Incompatibility of long-period neutron star precession with creeping neutron vortices

Abstract

Aims. To determine whether "vortex creep" in neutron stars, the slow motion of neutron vortices with respect to pinning sites in the core or inner crust, is consistent with observations of long-period precession. Methods. Using the concept of vortex drag, I discuss the precession dynamics of a star with imperfectly-pinned (i.e., "creeping") vortices.Results. The precession frequency is far too high to be consistent with observations, indicating that the standard picture of the outer core (superfluid neutrons in co-existence with type II, superconducting protons) should be reconsidered. There is a slow precession mode, but it is highly over-damped and cannot complete even a single cycle. Moreover, the vortices of the inner crust must be able to move with little dissipation with respect to the solid.

Influence of the small-scale magnetic field on the evolution of the angle between the magnetic moment and rotation axis of radio pulsars with superfluid cores

“…However, we assumed that either the core protons are in a superconducting state of the first kind (the question of the type of superconductivity occurring is still under discussion; see, e.g., [22]) or the Abrikosov vortices interact only weakly with the vortex lines of the neutron fluid. The strong coupling of the proton and neutron vortices would result in a very fast precession of the neutron star, in contradiction with observations (see, e.g., [23]). …”

Spectral and temporal variations of the isolated neutron star RX J0720.4-3125: new XMM-Newton observations

“…However, the presence of such a small emitting region produces a pulse shape which is not in agreement with the observed one. It is still under debate whether free precession can continiue for more than a few hundred cycles (Link 2006). If this is the case and RX J0720.4−3125 is precessing, then the precession needs to be powered by some mechanism.…”

Pulsar state switching, timing noise and free precession

“…However, it has been argued that there will be an even stronger interaction between the crust and fluid core if the interior protons are superconducting: then the magnetic flux tubes that make up the superconducting field will interact strongly with the neutron vortices. Link has argued that this interaction is so strong that there will be no long‐lived long‐period free precession, forcing one to conclude that either the pulsar observations do not correspond to precession, or else the neutron vortices do not coexist with the magnetic flux tubes (Link 2003, 2006). In the latter case, one might conclude that the interior neutrons are normal, returning us to the I C / I ★ ∼ 1 case described above.…”

“…In the case of superconductivity, these results are modified by the replacement of one factor of B with H c , the ‘critical field’, believed to approximately equal to 10 15 G, independent of the star’s individual field strength B (Baym et al 1969). The above results are then amended to This leads to or, in terms of spin‐down age, [As mentioned previously, given that we are attempting to model long‐period free precession, the existence of superconductivity is problematic, as a strong interaction between the magnetic flux tubes (not included in our model) that are believed to make up the magnetic field in a superconducting star with a coexisting neutron superfluid vortex array would tend to increase the precession frequency to very high values, or else damp the precession very rapidly (Jones & Andersson 2001; Link 2003, 2006). A possible resolution would be for the neutrons to be normal.…”