Neutron stars have masses up to about 2.5 times the mass of our Sun, but their radii are nearly 50 thousand times smaller. This means that the matter that makes up neutron stars must be squeezed incredibly close together. In fact, neutron stars present the densest observable form of matter in the Universe. While in black holes matter is even more tightly packed, it is all hidden behind an event horizon and hence beyond our reach. For neutron stars, however, we can observe radiation from their surface and use that to infer what they look like on the inside. Neutron stars allow us to study how matter behaves when it is crushed to enormously high densities, conditions that we cannot replicate and study in laboratories on Earth.

Neutron stars are thought to have a 1-km thick solid crust, below which matter occurs in liquid form. It is generally thought that this liquid must be superfluid, meaning that it swirls around freely without experiencing any friction. On Earth, such fascinating, odd behavior is observed when liquid helium is cooled down to extremely low temperatures of about -270 degrees Celsius. There are, however, many uncertainties about how a superfluid works at the millions of degrees Celsius temperatures inside neutron stars. As a result, it is far from understood what the superfluid properties of neutron stars are exactly. This is very important to establish, as it likely has a profound effect on how various neutron star properties, such as their magnetic field strength, spin and interior temperature, evolve over their lifetime. Moreover, several observable phenomena of neutron stars, such as occasional distortions in their rotation called glitches or sudden cracking of their crust analogous to Earth quakes, likely depend on the detailed properties of their interior superfluid.

Since we cannot perform experiments on Earth that teach us how superfluids inside neutron stars may work, we need to reverse the problem. One promising way to do this is to track how a neutron star cools down after it has been heated up by gobbling up gas from a companion star. We call this cannibalistic action accretion and often this happens in spurts that we call  outbursts. The superfluid properties of its interior should influence both the extend to which the neutron star can be heated during accretion outbursts, and how its temperature then subsequently decreases when accretion stops and hence there is no more heat generation. One such neutron star that we previously established to be heated and subsequently cooling down is called HETE J1900.1-2455. In a previous study we tracked its temperature decrease and our modeling suggested that the neutron star would cool further. After having waited a few years, we took a new observation of the source with the Chandra X-ray satellite to verify this.

Chandra pointed towards our neutron star for over half a day and surprisingly saw almost nothing! HETE J1900.1-2455 turned out to have cooled so much that it was hardly generating any detectable X-ray emission, even for the very sensitive X-ray detector of Chandra. In the 16 hours that Chandra started at it, a mere 5 X-ray light particles were capture from the neutron star. In order words, only every 3 hours was the Chandra detector hit by an X-ray light particle from HETE J1900.1-2455. Perhaps somewhat counter-intuitive, the fact that we detected so little light from the source actually provides us with very interesting constraints on its interior properties. Performing a rigorous and conservative analysis (based on those 5 light particles!) we inferred the likely temperature of the neutron star and performed highly advanced simulations that take into account the physical properties of neutron stars, including their interior superfluid.

Our analysis revealed that the low temperature of HETE J1900.1-2455 must imply that it is able to cool its interior extremely rapidly. Two exciting possibilities that would allow for this is either that the neutron star is very massive or that a large fraction of its core is not superfluid. The latter option would contrast the general belief and would have profound implications for our understanding of many observable properties of neutron stars. The former option would be a very important finding because the most massive neutron stars put the strongest constraints on the behavior of ultra-dense matter.  Although both options are equally exciting, our current observations do not allow us to distinguish between the two. However, because of the significant science impact, Chandra will again point towards HETE J1900.1-2455 to allow us to obtain a new temperature measurement. This time, it will not stare at the neutron star for 16 hours, but for 6 full days!! If its temperature remained stable, we will then collect 50 X-ray light particles and be able to obtain a much more reliable temperature estimate. If it cooled further, however, we might not see any light at all. Ironically, the latter case would be the most exciting outcome. Part of the data has recently been taken and the remaining observations will come in soon. I can wait to find out what we will (not) see in those new Chandra observations!

Degenaar, Page, van den Eijnden, Beznogov, Renolds, Wijnands 2021, MNRAS 508, 882: Constraining the properties of dense neutron star cores: the case of the low-mass X-ray binary HETE J1900.1-2455

Paper link: ADS