Propagation of Magnetized Neutron Stars through the Interstellar Medium

This work investigates the propagation of magnetized, isolated old neutronstars through the interstellar medium. We performed axisymmetric nonrelativistic magnetohydrodynamic (MHD) simulations of the propagation of anon-rotating star with dipole magnetic field aligned with its velocity throughthe interstellar medium (ISM). Effects of rotation will be discussed in asubsequent work. We consider two cases: (1) where theaccretion radius $R_{acc}$is comparable to the magnetic standoff distance or Alfv\'en radius $R_A$ andgravitational focusing is important; and (2) where $R_{acc}<< R_A$ and themagnetized star interacts with the ISM as a ``magnetic plow'', withoutsignificant gravitational focusing. For the first case simulations were done ata low Mach number ${\cal M}=3$ for a range of values of the magnetic field$B_*$. For the second case, simulations were done for higher Mach numbers,${\cal M}=10,$ 30, and 50. In both cases, the magnetosphere of the starrepresents an obstacle for the flow, and a shock wavestands in front of thestar. Magnetic field lines are stretched downwind from the star and form ahollow elongated magnetotail. Reconnection of the magnetic field is observed inthe tail which may lead to acceleration of particles. Similar powers areestimated to be released in the bow shock wave and in the magnetotail. Theestimated powers are, however, below present detection limits. Results of oursimulations maybe applied to other strongly magnetized stars, for example,white dwarfs and magnetic Ap stars. Future more sensitive observations mayreveal long magnetotails of magnetized stars moving through the ISM.Comment: 17 pages, 23 figures(2,3,4,5,6,8,9,15,16,17 color), submitted to Ap