Pulsars are spinning neutron stars — tiny, compacted remnants of once-massive stars. Pulsars spin rapidly, beaming radiation into space like a lighthouse. For this reason, they act as beacons, making them indispensable tools in astronomers’ arsenal when surveying the galaxy. But a series of new studies, which mapped the surface of a pulsar for the very first time, shows that we still have much to learn about these mysterious objects.
The groundbreaking studies were performed by two groups, one led by researchers at the University of Amsterdam, and the other led by astronomers at the University of Maryland. Although their roles differed, both teams examined X-ray light from the pulsar J0030+0451, which lies about 1,100-light-years away in the constellation Pisces.
J0030’s X-rays were detected and analyzed by the aptly named Neutron star Interior Composition Explorer (NICER) instrument, aboard the International Space Station where there isn’t an atmosphere to cloud measurements.
“From its perch on the space station, NICER is revolutionizing our understanding of pulsars,” said Paul Hertz, astrophysics division director at NASA Headquarters in Washington. “Pulsars were discovered more than 50 years ago as beacons of stars that have collapsed into dense cores, behaving unlike anything we see on Earth. With NICER we can probe the nature of these dense remnants in ways that seemed impossible until now.”
Inside neutron stars, intense gravitational forces crush protons and electrons together, turning them into neutrons. Such stars pack more mass than the sun into a sphere no larger than Manhattan, making them some of the densest objects in the universe. The new study determined that the pulsar has a mass between 1.3 and 1.4 times that of the sun, crammed into a sphere roughly 16 miles (26 km) in diameter.
The disproportionate mass-size ratio wasn’t surprising. What was really unexpected was the location of J0030’s hotspots.
Pulsars spin between 7 and 40,000 times a minute and form intense magnetic fields. The rapid spin and magnetic fields generate powerful beams of electromagnetic radiation, and as the pulsar rotates, these beams sweep the sky like a lighthouse. Scientists have always thought that these beams are fired from two hotspots, one for each magnetic pole.
But, at least in J0030’s case, there are two or three of these hotspots, all located in the southern hemisphere. There was none in the northern hemisphere, as textbooks suggest.
In order to map the pulsar’s hotspots, the researchers had to compute where the X-rays received by NICER originated on the neutron star’s surface — and this required computations that would have taken a normal computer about a decade to complete. Luckily, this turned out fine in just a month using the Dutch national supercomputer Cartesius.
There are at least two features that make this investigation unique. First of all, it’s the first time that astronomers have been able to detect photons from a pulsar this fast — NICER measures radiation on a photon-by-photon basis with an unparalleled precision of 100 nanoseconds — which enabled the computer model to account for the rotation of the pulsar. Secondly, the model also considered the fact that the X-ray radiation can also come from the side of the pulsar, thanks to the curvature of space-time.
These results indicate that a pulsar’s magnetic field is much more complex than previously assumed — or at least not as simple as the traditional two-pole model. Now, scientists will have to repeat the accomplishment with other pulsars. Such investigations may help answer many burning questions. For example, astronomers would like to know what exactly mater looks like in the ultra-dense core of a neutron star.
“It’s remarkable, and also very reassuring, that the two teams achieved such similar sizes, masses and hot spot patterns for J0030 using different modeling approaches,” said Zaven Arzoumanian, NICER science lead at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “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?”
The findings appeared in The Astrophysical Journal Letters.