&Bullet; physics 14, 64
A satellite experiment has shown that the heaviest known neutron star is unexpectedly large, suggesting that the matter in the star’s inner core is less “squeezable” than some models predict.
Neutron stars are “cosmic zombies” – corpses of massive stars that collapsed in violent explosions after they ran out of fuel. By studying these ultra-dense objects, researchers hope to understand how matter behaves under extreme conditions, which could help solve some of physics’ greatest puzzles. As announced at a press conference during the APS meeting in April 2021, NASA’s Neutron Star Interior Composition Explorer (NICER), an X-ray telescope on the International Space Station, measured the size of the heaviest known neutron star. The surprisingly large radius measured for this star implies a stiffer than expected state of matter in the core, which makes models that predict a “squishy” center unfavorable.
Neutron stars are the densest observable objects in the cosmos – they pack twice the mass of the sun into a sphere that is as wide as a big city. In the star’s outer core, the great pressure breaks nuclei into nucleons and crushes protons and electrons together, leaving a sea of mostly neutrons. However, researchers are unsure of what is happening in the star’s inner core. Do neutrons persist or do they break down into their quark components? Do these quarks interact to form exotic particles?
Since no laboratory experiment can reproduce the conditions of neutron stars, the only way to study this extraordinary state of matter is to observe neutron stars themselves and to infer what is going on inside the star from basic properties such as mass and size. However, such measurements are not an easy task. To date, approximately two thousand neutron stars have been discovered, but only a handful of them have been magnified, typically by monitoring the X-ray emission of the gas surrounding the star.
NICER has developed a unique sizing method applicable to rapidly rotating neutron stars known as pulsars. As pulsars spin, hot spots on their surface emit X-rays that scan the cosmos like lighthouse rays. The experiment monitors the oscillatory X-ray brightness of the pulsar and “stamps” the arrival of each X-ray photon with an accuracy of about 100 ns. The path these photons take is distorted by the gravitational distortion of spacetime around the star, so some hot spots remain visible even as they rotate to the other side of the star.
From the X-ray timestamp data, the researchers reconstruct the gravitational potential and deduce the star size from it. “The combination of X-ray spectroscopy capabilities with timing is a unique feature of NICER,” which allows researchers to fully use information about the star’s spin to constrain its properties, says Zaven Arzoumanian, NICER science director at Goddard Space Flight Center of NASA in Maryland.
The NICER Collaboration first used this method in 2019 to measure PSR J0030, a pulsar 1000 light years from Earth. Weighing 1.4 solar masses, J0030 was found to be about 26 km in diameter. For the new measurement, the collaboration turned to the most massive known neutron star, PSR J0740, in the “Giraffe” constellation. Almost four times further away than J0030, J0740 is 20 times weaker and was therefore a “route target for the experiment,” says Arzoumanian. But its mass (2.1 solar masses) makes this pulsar “so extraordinary” that the team decided to spend a lot of time measuring it, he says.
The collaboration hired two independent teams to analyze the data using different assumptions, such as the x-ray background in the sky and instrument calibration. The teams, led by Anna Watts of the University of Amsterdam and Cole Miller of the University of Maryland in College Park, came up with similar values for the star’s most likely diameter: 25 and 27 km, both close to the lighter pulsar measured earlier.
“This is a remarkable result,” says Sanjay Reddy, a theorist at the University of Washington in Seattle. He says the size measurement will allow researchers to examine different options for the star’s interior. In some models, neutrons decay into free-roaming quarks, resulting in a squishy, compressible core. These models provide the counterintuitive prediction that neutron stars should get smaller with increasing mass. Other models, in which some neutrons persist or the quarks interact strongly, predict a form of matter that is difficult to compress. The similar diameters for the J0030 and J0740 “put the muddy models at a great disadvantage,” says Reddy.
Together, the two measurements from J0740 and J0030 offer another thought-provoking conclusion, says Watts. In textbooks, pulsars are represented as perfectly symmetrical magnetic dipoles with hotspots at each pole of the pulsar. For both stars studied by NICER, however, the hot spots appear to be on the same hemisphere, suggesting a much more complex and asymmetrical field configuration. After seeing this asymmetry in the first two stars that mapped the hot spots, “this beautiful dipole cartoon made of pulsars is probably wrong,” says Watts.
With NICER’s goal of measuring more pulsars and the prospect of seeing many neutron star fusions through gravitational wave detection, we are entering a “golden age” for neutron star physics, says Reddy. He says that understanding the nature of matter in these stars will affect many fundamental questions, as neutron stars play a role in synthesizing the heaviest elements and are promising targets for dark matter detection.
Matteo Rini is the editor of physics.