These Three Neutron Stars Shouldn't Be So Cold

Neutron stars are among the densest objects in the Universe, second only to black holes. Like black holes, neutron stars are what remains after a star reaches the end of its life cycle and undergoes gravitational collapse. This produces a massive explosion (a supernova), in which a star sheds its outer layers and leaves behind a super-compressed stellar remnant. In fact, scientists speculate that matter at the center of the star is compressed to the point that even atoms collapse and electrons merge with protons to create neutrons.

Traditionally, scientists have relied on the “Equation of State” – a theoretical model that describes the state of matter under a given set of physical conditions – to understand what physical processes can occur inside a neutron star. But when a team led by scientists from the Spanish National Research Council (CSIC) examined three exceptionally young neutron stars, they noticed they were 10-100 times colder than other neutron stars of the same age. For this, the researchers concluded that these three stars are inconsistent with most of the proposed equations of state.

The team consisted of astrophysicists from the Institute of Space Sciences (ICE-CSIS) in Barcelona, the Institute d’Estudis Espacials de Catalunya (IEEC), and the Department of Applied Physics at the University of Alacant. Alessio Marino, a postdoctoral fellow in astrophysics at the ICE and IEEC, was the lead author of the team’s paper (“Constraints on the dense matter equation of state from young and cold isolated neutron stars“), which recently appeared in Nature Astronomy.

While astronomers are still unsure which equation of state models are correct for neutron stars, the laws of physics dictate that all neutron stars must obey the same one. What’s more, the cool nature of neutron stars is a reliable method for determining their age – the older they are, the cooler they get. While they are difficult to study invisible light, their rotating nature and magnetic fields (which funnel energy towards the magnetic poles) produce X-ray pulses that can be observed.

After consulting data from the ESA’s XMM-Newton and NASA’s Chandra missions, the team found evidence of three neutron stars. The extreme sensitivity of these telescopes not only allowed the team to detect these neutron stars but also to collect enough light to determine their temperatures and other properties. According to astrophysicist Nanda Rea, whose research group at the ICE-CSIC and the IEEC led the investigation, the results were very surprising:

“The young age and the cold surface temperature of these three neutron stars can only be explained by invoking a fast cooling mechanism. Since enhanced cooling can be activated only by certain equations of state, this allows us to exclude a significant portion of the possible models,”

“Neutron star research crosses many scientific disciplines, spanning from particle physics to gravitational waves. The success of this work demonstrates how fundamental teamwork is to advancing our understanding of the Universe.”

To this end, Rea and her colleagues – Alessio Marino, Clara Dehman, and Konstantinos Kovlakas – benefited from their combined and complementary expertise. Marino, a postdoctoral fellow with the ICE-CSIS and IEEC, led the team’s efforts to deduce the neutron stars’ other physical properties. In addition to determining their temperature from the X-rays emitted, the sizes and speeds of the surrounding supernova remnants gave an accurate indication of their ages.

This was followed by Clara, a Postdoctoral Researcher at the University of Alacant, computing the neutron stars’ “cooling curves” of neutron stars based on a range of masses and magnetic field strengths. This consisted of plotting what each “equation of state” model predicts for how a neutron star’s temperature (as indicated by its brightness) changes over time. Last, Kovlakas, a postdoctoral fellow at the ICE-CSIC and IEEC, led a statistical analysis that used machine learning to match the simulated cooling curves with the properties of the three neutron stars.

These simulations revealed that without a fast cooling mechanism, none of the equations of state matched the data. What’s more, the team concluded that the properties of these stars are inconsistent with about 75% of known neutron star models. By narrowing the range of possibilities, astronomers are one step closer to learning which neutron star equation of state governs them all. This could also have important implications for understanding how the fundamental laws of the Universe – General Relativity and Quantum Mechanics – fit together.

This makes neutron stars a perfect laboratory for testing the laws of physics since they have densities and gravitational forces far beyond anything that can be recreated on Earth. Much like black holes, these objects are where the laws of physics begin to break down, where the most profound breakthroughs in our understanding of them can often be found!

Further Reading: ESA, Nature Astronomy