Now 50 years since the existence of the neutron star crust was proposed, we review the current understanding of the nuclear physics of the outer layers of accreting neutron stars. Nuclei produced during nuclear burning replace the nascent composition of the neutron star ocean and crust. Non-equilibrium nuclear reactions driven by compression alter the outer thermal structure and chemical composition, leaving observable imprints on astronomical phenomena. As observations of bursting neutron stars and cooling neutron stars have increased, the recent volume of astronomical data allows new insights into the microphysics of the neutron star interior and the possibility to test nuclear physics input in model calculations. Despite numerous advances in our understanding of neutron star interiors and observed neutron star phenomena, many challenges remain in the astrophysics theory of accreting neutron stars, the nuclear theory of neutron-rich nuclei, and the reach and precision of terrestrial nuclear physics experiments. Nuclear Physics of the Outer Layers of Accreting Neutron StarsFor example, current investigations of the critical reaction rates in X-ray bursts [9,27], studies of key properties of nuclei in the accreted crust [25,26], and nuclear reaction network calculations of accreted crust compositions [22,24,28], all promise to improve the observational constraints on dense matter derived from accreting neutron stars.We begin in section 2 by briefly outlining the neutron star structure. Sections 3 and 4 describe the original composition of the neutron star crust and the accretion process which drives the system from equilibrium. In section 5, we discuss the nuclear burning that can occur on the surface of accreting neutron stars and the nuclei produced during the different possible burning regimes. In actively accreting neutron stars, the ashes of prior surface nuclear burning are compressed to greater depths by newly accreted material. We discuss in section 6 the nuclear interactions involving the ashes that take place as the ambient mass density increases. In section 7, we investigate the impact of interior nuclear interactions on observable neutron star phenomena. We summarize and discuss prospects for future work in section 8.

The study of long-term evolution of neutron star (NS) magnetic fields is key to understanding the rich diversity of NS observations, and to unifying their nature despite the different emission mechanisms and observed properties. Such studies in principle permit a deeper understanding of the most important parameters driving their apparent variety, e.g. radio pulsars, magnetars, x-ray dim isolated neutron stars, gamma-ray pulsars. We describe, for the first time, the results from self-consistent magneto-thermal simulations considering not only the effects of the Hall-driven field dissipation in the crust, but adding a complete set of proposed driving forces in a superconducting core. We emphasize how each of these core-field processes drive magnetic evolution and affect observables, and show that when all forces are considered together in vectorial form, the net expulsion of core magnetic flux is negligible, and will have no observable effect in the crust (consequently in the observed surface emission) on megayear time-scales. Our new simulations suggest that strong magnetic fields in NS cores (and the signatures on the NS surface) will persist long after the crustal magnetic field has evolved and decayed, due to the weak combined effects of dissipation and expulsion in the stellar core.

We report on 3.5 years of Chandra monitoring of the Galactic Centre magnetar SGR J1745−2900 since its outburst onset in April 2013. The magnetar spin-down has shown at least two episodes of period derivative increases so far, and it has slowed down regularly in the past year or so. We observed a slightly increasing trend in the time evolution of the pulsed fraction, up to ∼ 55 per cent in the most recent observations. SGR J1745−2900 has not reached the quiescent level yet, and so far the overall outburst evolution can be interpreted in terms of a cooling hot region on the star surface. We discuss possible scenarios, showing in particular how the presence of a shrinking hot spot in this source is hardly reconcilable with internal crustal cooling and favours the untwisting bundle model for this outburst. Moreover, we also show how the emission from a single uniform hot spot is incompatible with the observed pulsed fraction evolution for any pair of viewing angles, suggesting an anisotropic emission pattern.