Pulsar timing irregularities and the imprint of magnetic field evolution

Abstract

Context. The rotational evolution of isolated neutron stars is dominated by the magnetic field anchored to the solid crust of the star. Assuming that the core field evolves on much longer timescales, the crustal field evolves mainly though Ohmic dissipation and the Hall drift, and it may be subject to relatively rapid changes with remarkable effects on the observed timing properties. Aims. We investigate whether changes of the magnetic field structure and strength during the star evolution may have observable consequences in the braking index n. This is the most sensitive quantity to reflect small variations of the timing properties that are caused by magnetic field rearrangements. Methods. We performed axisymmetric, long-term simulations of the magneto-thermal evolution of neutron stars with state-of-the-art microphysical inputs to calculate the evolution of the braking index. Relatively rapid magnetic field modifications can be expected only in the crust of neutron stars, where we focus our study. Results. We find that the effect of the magnetic field evolution on the braking index can be divided into three qualitatively different stages depending on the age and the internal temperature: a first stage that may be different for standard pulsars (with n ∼ 3) or low field neutron stars that accreted fallback matter during the supernova explosion (systematically n < 3); in a second stage, the evolution is governed by almost pure Ohmic field decay, and a braking index n > 3 is expected; in the third stage, at late times, when the interior temperature has dropped to very low values, Hall oscillatory modes in the neutron star crust result in braking indices of a high absolute value and both positive and negative signs. Conclusions. Current magneto-thermal evolution models predict a large contribution to the timing noise and, in particular, to the braking index, from temporal variations of the magnetic field. Models with strong ( 10 14 G) multipolar or toroidal components, even with a weak (∼10 12 G) dipolar field are consistent with the observed trend of the timing properties.

On the diversity of compact objects within supernova remnants – II. Energy-loss mechanisms

“…Thus, a magnetar-scale field of 10 15 G can evolve on the order of 1 − 100 kyr. Therefore the Hall effect provides the dominant mechanism for the field evolution of highly magnetized, young NSs on timescales comparable with the observed ages of SNRs (Geppert & Rheinhardt 2002;Pons, Vigano & Geppert 2012).…”

“…Pons, Vigano & Geppert (2012) has analyzed the properties of a sample of 118 NSs, most of which are recycled millisecond PSRs with measured braking index and relatively low magnetic fields. Their Figure 2 shows the evolutionary properties of this sample of low-field NSs.…”

“…However, this is a simplification due to the dependence of σ0 on electron density and temperature. Changes in these quantities can cause variations in σ0 over orders of magnitude (Pons, Vigano & Geppert 2012), and for low-field NSs with ages ∼ 10 to 100 kyr, Ohmic dissipation is expected to dominate the magnetic field evolution. In contrast, for high field NSs, the timescale for Hall drift in terms of the electron number density ne and charge e is kyr with B14 the magnetic field in units of 10 14 G, and electron densities between 10 35 and 10 37 cm −3 .…”

Fallback Disks, Magnetars and Other Neutron Stars

“…If seen as isolated pulsars, SGR 0418+5729 must have experienced rapid decay of the dipole field, while PSR J1734-3333 must have a rapidly growing dipole field at present. If further the total magnetic field is in the magnetar range, such ongoing growth of the dipole field is difficult to understand, unless, perhaps, the dipole field was buried and is now reemerging (Pons et al 2012). …”

Origin of Magnetar-Scale Crustal Field in PSR J1852+0040 and ‘Frozen’ Magnetars

“…In the modern picture of unified NS population (see, for example, Pons, Viganò, & Geppert 2012 for description of different possibilities and evolutionary histories, especially their Figure 2 for evolutionary tracks), such sources are quite natural. However, they must be relatively rare.…”