Modeling of Disk-Star Interaction: Different Regimes of Accretion and Variability

Abstract

The appearance and time variability of the accreting millisecond X‐ray pulsars (hereafter AMXPs, e.g. [27]) depends strongly on the accretion rate and the effective viscosity and magnetic diffusivity of the disk‐magnetosphere boundary. The accretion rate is the main parameter which determines the location of the magnetospheric radius of the neutron star. We introduce a classification of accreting neutron stars as a function of the accretion rate and show the corresponding stages obtained from our global 3D magnetohydrodynamic (MHD) simulations and from our axisymmetric MHD simulations. We discuss the expected variability features in these stages of accretion, both periodic and quasi‐periodic (QPOs). We conclude that the periodicity may be suppressed at both very high and very low accretion rates. In addition the periodicity may disappear when ordered funnel flow accretion is replaced by disordered accretion through the interchange instability.