Rare isotope studies to understand the cooling of neutron star crusts

The densest forms of matter anywhere in the universe is found inside neutron stars. Space-based X-ray observations make these systems unique dense matter laboratories, especially if the neutron star is located in a binary stellar system. This is where matter from a companion star may fall onto the neutron star surface over extended periods of time, a process called accretion. One unique glimpse into the properties of dense matter is provided by observations of the surface temperature of neutron stars that were heated during an accretion episode, and cool during phases of quiescence that can last many years. Such observations can be used to determine the internal composition and structure of neutron stars, and whether they contain superfluid neutrons or nuclear pasta. Interpreting the observations requires understanding of the nuclear reactions that heat and cool the neutron star during accretion. These reactions involve neutron-rich rare isotopes that form a crust around the neutron star.

We developed an experimental approach that takes advantage of the Lab’s ability to produce the right neutron-rich rare isotopes to determine the rate of key nuclear reactions. Of interest are particular reactions, called Urca reactions, which limit how hot the neutron-star surface can get. These Urca reactions have been identified theoretically in previous JINA-CEE work. They consist of alternating electron captures and beta decays, that convert a nucleus into its neighbor on the chart of nuclides, and back to the original nucleus, with the cycle repeating as long as it is hot enough. In each cycle, neutrinos are emitted that escape the dense neutron star readily due to quantum effects, and carry away energy, cooling the star.

The experiment studied the beta decay of the vanadium-61 isotope, which has a short half-life of only 48 milliseconds. Vanadium-61 is part of a particularly prolific Urca cooling cycle for neutron stars where X-ray bursts produce heavy nuclei on the surface that are then incorporated into the neutron star crust. The experiment was conducted at NSCL in two parts – in the first part the vanadium-61 beam was stopped inside the SuN detector. In the second part, the beam was stopped inside the NERO neutron detector.

As a result, the now well-determined cooling strength is significantly lower than predicted, but significant enough to require inclusion in models that are used to interpret X-ray observations. The technique can readily be used to investigate all Urca cooling reactions. Two followup experiments have already been carried out at NSCL and future experiments are planned at the FRIB facility where all nuclei along the dripline up to nickel, that form the (outer) crust of neutron stars can be produced and studied.

The work was the thesis project of JINA-CEE graduate student Wei Jia Ong, now a Lawrence Fellow at Lawrence Livermore National Laboratory. The work was performed by a JINA-CEE collaboration of multiple experimental groups to combine various detector systems, nuclear theorists, and astrophysicists. The results have been published in Phys. Rev. Lett.