Formation of Semi-relativisitc Jets from Magnetospheres of Accreting Neutron Stars: Injection of Hot Bubbles into a Magnetic Tower

Abstract

We present the results of 2.5-dimensional resistive magnetohydrodynamic (MHD) simulations of the magnetic interaction between a weakly magnetized neutron star and its accretion disk. General relativistic effects are simulated by using the pseudo-Newtonian potential. We find that well-collimated jets traveling along the rotation axis of the disk are formed by the following mechanism: (1) The magnetic loops connecting the neutron star and the disk are twisted due to the differential rotation between the neutron star and the disk. (2) Twist injection from the disk initiates expansion of the loop. (3) The expanding magnetic loops create a magnetic tower in which accelerated disk material travel as collimated bipolar jets. The propagation speed of the working surface of the jet is the order of 10% of the speed of light ($\sim 0.1c$). (4) Magnetic reconnection taking place inside the expanding magnetic loops injects hot bubbles intermittently into the magnetic tower. The ejection speed of the bubble is the order of the local Alfv\'{e}n speed of the launching point and $\sim 0.2c$ in our simulations. (5) The hot bubbles moving inside the tower catch up with the working surface of the jet. High energy electrons created by the magnetic reconnection are a plausible source of radio emission. Our model can explain the formation process of a narrow jet from a weakly magnetized ($|{\boldmath$B_{*}$}|\le 10^{9}$ gauss) neutron star and the correlation between radio flares of the core and of the lobe observed in Sco X-1.