers spot a pulsar if one or both beams happen to pass over Earth.

J0030 is oriented with its northern hemisphere facing Earth. As a result, the teams anticipated finding a hotspot near the north pole. Mapping the hotspots required supercomputer modeling to determine where the X-rays NICER received from the pulsar originated on the star’s surface. The task would have taken a decade for normal desktop computers to complete, but the supercomputers finished in less than a month.

The pulsar J0030 appears to have two to three hotspots on its southern hemisphere only – a finding astronomers didn’t expect.
NASA’s Goddard Space Flight Center/CI Lab

A new picture

What the teams found presented a different picture: J0030 has two or three hotspots, all of which are located in the southern hemisphere. The researchers at the University of Amsterdam believe the pulsar has one small, circular spot and one thin, crescent-shaped spot spinning around its lower latitudes. The University of Maryland team discovered that the X-rays could be coming from two oval spots in the star’s southern hemisphere, as well as one cooler spot near the star’s south pole.

Neither result is the simple picture astronomers expected, indicating that the pulsar’s magnetic field, which causes the hotspots, is likely even more complex than originally assumed. While the result certainly leaves astronomers wondering, “It tells us NICER is on the right path to help us answer an enduring question in astrophysics: What form does matter take in the ultra-dense cores of neutron stars?” NICER science lead and study co-author Zaven Arzoumanian said in a press release.  

With this accomplishment, astronomers will now look to duplicate it using more pulsars, building up a better understanding of what these strange stars look like and how they work.

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