Unifying the observational diversity of isolated neutron stars via magneto-thermal evolution models

Abstract

Observations of magnetars and some of the high magnetic field pulsars have shown that their thermal luminosity is systematically higher than that of classical radiopulsars, thus confirming the idea that magnetic fields are involved in their X-ray emission. Here we present the results of 2D simulations of the fully-coupled evolution of temperature and magnetic field in neutron stars, including the state-of-the-art kinetic coefficients and, for the first time, the important effect of the Hall term. After gathering and thoroughly re-analysing in a consistent way all the best available data on isolated, thermally emitting neutron stars, we compare our theoretical models to a data sample of 40 sources. We find that our evolutionary models can explain the phenomenological diversity of magnetars, high-B radio-pulsars, and isolated nearby neutron stars by only varying their initial magnetic field, mass and envelope composition. Nearly all sources appear to follow the expectations of the standard theoretical models. Finally, we discuss the expected outburst rates and the evolutionary links between different classes. Our results constitute a major step towards the grand unification of the isolated neutron star zoo.with the very active magnetars, showing intermediate luminosities, and sporadic magnetar-like activity. The radioquiet X-ray Isolated NSs (XINSs) are a class of relatively old, nearby cooling NSs, with the cleanest detected thermal emission and a relatively large magnetic field. Last, Central Compact Objects (CCOs, Gotthelf et al. 2013) represent a handful of puzzling, radio-quiet sources which in some cases combine a very weak external magnetic field with a relatively large luminosity and evidence for anisotropic surface temperature distribution.Although isolated NSs have been divided into all these observational classes for historical reasons, a unifying vision considers them as different manifestations of the same underlying physics (Kaspi 2010). In this context, one of the main theoretical tasks is to explain the varied phenomenology of their X-ray emission. X-ray spectra carry precious information about the surface temperature and the physics driving the cooling of the NSs. The detected X-ray flux, if accompanied by a reliable distance measurement, leads to an estimate of the bolometric luminosity, which can be confronted with cooling models to infer properties of dense matter in the NS interior. In addition, timing properties (period

Magnetic field evolution in superconducting neutron stars

“…Old millisecond pulsars thought to be formed in low-mass X-ray binaries have fields between 10 8 − 10 10 G, while the fields of 'classical' rotation-powered pulsars range between 10 10 − 10 13 G. A third class of slowrotating, highly magnetised neutron stars, so-called magnetars, reaches field strengths up to 10 15 G. This class is believed to include both, soft gamma repeaters and anomalous X-ray pulsars. Understanding the long-term evolution of the stars' magnetic fields might be key to establishing connections between the different classes and forming a unified picture of the neutron star 'zoo' (Kaspi 2010;Viganò et al 2013;Harding 2013).…”

“…The actual mechanisms, causing the magnetic field to change on these rather short timescales, are only poorly understood and there is no definitive answer to the question of which part of the neutron star dominates the magnetic field evolution. Most theoretical studies and numerical simulations focus on the crust as the source of the field decay and neglect the core contribution (Pons & Geppert 2007;Viganò et al 2013;Gourgouliatos & Cumming 2014). However, one could argue that the core, which carries the majority of the star's inertia and magnetic energy, should also play a role in the magnetic field evolution.…”

Modelling of the surface emission of the low magnetic field magnetar SGR 0418+5729

“…In particular, Turolla et al (2011), using the magnetothermal code by Pons et al (2009), found that the quiescent luminosity of the source and its timing properties are compatible with those of an old NS born with a super-strong magnetic field which underwent significant decay over a time ≈ 10 6 yr. More recently, following a refined timing solution , the realistic age 2 of SGR 0418 was estimated to be ∼ 550 kyr with the state-of-the-art magnetothermal evolution model of Viganò et al (2013). The initial strength of the dipolar component was estimated to be in the range B 0 dip ∼ 1 − 3 × 10 14 G (Turolla et al 2011;; a larger value would have spun down the pulsar too much during its estimated lifetime).…”

“…During the last few years, a number of investigations have been aimed at understanding the physical reasons for the diverse phenomenology of magnetars (Pons et al 2007(Pons et al , 2009; Aguilera et al 2008a,b;Kaspi 2010;Viganò et al 2013). In particular, a suite of magnetothermal simulations highlighted the importance of a hidden toroidal field in determining the observational manifestations of a NS Viganò et al 2013).…”

“…In particular, a suite of magnetothermal simulations highlighted the importance of a hidden toroidal field in determining the observational manifestations of a NS Viganò et al 2013). A NS with a low dipolar component of the magnetic field (as inferred from timing measurements) could still display an outbursting behaviour and an enhanced quiescent X-ray luminosity if endowed with a much stronger internal toroidal field (Turolla et al 2011;Rea et al 2012).…”