120
P. Esposito et al.
The inspection of a range of theoretical models, as well as observations, has
shown that the magnetic field has little effect on the luminosity for ‘weakly’ magnetized neutron stars with B < 10 13 G. These objects, of which the radio pulsars are
the most notable representatives, have thermal luminosities that are compatible with
those predicted by standard non-magnetic cooling models. Overall, the magnetothermal simulations can broadly reproduce the observed X-ray luminosities for a
range of initial magnetic field strengths, envelope compositions, and neutron star
masses. As the neutron stars age and become colder, they also spin down, primarily
due to dipolar radiation losses. In the absence of field decay, pulsars should follow
linear tracks in the P – ˙
P diagram (see dashed lines in the lower panel of Fig. 3.8).
However, when magnetic field dissipation is taken into account, evolutionary tracks
in the P – ˙
P diagram bend down (Fig. 3.8).
Comparing observations and theoretical modelling of the neutron star magnetothermal evolution, considering both the luminosity and the rotational period properties, we can gather that objects like the traditional rotation-powered radio pulsars
were born with magnetic fields in the range of a few 10 12 –10 13 G. When they cool
and slow down, they eventually become invisible in both the radio and the X-ray
bands, and hence they lack observable counterparts. On the other hand, pulsars born
with fields exceeding the 10 14 G, will be observed now as young magnetars or highB pulsars (depending on the strength and the configuration of the field at birth), and
have as descendants the objects known as X-ray dim isolated neutron stars. These
simulations point to evolutionary connections (some of which have been suspected
for long) between apparently different groups of pulsars: Most likely, they are all
essentially the same kind of objects, but they were born with different magnetic
field strength and geometry, and are observed at different evolutionary stages of
their life.
3.2.5 Low-B Magnetars and High-B Pulsars
Recently, the long standing belief that magnetars must posses supercritical magnetic
fields 2 has been challenged by the discovery of full-fledged magnetars with a
dipole magnetic field well in the range of ordinary radio pulsars: SGR 0418+5729,
Swift J1822.3−1606, and 3XMM J1852+0033 (Rea et al. [182, 183, 186]; see
Turolla and Esposito [217] for a review). Those three magnetars are in fact not
dissimilar from the other members of the class, except for the strength of the dipole
magnetic field B p estimated from the spin parameters, in the range (0.6–4)×10 13 G.
2 The electron quantum critical magnetic field B Q = m 2
e c 2 /(¯ he) 4.4 × 10 13 G was traditionally
considered the threshold above which magnetars could be found.
P. Esposito et al.
The inspection of a range of theoretical models, as well as observations, has
shown that the magnetic field has little effect on the luminosity for ‘weakly’ magnetized neutron stars with B < 10 13 G. These objects, of which the radio pulsars are
the most notable representatives, have thermal luminosities that are compatible with
those predicted by standard non-magnetic cooling models. Overall, the magnetothermal simulations can broadly reproduce the observed X-ray luminosities for a
range of initial magnetic field strengths, envelope compositions, and neutron star
masses. As the neutron stars age and become colder, they also spin down, primarily
due to dipolar radiation losses. In the absence of field decay, pulsars should follow
linear tracks in the P – ˙
P diagram (see dashed lines in the lower panel of Fig. 3.8).
However, when magnetic field dissipation is taken into account, evolutionary tracks
in the P – ˙
P diagram bend down (Fig. 3.8).
Comparing observations and theoretical modelling of the neutron star magnetothermal evolution, considering both the luminosity and the rotational period properties, we can gather that objects like the traditional rotation-powered radio pulsars
were born with magnetic fields in the range of a few 10 12 –10 13 G. When they cool
and slow down, they eventually become invisible in both the radio and the X-ray
bands, and hence they lack observable counterparts. On the other hand, pulsars born
with fields exceeding the 10 14 G, will be observed now as young magnetars or highB pulsars (depending on the strength and the configuration of the field at birth), and
have as descendants the objects known as X-ray dim isolated neutron stars. These
simulations point to evolutionary connections (some of which have been suspected
for long) between apparently different groups of pulsars: Most likely, they are all
essentially the same kind of objects, but they were born with different magnetic
field strength and geometry, and are observed at different evolutionary stages of
their life.
3.2.5 Low-B Magnetars and High-B Pulsars
Recently, the long standing belief that magnetars must posses supercritical magnetic
fields 2 has been challenged by the discovery of full-fledged magnetars with a
dipole magnetic field well in the range of ordinary radio pulsars: SGR 0418+5729,
Swift J1822.3−1606, and 3XMM J1852+0033 (Rea et al. [182, 183, 186]; see
Turolla and Esposito [217] for a review). Those three magnetars are in fact not
dissimilar from the other members of the class, except for the strength of the dipole
magnetic field B p estimated from the spin parameters, in the range (0.6–4)×10 13 G.
2 The electron quantum critical magnetic field B Q = m 2
e c 2 /(¯ he) 4.4 × 10 13 G was traditionally
considered the threshold above which magnetars could be found.