Magnetic accretion disk-outflow model for the state transition in X-ray binaries

Abstract

Context. The hard-to-soft state transition of the outbursts in X-ray binaries (XRBs) is triggered by the rising of the mass accretion rate as a result of the disk instability. The hard X-ray transition luminosity is found to be tightly correlated to the soft X-ray peak luminosity in the soft state, the physical origin of which is still a mystery. Aims. In order to explain the observed correlation between the hard X-ray transition luminosity and the soft X-ray peak luminosity in the soft state, we construct a magnetic disk-outflow model for the state transition in XRBs. Methods. We assumed that the large-scale magnetic field in the outer thin disk is formed through an inverse cascade of the field generated by the small-scale dynamo, which is then advected by the inner advection-dominated accretion flow (ADAF). The advected field accelerates a fraction of the gas in the ADAF into the outflows. We calculated the transition luminosity of an ADAF that is driven by these magnetic outflows, which vary with the mass accretion rate of the outer disk. Results. During the outbursts, the heating front moves inward, and the field strength at the heating front of the outer disk is proportional to the accretion rate of the disk. Much angular angular momentum of the inner ADAF is carried away by the outflows for a stronger magnetic field, which leads to a high radial velocity of the ADAF. This increases the critical mass accretion rate of the ADAF with the field strength, and it therefore leads to a correlation between transition luminosity and the peak luminosity in the thermal state. We found that the values of the viscosity parameter α of the neutron star XRBs are systematically higher for those of the black hole (BH) XRBs (α ∼ 0.05−0.15 for BHs, and α ∼ 0.15−0.4 for neutron stars). Our model predicts that the transition luminosity may be higher than the peak luminosity provided α is sufficiently high, which is able to explain a substantial fraction of outbursts in BHXRBs that do not reach the thermally dominant accretion state.