- Magnetic angle evolution in accreting neutron stars(arXiv)
Abstract : The rotation of a magnetised accreting neutron star (NS) in a binary system is described by its spin period and two angles: spin inclination α with respect to the orbital momentum and magnetic angle χ between the spin and the magnetic moment. Magnetospheric accretion spins the NS up and adjusts its rotation axis, decreasing α to nearly perfect alignment. Its effect upon the magnetic angle is more subtle and relatively unstudied. In this work, we model the magnetic angle evolution of a rigid spherical accreting NS. We find that the torque spinning the NS up may affect the magnetic angle while both α and χsignificantly deviate from zero, and the spin-up torque varies with the phase of the spin period. As the rotation axis of the NS is being aligned with the spin-up torque, the magnetic axis becomes misaligned with the rotation axis. Under favourable conditions, magnetic angle may increase by Δχ∼15∘−20∘. This orthogonalisation may be an important factor in the evolution of millisecond pulsars, as it partially compensates the χ decrease potentially caused by pulsar torques. If the direction of the spin-up torque changes randomly with time, as in wind-fed high-mass X-ray binaries, both the rotation axis of the NS and its magnetic axis become involved in a non-linear random-walk evolution. The ultimate attractor of this process is a bimodal distribution in χ peaking at χ=0∘ and χ=90
2. Expanded atmospheres and winds in Type I X-ray bursts from accreting neutron stars(arXiv)
Abstract : We calculate steady-state models of radiation-driven super-Eddington winds and static expanded envelopes of neutron stars caused by high luminosities in type I X-ray bursts. We use flux-limited diffusion to model the transition from optically thick to optically thin, and include effects of general relativity, allowing us to study the photospheric radius close to the star as the hydrostatic atmosphere evolves into a wind. We find that the photospheric radius evolves monotonically from static envelopes (rph≲50−70 km) to winds (rph≈100−1000 km). Photospheric radii of less than 100 km, as observed in most photospheric radius expansion bursts, can be explained by static envelopes, but only in a narrow range of luminosity. In most bursts, we would expect the luminosity to increase further, leading to a wind with photospheric radius ≳100 km. In the contraction phase, the expanded envelope solutions show that the photosphere is still ≈1 km above the surface when the effective temperature is only 3% away from its maximum value. This is a possible systematic uncertainty when interpreting the measured Eddington fluxes from bursts at touchdown. We also discuss the applicability of steady-state models to describe the dynamics of bursts. In particular, we show that the sub to super-Eddington transition during the burst rise is rapid enough that static models are not appropriate. Finally, we analyze the strength of spectral shifts in our models. Expected shifts at the photosphere are dominated by gravitational redshift, and are therefore predicted to be less than a few percen
3. β-decay of 61V and its Role in Cooling Accreted Neutron Star Crusts(arXiv)
Author : W. -J. Ong, E. F. Brown, J. Browne, S. Ahn, K. Childers, B. P. Crider, A. C. Dombos, S. S. Gupta, G. W. Hitt, C. Langer, R. Lewis, S. N. Liddick, S. Lyons, Z. Meisel, P. Möller, F. Montes, F. Naqvi, J. Pereira, C. Prokop, D. Richman, H. Schatz, K. Schmidt, A. Spyrou
Abstract : The interpretation of observations of cooling neutron star crusts in quasi-persistent X-ray transients is affected by predictions of the strength of neutrino cooling via crust Urca processes. The strength of crust Urca neutrino cooling depends sensitively on the electron-capture and β-decay ground-state to ground-state transition strengths of neutron-rich rare isotopes. Nuclei with mass number A=61 are predicted to be among the most abundant in accreted crusts, and the last remaining experimentally undetermined ground-state to ground-state transition strength was the β-decay of 61V. This work reports the first experimental determination of this transition strength, a ground-state branching of 8.1+2.2−2.0%, corresponding to a log ft value of 5.5+0.2−0.2. This result was achieved through the measurement of the β-delayed γ rays using the total absorption spectrometer SuN and the measurement of the β-delayed neutron branch using the neutron long counter system NERO at the National Superconducting Cyclotron Laboratory at Michigan State University. This method helps to mitigate the impact of the Pandemonium effect in extremely neutron-rich nuclei on experimental results. The result implies that A=61nuclei do not provide the strongest cooling in accreted neutron star crusts as expected by some predictions, but that their cooling is still larger compared to most other mass numbers. Only nuclei with mass numbers 31, 33, and 55 are predicted to be cooling more strongly. However, the theoretical predictions for the transition strengths of these nuclei are not consistently accurate enough to draw conclusions on crust cooling. With the experimental approach developed in this work all relevant transitions are within reach to be studied in the future.