The many lives of magnetized neutron stars

Abstract

The magnetic field strength at birth is arguably one of the most important properties to determine the evolutionary path of a neutron star. Objects with very high fields, collectively known as magnetars, are characterized by high X-ray quiescent luminosities, occurrence of outbursts, and, for some of them, sporadic giant flares. While the magnetic field strength is believed to drive their collective behaviour, however, the diversity of their properties, and, especially, the observation of magnetar-like bursts from "low-field" pulsars, has been a theoretical puzzle. In this review, we discuss results of long-term simulations following the coupled evolution of the X-ray luminosity and the timing properties for a large, homogeneous sample of X-ray emitting isolated neutron stars, accounting for a range of initial magnetic field strengths, envelope compositions, and neutron star masses. In addition, by following the evolution of magnetic stresses within the neutron star crust, we can also relate the observed magnetar phenomenology to the physical properties of neutron stars, and in particular to their age and magnetic field strength and topology. The dichotomy of "high-B" field pulsars versus magnetars is naturally explained, and occasional outbursts from old, low B-field neutron stars are predicted. We conclude by speculating on the fate of old magnetars, and by presenting observational diagnostics of the neutron star crustal field topology.1 Note that it is important that the toroidal component is dipolar and dominant over the poloidal one. If the toroidal component were, e.g., quadrupolar, much smaller effects would be seen in the resulting temperature distribution and hence the pulsed profiles.

Evolution of neutron star magnetic fields

“…Most often magnetic field does not decay rapidly enough in the crust, and diffuses back to the surface on timescales of 10 4 -10 6 years [36][37][38][39]. It occurs due to a combination of three physical effects [39]: (1) condensed poloidal field forms strong toroidal field, (2) interaction between poloidal and toroidal fields creates additional forces which push poloidal field towards the surface (re-emergence on Hall timescale) and (3) poloidal fields generally diffuse in the crust and thus re-emerge toward the surface (re-emergence on Ohmic timescale). In the case of a supernova with strong fall-back, even magnetar-strength fields could be buried, leading to "hidden magnetar" cases [40].…”

“…Magnetic fields play a significant role in many areas of modern astrophysics, manifesting themselves in different energetic phenomena ranging from solar flares to gamma-ray and fast radio bursts. The strongest magnetic fields are found in neutron stars (NSs), where they control whether NSs are seen as normal radio pulsars, magnetars, dim isolated cooling NSs, or something else [1,2]. Magnetic fields and neutron stars are therefore intertwined, and understanding the formation and evolution of NS magnetic fields will significantly advance our understanding of the NS population, and vice versa, systematic study of the NS population will help to understand the formation and evolution of their magnetic fields.…”

Simulated magnetic field expulsion in neutron star cores

“…Figure 2 (right) shows the magnetic field initial conditions for a large-scale dipolar magnetic configuration with a strong dipolar component confined to the NS crust. The toroidal field is confined to the last closed field line region saddling the crust-core interface, with magnitude 10 15 G. The initial poloidal field strength at the pole is 10 14 G. The crustconfined dipole adds a significant amount of magnetic energy in the crust; such configurations have been extensively used to unify NS observations (cite), and are widely considered to be candidate topologies for high-field sources (Perna et al 2014).…”

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

“…The problem of identification (the detectability of millisecond periods for complicated field topology in the crust, pulse profiles, etc.) can also prevent easy discovery of 'frozen magnetars' (but see a recent discussion in Perna et al 2013). On the other hand, an additional energy source-field decay -can help such sources to stay bright for a longer time, which favours their discovery (Heyl & Kulkarni 1998).…”