Neutron Stars Might Be Squishy Inside

Astronomers have found a way to peer inside neutron stars and glimpse the exotic matter hiding in their cores. By pinning down the properties of the closest and brightest neutron star yet, Devarshi Choudhury (University of Amsterdam) and colleagues have ruled out both the plainest and the strangest ideas describing the dense matter inside these exotic objects.

Under Pressure

Neutron stars are crushed stellar remnants that have already lost their first battle against gravity. Upon the star’s collapse, the core’s atoms broke down into neutrons; only a rule that helps govern the world of the very small (the Pauli exclusion principle) prevents the neutrons from getting too friendly with one another. That pressure prevents further caving-in to gravity.

So far so good — physicists understand that part pretty well. But things get weirder at the center, where the pressure can grow to the limits of unbearable. The key to understanding how extreme things can become in the core is to figure out the equation of state that describes this dense material. If the equation of state says that this matter is “stiff,” then increasing a neutron star’s mass, and thus its internal pressure, wouldn’t cause many (or any) interior changes. All that would change is its size — add more mass and the neutron star grows.

On the other hand, the equation of state might instead say the interior is “squishy.” In that case, an increase in mass, and thus pressure, would alter the densest matter in some way, perhaps squeezing quarks out of the neutrons or maybe creating more exotic particles such as hyperons. Depending on how the matter changes under pressure, the neutron star may stay the same size as mass is added, or it might even shrink.

Of course, we can’t add mass to neutron stars ourselves — even companion stars can take a long time to add enough material to make a difference. Instead, astronomers have set about measuring the mass and radius of several spinning neutron stars, known as pulsars, using the Neutron Star Interior Composition Explorer (NICER) observatory on the International Space Station.

Nice and Precise

NICER observes X-rays from pulsars that spin quickly, in turn generating powerful magnetic fields. Extremely hot regions near the pulsars' magnetic poles whirl around like frenetic lighthouses. By precisely clocking the arrival of each pulse of light, astronomers can learn a lot about the objects emitting them.

NICER scientists have now added data from the closest pulsar on their list, PSR J0437–4715, 510 light-years away. It’s fast, pulsing 174 times per second — quicker than the whirling knives of a kitchen blender — and it’s bright, which makes for precise measurements. The astronomers find the pulsar’s mass to be right around 1.4 times the Sun’s mass (the addition of previously gathered radio data aided that measurement). The pulsar’s radius is between 10.7 and 12.3 kilometers (6.6–7.6 miles), which means it would fit inside the length of Manhattan.

Squishy Matter

This pulsar has placed some of the best limits yet on what kinds of material might exist inside neutron stars, ruling out scenarios on both extremes.

“Our new result points us towards slightly softer (squisher!) equations of state,” says team member Anna Watts (also at University of Amsterdam). “But the very softest are still unlikely, as are now the stiffest.” The results agree with gravitational-wave measurements of two neutron stars observed colliding in 2017.

So how exotic can a neutron star get? Combining the mass-radius measurements of several neutron stars excludes the idea of cores dominated by plain ol' neutrons, which would be described by a stiffer equation of state. The data do allow some (but not all) scenarios in which increased pressure might squeeze quarks out of the neutrons. But, as the plot above shows, a lot of possible scenarios are still in the running.

Sky & Telescope recently reported on a study that purportedly put stringent limits on the possible equations of state for neutron stars. However, Nathan Rutherford (University of New Hampshire) says it's not easy to combine that result — which relied on temperature and magnetic field measurements of three unusually cool neutron stars — with this new result, which is based on mass and radius measurements.

The cold-neutron-star study also considered a smaller family of possibilities, Watts adds, although there are possibilities for expanding that work. There's simply a lot of wiggle room for what's possible within these exotically compact objects.

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The study described above, led by Choudhury, will appear in the Astrophysical Journal Letters, as will a deeper dive into the methods for determining the pulsar’s mass and distance, led by Daniel Reardon (Swinburne University of Technology, Australia), and its equation of state, led by Nathan Rutherford (University of New Hampshire).