Abstract
Accreting neutron stars host a number of astronomical observables which can be used to infer the properties of the underlying dense matter. These observables are sensitive to the heating and cooling processes taking place in the accreted neutron star (NS) crust. Within the past few years it has become apparent that electron-capture/beta-decay (urca) cycles can operate within the NS crust at high temperatures. Layers of nuclei undergoing urca cycling can create a thermal barrier, or Great Wall, between heating occurring deep in the crust and the regions above the urca layers. This paper briefly reviews the urca process and the implications for observables from accreting neutron stars.
Highlights
Describing how matter behaves across the range of temperatures and densities occurring in nature is the ultimate goal of much of modern physics research
The only avenue presently available to study matter at the highest densities, near and above those of an atomic nucleus, at relatively low temperatures is the study of neutron stars
Neutron stars accreting material from a binary companion are useful in this regard, as their observable phenomena provide clues as to the underlying structure of the neutron star (NS)
Summary
Describing how matter behaves across the range of temperatures and densities occurring in nature is the ultimate goal of much of modern physics research. Neutron stars accreting material from a binary companion are useful in this regard, as their observable phenomena provide clues as to the underlying structure of the neutron star (NS). Gleaning information from such observables requires comparisons between the observed data and results from astrophysics model calculations. The nuclear reactions of the outer crust and above can be constrained using indirect measurements at present stable and radioactive ion-beam facilities. The nuclear physics of particular interest for this work is the phenomenon of electron-capture/β--decay cycling, known as urca cooling, which was recently found to be active in the crusts of accreting neutron stars. Prior to describing urca cooling and its impact, a brief discussion of reactions in the outer NS (shown schematically in Figure 1) will help provide context
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