Variable opacity and accretion-column regimes of magnetized neutron stars

Abstract: Radiation-supported accretion columns of magnetized neutron stars can operate in several distinct regimes. As the accretion rate increases, magnetospheric accretion may proceed through hot-spot emission, efficiently cooling radiative shocks, and advective sinking columns. The structure of the accretion flow is determined by the global parameters such as magnetic field strength and mass accretion rate, but is also affected by opacity variations in strong magnetic fields. A decrease of opacity for photons with energy below the cyclotron energy is able to substantially augment the classical critical accretion rate separating efficiently cooling solutions from advective solutions. We propose an approach to determine the critical accretion rate above which the Basko-Sunyaev sinking solution becomes unavoidable. We also use the Basko-Sunyaev sinking solution to estimate the plasma temperature and compare the characteristic photon energy with the local cyclotron energy. For a combination of moderate magnetic fields and high accretion rates, the characteristic photon energy in advective columns typically exceeds the cyclotron energy. In this case, the opacity is not reduced by the magnetic field, and tall accretion columns are formed. Strong magnetic-field systems may retain short, efficiently cooling accretion columns over a wide range of super-Eddington accretion rates. We identify a region in accretion rate and magnetic-field strength where both efficiently cooling and advective solutions may coexist. In this region, the column may switch or oscillate between the regimes and geometries. The typical time of such oscillations are close to the replenishment time of the column, or to the thermal time near its bottom. The quasi-periodic oscillations, observed in some super-Eddington objects in the 1-100 mHz frequency range, may be related to such relaxation cycles.