Neutron stars are sometimes referred to as “dead stars”, because they cannot burn nuclear fuel in their interior as other stars do. Nevertheless, neutron stars can generate energy by accreting gas from a companion star in an X-ray binary.

When a neutron star accretes material, its ~1 km thick crust is compressed. This compression induces nuclear reactions such as atomic nuclei capturing electrons and nuclear fusion reactions. Theoretical calculations and accelerator experiments (e.g., such as done at the nuclear superconducting cyclotron laboratory at Michigan State University), provide a handle on how much heat should be generated inside neutron stars as a result of accretion. These results can then be compared with astrophysical observations.

Neutron stars often feast on their companion only for a few weeks/months (in exceptional cases years), after which they typically sag for years/decades before becoming active again. It is during these periods of quiescence that heat radiation from the neutron star (which have a temperature of millions of degrees Celsius) can be detected with sensitive X-ray satellites such as Chandra, XMM-Newton, or Swift. Since neutron stars do not generate energy when they are not accreting, they will gradually cool as they radiate their heat away as X-rays. Excitingly, dedicated efforts have allowed to detect the cooling trajectories of 7 neutron stars by now.

In this area of research there is thus a direct interplay between astrophysical observations of neutron stars, theoretical calculations about their interior, and nuclear accelerator experiments that mimic the nuclear reactions that take place in their outer layers. Interestingly, our X-ray observations seem to suggest that neutron stars are heated to a larger extend than is currently accounted for by nuclear heating models. It appears that there is a puzzling source of extra energy generation in the outermost layers of the neutron star, referred to as a “shallow heat source“, the origin of which remains to be established.

So far, it appears that the amount of extra heating differs between sources. It is not clear, however, if this is due to their different outburst properties (e.g., length, duration, total accreted mass, accretion geometry), or relates to different neutron star parameters (e.g., rotation period, age, mass, radius). A powerful way to break these degeneracies would be to observe cooling curves for one particular source after different types of outbursts. If the shallow heating is different for each outburst, it is likely related to the accretion properties — else we need to seek its origin in the properties of the neutron star itself.

We took up this challenge by studying the prolific neutron star X-ray binary Aql X-1, which is active approximately once every year and exhibits a wide range of outburst properties. In particular, we exploited the unique flexibility and good sensitivity of the Swift satellite to study the temperature evolution of the neutron star after different outbursts. The first test was to prove whether cooling is actually observable for this particular neutron star, which definitely appears to be the case. Future observations of Aql X-1 aiming to study the cooling trajectories after different outbursts are therefore a very promising avenue to elucidate the origin of the shallow heat generation in neutron star crusts that puzzles astrophysical observers, physicists and nuclear experimentalists.

Waterhouse, Degenaar, Wijnands, Brown, Miller, Altamirano, Linares 2016, MNRAS 456, 4001: Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

Paper link: ADS

Long-term activity of Aql X-1 as seen through daily monitoring observations with Swift/BAT (launched in 2005) and MAXI (launched in 2009). This illustrates the frequent activity of this neutron star X-ray binary. Figure from Waterhouse et al. 2016.