126
P. Esposito et al.
surface, but is dominated (∼90%) by multipolar components, which vanish rapidly
with the distance, so that at the magnetospheric radius (≈100 neutron star radii) the
magnetic field is virtually the dipolar one, low enough for the accretion to proceed.
It is interesting to note that this hypothesis does not require for the neutron star only
a superstrong magnetic field (a purely dipolar magnetic field would not work), but a
complex magnetic field configuration similar to that envisaged for magnetars.
Finally, there is a peculiar binary source in which the presence of an ultramagnetized neutron star has been suggested not as a wildcard to explain its
properties, but because it actually showed one of the specific characteristics of
magnetars: LS I +61 ◦ 303. It is one of the few gamma-ray (TeV) binaries [54] and is
in a orbit of 27 days with a 10–15-M Be star. LS I +61 ◦ 303 is also a periodic
(27 d) and variable radio source (it is often referred to as a microquasar) and,
since no direct evidence for the presence of a neutron star has been obtained so
far (for example, pulsations, mass limits or thermonuclear bursts), the nature of
its compact object is still debated. In 2008, Swift triggered on a short, SGR-like
burst from LS I +61 ◦ 303. With a duration of ∼0.2–0.3 s, a blackbody spectrum with
temperature of 7.5 keV, and luminosity of ∼2 × 10 37 erg s −1 , the event had the
characteristics of a magnetar burst [216]. A second magnetar-like burst was detected
again by Swift in 2012 [23]. Torres et al. [216] discussed the implication of the
presence of a magnetar in LS I +61 ◦ 303 and showed that it would be compatible
with the properties of the source.
3.3 Final Remarks
Until about a decade or little more ago, magnetars were regarded as a sort of
astrophysical oddities and only comparatively few small groups of astronomers were
interested in them. In recent times however, a number of surprising observational
discoveries connected more strictly magnetars to the other classes of neutron stars
and made them hard to be ignored by the large community studying pulsars
at different wavelengths. In fact, it has been discovered that magnetars can be
radio pulsars and also that ‘ordinary’ X-ray and radio pulsars, as well as other
sources, such as the peculiar neutron star in the supernova remnant RCW 103,
can behave like magnetars, showing the whole array of magnetar activity: bursts,
outbursts, dramatic and abrupt pulse profile changes and other timing anomalies.
We have also learnt that magnetars can populate unexpected regions of the P – ˙
P
diagram, with dipole magnetic fields measured from the rotation parameters that
are comparable to or lower than those of the radio pulsars, disguising at the same
time much stronger nondipolar magnetic field components. On the other hand, there
are mounting evidences that magnetar (nondipolar) magnetic fields may be present
also in other classes of isolated and binary neutron stars; in the future, they might
manifest magnetar behaviour. Summarising, episodes of magnetar activity and their
frequency seem to be related to the total magnetic energy stored in the internal field
of a neutron star but, while a huge tank of this energy is typically associated to the
P. Esposito et al.
surface, but is dominated (∼90%) by multipolar components, which vanish rapidly
with the distance, so that at the magnetospheric radius (≈100 neutron star radii) the
magnetic field is virtually the dipolar one, low enough for the accretion to proceed.
It is interesting to note that this hypothesis does not require for the neutron star only
a superstrong magnetic field (a purely dipolar magnetic field would not work), but a
complex magnetic field configuration similar to that envisaged for magnetars.
Finally, there is a peculiar binary source in which the presence of an ultramagnetized neutron star has been suggested not as a wildcard to explain its
properties, but because it actually showed one of the specific characteristics of
magnetars: LS I +61 ◦ 303. It is one of the few gamma-ray (TeV) binaries [54] and is
in a orbit of 27 days with a 10–15-M Be star. LS I +61 ◦ 303 is also a periodic
(27 d) and variable radio source (it is often referred to as a microquasar) and,
since no direct evidence for the presence of a neutron star has been obtained so
far (for example, pulsations, mass limits or thermonuclear bursts), the nature of
its compact object is still debated. In 2008, Swift triggered on a short, SGR-like
burst from LS I +61 ◦ 303. With a duration of ∼0.2–0.3 s, a blackbody spectrum with
temperature of 7.5 keV, and luminosity of ∼2 × 10 37 erg s −1 , the event had the
characteristics of a magnetar burst [216]. A second magnetar-like burst was detected
again by Swift in 2012 [23]. Torres et al. [216] discussed the implication of the
presence of a magnetar in LS I +61 ◦ 303 and showed that it would be compatible
with the properties of the source.
3.3 Final Remarks
Until about a decade or little more ago, magnetars were regarded as a sort of
astrophysical oddities and only comparatively few small groups of astronomers were
interested in them. In recent times however, a number of surprising observational
discoveries connected more strictly magnetars to the other classes of neutron stars
and made them hard to be ignored by the large community studying pulsars
at different wavelengths. In fact, it has been discovered that magnetars can be
radio pulsars and also that ‘ordinary’ X-ray and radio pulsars, as well as other
sources, such as the peculiar neutron star in the supernova remnant RCW 103,
can behave like magnetars, showing the whole array of magnetar activity: bursts,
outbursts, dramatic and abrupt pulse profile changes and other timing anomalies.
We have also learnt that magnetars can populate unexpected regions of the P – ˙
P
diagram, with dipole magnetic fields measured from the rotation parameters that
are comparable to or lower than those of the radio pulsars, disguising at the same
time much stronger nondipolar magnetic field components. On the other hand, there
are mounting evidences that magnetar (nondipolar) magnetic fields may be present
also in other classes of isolated and binary neutron stars; in the future, they might
manifest magnetar behaviour. Summarising, episodes of magnetar activity and their
frequency seem to be related to the total magnetic energy stored in the internal field
of a neutron star but, while a huge tank of this energy is typically associated to the