3 Magnetars: A Short Review and Some Sparse Considerations
99
opposite possibility, too, was considered: that AXPs might be SGR progenitors in
which the magnetic field decay has just begun (e.g. Gavriil et al. [73]).
The magnetar hypothesis was arguably the most successful in explaining the
salient features of SGRs and AXPs, but some skepticism about the scenario persisted
for long, in particular for the AXPs, for which several alternative models could not
be excluded [31, 151]. The magnetar model really started to become the mainstream
when the period derivative of an SGR was measured for the first time [130]. Using
RossiXTE, it was established that SGR 1806–20 was pulsating at P = 7.47 s and
the period was increasing at the rate ˙
P = 2.6 × 10 −3 s yr −1 : with the standard
assumption of a magnetic dipole rotating in vacuum routinely used for standard
pulsar, the values correspond to a surface magnetic field of B 3.2 × 10 19
√
P ˙
P =
8 × 10 14 G at the neutron star’s equator. Moreover, the release of magnetic energy
was necessary to power the X-ray emission of SGR 1806–20, since the spin-down
energy loss of the neutron star was two orders of magnitude lower than the observed
X-ray luminosity.
Few years later, the detection of SGR-like bursts from the AXP 1E 1048-1–5937
[73] confirmed the suspect that also AXPs could harbour magnetars. Since then,
with the discovery of many new magnetars showing a variegated phenomenology
and observations of strong bursting activity and powerful flares from AXPs, as well
as prolonged periods of quiescence in once-active SGRs, the AXP–SGR dichotomy
seems completely obsolete [125, 153].
3.2 Observational Characteristics
3.2.1 Persistent Emission
3.2.1.1 X-Ray Emission
The X-ray emission from magnetars is gently modulated at the pulsar spin period,
with generally one or two broad sine components and substantial pulsed fractions
(the fraction of the flux that changes along the rotation cycle) of 10–30%. The pulse
profiles may be energy dependent and may display dramatic changes with time,
especially in connection with strong bursting/outbursting activity (see Sect. 3.2.2).
As an example, Fig. 3.1 shows a multi-epoch and multi-instrument pulse profile of
1RXS J170849.0–400910 (see [89] for details).
Part of the X-ray luminosity of magnetars in quiescence has a thermal origin and
can be fit by a blackbody with temperature kT ≈ 0.3–1 keV, much higher than the
typical values for rotation-powered pulsars; magnetar also tend to be more luminous
than rotation-powered pulsars of similar characteristic age. Indeed, in magnetars the
neutron star surface is believed to be particularly hot because of the extra-heating
by the magnetic field decay (e.g. Aguilera et al. [1]).
The size of the region of the thermal emission inferred from a blackbody fit
is generally much smaller than the surface of the star, possibly suggesting that a
99
opposite possibility, too, was considered: that AXPs might be SGR progenitors in
which the magnetic field decay has just begun (e.g. Gavriil et al. [73]).
The magnetar hypothesis was arguably the most successful in explaining the
salient features of SGRs and AXPs, but some skepticism about the scenario persisted
for long, in particular for the AXPs, for which several alternative models could not
be excluded [31, 151]. The magnetar model really started to become the mainstream
when the period derivative of an SGR was measured for the first time [130]. Using
RossiXTE, it was established that SGR 1806–20 was pulsating at P = 7.47 s and
the period was increasing at the rate ˙
P = 2.6 × 10 −3 s yr −1 : with the standard
assumption of a magnetic dipole rotating in vacuum routinely used for standard
pulsar, the values correspond to a surface magnetic field of B 3.2 × 10 19
√
P ˙
P =
8 × 10 14 G at the neutron star’s equator. Moreover, the release of magnetic energy
was necessary to power the X-ray emission of SGR 1806–20, since the spin-down
energy loss of the neutron star was two orders of magnitude lower than the observed
X-ray luminosity.
Few years later, the detection of SGR-like bursts from the AXP 1E 1048-1–5937
[73] confirmed the suspect that also AXPs could harbour magnetars. Since then,
with the discovery of many new magnetars showing a variegated phenomenology
and observations of strong bursting activity and powerful flares from AXPs, as well
as prolonged periods of quiescence in once-active SGRs, the AXP–SGR dichotomy
seems completely obsolete [125, 153].
3.2 Observational Characteristics
3.2.1 Persistent Emission
3.2.1.1 X-Ray Emission
The X-ray emission from magnetars is gently modulated at the pulsar spin period,
with generally one or two broad sine components and substantial pulsed fractions
(the fraction of the flux that changes along the rotation cycle) of 10–30%. The pulse
profiles may be energy dependent and may display dramatic changes with time,
especially in connection with strong bursting/outbursting activity (see Sect. 3.2.2).
As an example, Fig. 3.1 shows a multi-epoch and multi-instrument pulse profile of
1RXS J170849.0–400910 (see [89] for details).
Part of the X-ray luminosity of magnetars in quiescence has a thermal origin and
can be fit by a blackbody with temperature kT ≈ 0.3–1 keV, much higher than the
typical values for rotation-powered pulsars; magnetar also tend to be more luminous
than rotation-powered pulsars of similar characteristic age. Indeed, in magnetars the
neutron star surface is believed to be particularly hot because of the extra-heating
by the magnetic field decay (e.g. Aguilera et al. [1]).
The size of the region of the thermal emission inferred from a blackbody fit
is generally much smaller than the surface of the star, possibly suggesting that a