Spin-down of a pulsar with a yielding crust

Abstract

In light of the discovery of the long-period radio pulsar PSR J0901-4046, it is interesting to revisit a question about how the magnetized neutron star slows down its rotation. In the case of a weak or liquid outer crust, the mechanism of spin-down becomes unclear because the braking stress cannot then be transmitted from the surface to the main bulk of the star. We show that even if the outer crust does not withstand the surface electromagnetic forces creating the braking torque, the stellar spin-down does not stop, and the matter rearranges so that the necessary electromagnetic forces form in more deep and rigid layers capable of withstanding these forces. The spin-down rate remains the same and corresponds to the transformation of the rotational energy of the neutron star into the energy of the generated relativistic electron-positron plasma. The solid iron surface of ultrastrongly magnetized PSR J0901-4046 appears not to be yielding and withstands the braking stress without breaking.