116
P. Esposito et al.
of current-carrying field lines decrease, as the magnetospheric twist gradually
dissipates and the rate of particles impacting the surface consequently declines
[35, 36].
3.2.3 Magnetar Formation
The generation of magnetar-like magnetic fields from the progenitor star is still
a debated and relatively open problem. All along, preliminary calculations have
shown that the effects of a turbulent dynamo amplification occurring in a newly
born neutron stars can indeed result in a magnetic field of up to a few 10 17 G. This
dynamo effect is expected to operate only in the first ∼10 s after the supernova
explosion of the massive progenitor, and if the proto-neutron star is born with
sufficiently small rotational periods (of the order of 1–2 ms). The resulting amplified
magnetic fields are expected to have a strong multipolar structure and toroidal
component (Duncan and Thompson [56], Thompson and Duncan [207]). This
formation scenario predicts two main observational consequences: (a) magnetars
should have large kick velocities, of the order of 10 3 km s −1 and (b) their associated
supernovae should be more energetic than ordinary core collapse-supernovae,
because of the additional rotational energy loss of such fast spinning proto-neutron
star.
However, this additional energy loss is not observed in the supernova remnants
surrounding magnetars [148, 227], nor a large kick velocity is observed in the
few cases where this could be measured. In particular, measured magnetar proper
motions are v = 212 ± 35 km s −1 for XTE J1810–197 [99], v = 280 ± 130 km s −1
for 1E 1547–5408 [46], v = 157 ± 17 km s −1 for 1E 2259+586 and v = 102 ±
26 km s −1 for 4U 0142+61 [203], while candidate proper motion velocities are
v = 350±100 km s −1 for SGR 1806–20 and v = 130±30 km s −1 for SGR 1900+14
[202]. All these values are well within the typical radio pulsar distribution (see also
the gray box on the Galactic Centre magnetar SGR 1745–2900).
The fact that the former predictions did not seem to be fulfilled is however
not sufficient to dismiss the dynamo formation mechanism (for example, about
the lack of evidence for a particularly energetic supernova, Dall’Osso et al. [39]
noted that most of the rotational energy of a proto-neutron star with internal toroidal
field ≈10 16 G should be released through gravitational waves, without supplying
substantial additional energy to the ejecta), but it has lent some support to other
formation scenarios. One alternative theory is based on magnetic flux conservation
arguments and postulates that the distribution of field strengths in neutron stars
simply reflects that of their progenitors. In this fossil field scenario, magnetars
would be the descendant of the massive stars with the highest magnetic fields [69].
P. Esposito et al.
of current-carrying field lines decrease, as the magnetospheric twist gradually
dissipates and the rate of particles impacting the surface consequently declines
[35, 36].
3.2.3 Magnetar Formation
The generation of magnetar-like magnetic fields from the progenitor star is still
a debated and relatively open problem. All along, preliminary calculations have
shown that the effects of a turbulent dynamo amplification occurring in a newly
born neutron stars can indeed result in a magnetic field of up to a few 10 17 G. This
dynamo effect is expected to operate only in the first ∼10 s after the supernova
explosion of the massive progenitor, and if the proto-neutron star is born with
sufficiently small rotational periods (of the order of 1–2 ms). The resulting amplified
magnetic fields are expected to have a strong multipolar structure and toroidal
component (Duncan and Thompson [56], Thompson and Duncan [207]). This
formation scenario predicts two main observational consequences: (a) magnetars
should have large kick velocities, of the order of 10 3 km s −1 and (b) their associated
supernovae should be more energetic than ordinary core collapse-supernovae,
because of the additional rotational energy loss of such fast spinning proto-neutron
star.
However, this additional energy loss is not observed in the supernova remnants
surrounding magnetars [148, 227], nor a large kick velocity is observed in the
few cases where this could be measured. In particular, measured magnetar proper
motions are v = 212 ± 35 km s −1 for XTE J1810–197 [99], v = 280 ± 130 km s −1
for 1E 1547–5408 [46], v = 157 ± 17 km s −1 for 1E 2259+586 and v = 102 ±
26 km s −1 for 4U 0142+61 [203], while candidate proper motion velocities are
v = 350±100 km s −1 for SGR 1806–20 and v = 130±30 km s −1 for SGR 1900+14
[202]. All these values are well within the typical radio pulsar distribution (see also
the gray box on the Galactic Centre magnetar SGR 1745–2900).
The fact that the former predictions did not seem to be fulfilled is however
not sufficient to dismiss the dynamo formation mechanism (for example, about
the lack of evidence for a particularly energetic supernova, Dall’Osso et al. [39]
noted that most of the rotational energy of a proto-neutron star with internal toroidal
field ≈10 16 G should be released through gravitational waves, without supplying
substantial additional energy to the ejecta), but it has lent some support to other
formation scenarios. One alternative theory is based on magnetic flux conservation
arguments and postulates that the distribution of field strengths in neutron stars
simply reflects that of their progenitors. In this fossil field scenario, magnetars
would be the descendant of the massive stars with the highest magnetic fields [69].