Neutron stars are some of the densest objects in the universe, with their core possibly composed of a thick soup of quarks or exotic particles that could not survive anywhere else. Recent observations by ESA’s XMM-Newton and NASA’s Chandra have revealed three unusually cold, young neutron stars, challenging current models by showing they cool much faster than expected. This finding has significant implications, suggesting that only a few of the many proposed neutron star models are viable, and pointing to a potential breakthrough in linking the theories of general relativity and quantum mechanics through astrophysical observations.
Neutron stars are formed in the last moments of the life of a very large star (with more than about eight times the mass as our Sun), when the nuclear fuel in its core eventually runs out. In a sudden and violent end, the outer layers of the star are ejected with monstrous energy in a supernova explosion, leaving behind spectacular clouds of interstellar material rich in dust and heavy metals. At the center of the cloud (nebula), the dense stellar core further contracts to form a neutron star.
The matter in the center of a neutron star is squeezed so hard that scientists still don’t know what form it takes. Neutron stars get their name from the fact that under this immense pressure, even atoms collapse: electrons merge with atomic cores, turning protons into neutrons. However, the extreme heat and pressure may stabilize more exotic particles that don’t survive anywhere else, or possibly melt particles together into a swirling soup of their constituent quarks.
The true neutron star equation of state, which describes what physical processes can occur inside a neutron star, is still unknown. While the behavior of individual neutron stars may depend on properties like their mass or how fast they spin, all neutron stars must obey the same equation of state.
The discovery of these three exceptionally young and cold neutron stars has allowed scientists to exclude most proposed equations of state. The young age and the cold surface temperature of these three neutron stars can only be explained by invoking a fast cooling mechanism, which allows scientists to exclude a significant portion of the possible models.
Uncovering the true neutron star equation of state also has important implications for the fundamental laws of the universe, as neutron stars are the best testing ground for stitching together the theory of general relativity with quantum mechanics. The sensitivity of XMM-Newton and Chandra made it possible to detect these neutron stars and collect enough light to determine their temperatures and other properties. The complementary expertise of a team of researchers was combined to draw conclusions about what these oddballs mean for the neutron star equation of state.
