3 Magnetars: A Short Review and Some Sparse Considerations
117
Counter-arguments have been put forward also in this scenario by Spruit [198],
who argued that the number of highly magnetic massive stars with B 1 kG is
not sufficient to explain the magnetar population. Furthermore, even assuming that
most of the magnetic flux is indeed conserved, magnetic fields higher than 10 14 G
seem unattainable.
Recent surveys of very massive stars have shown how in our Galaxy massive
stars tend to be in binaries [191]. Furthermore, a detailed radial velocity survey
of Westerlund 1, an open cluster of very massive stars which contains a magnetar,
CXOU J164710.2–455216, have discovered the possible companion massive star
that might have resulted by the disruption of a massive binary progenitor (Clark et
al. [33]; see Sect. 3.2.6 for more details). All these results are pointing to a further
element in magnetar formation: the evolution in a binary system of massive stars.
The binary scenario might overcome the problem of the spin down by the core–
envelope coupling. In particular, both mass transfer and stellar merger in compact
binaries may lead to substantial spin-up of the mass-gainer (or of the remnant of
the merger), favoring the amplification of the magnetic field via dynamo effects
[136]. Recent simulations have shown that gamma-ray bursts and hyper-luminous
supernovae can indeed be powered by recently formed millisecond magnetar [154],
although no direct or sound observational evidence of the existence of such fast
spinning and strongly magnetic neutron stars has been collected thus far.
3.2.4 Magnetic Field Evolution and the Neutron Star Bestiary
The evolution of magnetic fields in neutron stars has been extensively studied by
a number of authors in the past. In the neutron star solid crust, the field evolves
under the influence of the Lorentz force (causing the Hall drift) and the Joule
effect (responsible for Ohmic dissipation). The evolution in the liquid core is
very uncertain. In the core, soon after the neutron star birth (from hours to days)
protons undergo a transition to a type-II superconducting phase [14], in which
the magnetic field is confined to tiny flux tubes surrounded by nonmagnetized
matter. The dynamics of those flux tubes, likely coupled to the motion of superfluid
neutron vortices, is a complex problem that makes the magnetic field evolution
in the core formally difficult to tackle (see Elfritz et al. [59]). Most works (e.g.
[77, 78, 83, 84, 177, 178, 226]) considered mainly the magnetic evolution in the solid
crust, a ∼1-km-thick lattice of ions, where the electrical conduction is governed by
electrons.
The magnetic field evolution in a neutron star is strictly coupled to its thermal
evolution. In fact, the magnetic field influences the heating rate and, secondarily,
affects the rate of a few neutrino processes; on the other side, the conduction of heat
becomes anisotropic in the presence of a strong magnetic field. The simultaneous
study of the magnetic and temperature evolutions (magneto-thermal evolution)
was started by Pons and Geppert [175] and Aguilera et al. [1] with simplifying
assumptions, and later implemented in two-dimensional simulations of the fully
117
Counter-arguments have been put forward also in this scenario by Spruit [198],
who argued that the number of highly magnetic massive stars with B 1 kG is
not sufficient to explain the magnetar population. Furthermore, even assuming that
most of the magnetic flux is indeed conserved, magnetic fields higher than 10 14 G
seem unattainable.
Recent surveys of very massive stars have shown how in our Galaxy massive
stars tend to be in binaries [191]. Furthermore, a detailed radial velocity survey
of Westerlund 1, an open cluster of very massive stars which contains a magnetar,
CXOU J164710.2–455216, have discovered the possible companion massive star
that might have resulted by the disruption of a massive binary progenitor (Clark et
al. [33]; see Sect. 3.2.6 for more details). All these results are pointing to a further
element in magnetar formation: the evolution in a binary system of massive stars.
The binary scenario might overcome the problem of the spin down by the core–
envelope coupling. In particular, both mass transfer and stellar merger in compact
binaries may lead to substantial spin-up of the mass-gainer (or of the remnant of
the merger), favoring the amplification of the magnetic field via dynamo effects
[136]. Recent simulations have shown that gamma-ray bursts and hyper-luminous
supernovae can indeed be powered by recently formed millisecond magnetar [154],
although no direct or sound observational evidence of the existence of such fast
spinning and strongly magnetic neutron stars has been collected thus far.
3.2.4 Magnetic Field Evolution and the Neutron Star Bestiary
The evolution of magnetic fields in neutron stars has been extensively studied by
a number of authors in the past. In the neutron star solid crust, the field evolves
under the influence of the Lorentz force (causing the Hall drift) and the Joule
effect (responsible for Ohmic dissipation). The evolution in the liquid core is
very uncertain. In the core, soon after the neutron star birth (from hours to days)
protons undergo a transition to a type-II superconducting phase [14], in which
the magnetic field is confined to tiny flux tubes surrounded by nonmagnetized
matter. The dynamics of those flux tubes, likely coupled to the motion of superfluid
neutron vortices, is a complex problem that makes the magnetic field evolution
in the core formally difficult to tackle (see Elfritz et al. [59]). Most works (e.g.
[77, 78, 83, 84, 177, 178, 226]) considered mainly the magnetic evolution in the solid
crust, a ∼1-km-thick lattice of ions, where the electrical conduction is governed by
electrons.
The magnetic field evolution in a neutron star is strictly coupled to its thermal
evolution. In fact, the magnetic field influences the heating rate and, secondarily,
affects the rate of a few neutrino processes; on the other side, the conduction of heat
becomes anisotropic in the presence of a strong magnetic field. The simultaneous
study of the magnetic and temperature evolutions (magneto-thermal evolution)
was started by Pons and Geppert [175] and Aguilera et al. [1] with simplifying
assumptions, and later implemented in two-dimensional simulations of the fully