2009MNRAS.396..878H

The origin of ultra-intense magnetic fields on magnetars is a mystery in modern astrophysics. We model the core collapse dynamics of massive progenitor stars with high surface magnetic fields in the theoretical framework of a self-similar general polytropic magnetofluid under self-gravity with a quasi-spherical symmetry. With the specification of physical parameters such as mass density, temperature, magnetic field and wind mass-loss rate on the progenitor stellar surface and the consideration of a rebound shock breaking through the stellar interior and envelope, we find a remnant compact object (i.e. neutron star) left behind at the centre with a radius of ∼10⁶ cm and a mass range of ∼1-3M_☉. Moreover, we find that surface magnetic fields of such a type of compact objects can be ∼10¹⁴-10¹⁵ G, consistent with those inferred for magnetars which include soft gamma-ray repeaters and anomalous X-ray pulsars. The magnetic field enhancement factor critically depends on the self-similar scaling index n, which also determines the initial density distribution of the massive progenitor. We propose magnetized massive stars as magnetar progenitors based on the magnetohydrodynamic evolution of the gravitational core collapse and rebound shock. Our physical mechanism, which does not necessarily require ad hoc dynamo amplification within a fast spinning neutron star, favours the `fossil-field' scenario of forming magnetars from the strongly magnetized core collapse inside massive progenitor stars. The resulting magnetic field strength on the surface of the remnant proto-neutron star is proportional to the surface magnetic field strength of the progenitor and to the neutron star mass itself, while it anti-correlates with the progenitor stellar mass. With a range of surface magnetic field strengths over massive progenitor stars, our scenario allows a continuum of magnetic field strengths from pulsars to magnetars. The intense Lorentz force inside a magnetar may break the crust of a neutron star into pieces to various extents. Coupled with the magnetar spin, the magnetospheric configuration of a magnetar is most likely variable in the presence of exposed convection, differential rotation, equatorial bulge, bursts of interior magnetic flux ropes as well as rearrangement of broken pieces of the crust. Sporadic and violent releases of accumulated magnetic energies and a broken crust are the underlying causes for various observed high-energy activities of magnetars.