3 Magnetars: A Short Review and Some Sparse Considerations
121
Furthermore, this results also in a large characteristic age of >10 6 yr, two or three
orders of magnitudes larger than the typical magnetar ages, suggesting that these
low-field magnetars might be old objects. The small number of detected bursts (with
comparatively low energetics) and the low persistent luminosity in quiescence have
been taken as further hints that these might be worn-out magnetars, approaching the
end of their active life [218]. The ‘old magnetar’ scenario sounds appealing since
it offers an interpretation of the low-magnetic-field magnetars within an already
well established framework, validating the magnetar model also for (surface) field
strengths quite far away from those of canonical SGR/AXPs.
The crucial issue is whether a relatively low dipolar field is consistent with
the starquake models, in which the primary cause of the outbursts is an internal
deposition of energy following a crust failure once the magnetically induced shear
stress exceeds a critical value. The magnetic stress needed to break the crust is
strongly dependent on the density (it is much easier to break the outer crust than
the inner crust); moreover, the crust thickness grows as the temperature drops with
age. Detailed calculations show that a local magnetic field of ≈2 × 10 15 G should
be necessary to break the crust, but closer to the surface of the crust, due to the
smaller density, magnetic fields as low as ∼10 14 G may lead to crust fractures
[92, 135]. At any rate, the minimum requirement seems to be around 10 14 G. So,
can (and how) aged, cold and low-magnetic-field magnetars still produce bursts and
outbursts? This depends on the internal toroidal component of its magnetic field.
For this reason, objects with similar dipolar magnetic field strength as inferred from
their period and period derivative can display very different behaviours. In general
the toroidal component of the magnetic field is unmeasurable in a pulsar (but see
the gray box for SGR 0418+5729), but this reasoning help us understanding and
explaining the populations of active magnetars, low-magnetic-field magnetars, and
high-B pulsars. A rough prediction of the expected outburst rate for different initial
magnetic field configurations and life stages is given for standard assumptions in
Fig. 3.9. For an object similar to SGR 0418+5729, a rate of ≈10 −3 starquakes yr −1
is expected [169, 226]. Assuming that there are about 10 4 neutron stars in the
Galaxy with similar age, and that a (very approximatively) 10% of them were
born as magnetars, a naive extrapolation of this event rate to the whole neutron
star population leads to the occurrence of ∼1 low-magnetic-field-magnetar outburst
per year. Therefore, we expect that more and more objects of this class will be
discovered in the upcoming years. Similarly, we can anticipate some magnetar-like
events from only-sporadically-active sources labelled as belonging to other classes
of isolated neutron stars, as perhaps shown by the outbursts of PSR J1846–0258 and
PSR J1119–6127 [7, 74, 91].
121
Furthermore, this results also in a large characteristic age of >10 6 yr, two or three
orders of magnitudes larger than the typical magnetar ages, suggesting that these
low-field magnetars might be old objects. The small number of detected bursts (with
comparatively low energetics) and the low persistent luminosity in quiescence have
been taken as further hints that these might be worn-out magnetars, approaching the
end of their active life [218]. The ‘old magnetar’ scenario sounds appealing since
it offers an interpretation of the low-magnetic-field magnetars within an already
well established framework, validating the magnetar model also for (surface) field
strengths quite far away from those of canonical SGR/AXPs.
The crucial issue is whether a relatively low dipolar field is consistent with
the starquake models, in which the primary cause of the outbursts is an internal
deposition of energy following a crust failure once the magnetically induced shear
stress exceeds a critical value. The magnetic stress needed to break the crust is
strongly dependent on the density (it is much easier to break the outer crust than
the inner crust); moreover, the crust thickness grows as the temperature drops with
age. Detailed calculations show that a local magnetic field of ≈2 × 10 15 G should
be necessary to break the crust, but closer to the surface of the crust, due to the
smaller density, magnetic fields as low as ∼10 14 G may lead to crust fractures
[92, 135]. At any rate, the minimum requirement seems to be around 10 14 G. So,
can (and how) aged, cold and low-magnetic-field magnetars still produce bursts and
outbursts? This depends on the internal toroidal component of its magnetic field.
For this reason, objects with similar dipolar magnetic field strength as inferred from
their period and period derivative can display very different behaviours. In general
the toroidal component of the magnetic field is unmeasurable in a pulsar (but see
the gray box for SGR 0418+5729), but this reasoning help us understanding and
explaining the populations of active magnetars, low-magnetic-field magnetars, and
high-B pulsars. A rough prediction of the expected outburst rate for different initial
magnetic field configurations and life stages is given for standard assumptions in
Fig. 3.9. For an object similar to SGR 0418+5729, a rate of ≈10 −3 starquakes yr −1
is expected [169, 226]. Assuming that there are about 10 4 neutron stars in the
Galaxy with similar age, and that a (very approximatively) 10% of them were
born as magnetars, a naive extrapolation of this event rate to the whole neutron
star population leads to the occurrence of ∼1 low-magnetic-field-magnetar outburst
per year. Therefore, we expect that more and more objects of this class will be
discovered in the upcoming years. Similarly, we can anticipate some magnetar-like
events from only-sporadically-active sources labelled as belonging to other classes
of isolated neutron stars, as perhaps shown by the outbursts of PSR J1846–0258 and
PSR J1119–6127 [7, 74, 91].