Magnetic Field Evolution During Neutron Star Recycling

I describe work on two aspects of magnetic field evolution relevant for the “recycling” scenario for making millisecond radio pulsars. First, many of the theoretical ideas for bringing about accretion-induced field decay rely on dissipation of currents in the neutron star crust. I discuss field evolution in the crust due to the Hall effect, and outline when it dominates Ohmic decay. This emphasises the importance of understanding the impurity level in the crust. Second, I briefly discuss the progress that has been made in understanding the magnetic fields of neutron stars currently accreting matter in low mass X-ray binaries. In particular, thermonuclear X-ray bursts offer a promising probe of the magnetic field of these neutron stars. 1. The Crust as a Site for Dissipation of Currents While spin up is naturally expected from accretion of angular momentum, magnetic field decay in an accreting neutron star is much less well-understood (see Bhattacharya & Srinivasan 1995 for a review). The timescales for evolution of currents in the core of a neutron star is extremely long (Baym, Pethick, & Pines 1969; Goldreich & Reisenegger 1992, hereafter GR92). However, short decay times (∼< 107 yr) are possible in the neutron star crust (as emphasised very early on, e.g., Ewart et al. 1975), and this underlies one class of models for accretion-induced field decay. One idea is to place the currents supporting the stellar magnetic field entirely in the crust. This might be the case if the field is generated from thermomagnetic effects, for example (Blandford, Applegate, & Hernquist 1983). The evolution due to Ohmic decay is then straightforward to calculate, and has been followed in many papers (Urpin, Geppert, & Konenkov 1997, Konar & Bhattacharya 1997, and references therein). The role of accretion is to heat the crust, reducing the electrical conductivity. The amount by which the field decays depends on the accretion lifetime, depth of the currents, and how much current is advected and subsequently “frozen” into the superconducting core. Thermomagnetic processes may also destroy field in the reheated crust (Blondin & Freese 1986). A second proposal is that as a radio pulsar spins down, outward moving vortices in the superfluid and superconducting core push magnetic fluxoids into the crust, where magnetic energy dissipates (Srinivasan et al. 1990; Konar & Bhattacharya 1999). This model predicts changes in alignment of the magnetic field and spin