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Astronomers map a neutron star’s surface for the first time

The NICER equipment from NASA demonstrates that neutron stars are not as simple as previously imagined.

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

Pulsars are the universe’s lighthouses. These tiny, compact objects are neutron stars, which are the remains of once-massive stars that spin quickly and beaming radiation into space. For the first time, astronomers have carefully studied the surface of a 16-mile-wide pulsar. The discovery challenges astronomers’ textbook image of pulsar appearance and opens the door to knowing more about these extreme objects.

The Neutron star Interior Composition Explorer, or NICER, looks 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 studies 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.

Working on a Correct map

Pulsars are highly dense but extremely small objects, similar to black holes. Their massive 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 clock the arrival of X-rays from the pulsar with extreme precision (better than 100 nanoseconds ), the researchers were able to build a map of the star’s surface and measure its size with unprecedented accuracy.

The neutron star’s mass was calculated to be between 1.3 and 1.4 times that of the Sun. It is approximately 16 miles (26 kilometers) wide. (By contrast, our Sun stretches just over 864,000 miles [1.3 million km] across.)

Those statistics are hardly surprising. The researchers then attempted to map the position of hotspots on J0030’s surface. Pulsars are depicted in the textbook illustration as having two hotspots, one at each of their magnetic poles. As the star spins, the hotspots shoot radiation out into space in thin beams, like a lighthouse. If one or both beams happen to pass over Earth, astronomers observe a pulsar.

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

A  Brand new picture

What the investigators discovered presented a different picture: J0030 contains 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 researchers discovered that the X-rays could be originating from two oval spots in the star’s southern hemisphere, as well as one colder spot near the star’s south pole. 

Neither finding is the straightforward picture that scientists expected, implying that the pulsar’s magnetic field, which creates the hotspots, is likely more complex than previously thought. “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?” astronomers said. According to NICER science lead and study co-author Zaven Arzoumanian in a press release.

With this achievement, astronomers will now attempt to replicate it using more pulsars, developing a better understanding of what these unusual stars look like and how they function.

References: The Astrophysical Journal, NASA

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