64
M. Burgay et al.
(accretion-driven decay; [28]), or because of the variation in the rotational regime
(spin-driven decay; [29]), hence making the NS move downwards in the diagram.
As a result, when the accretion stops, the neutron star, spinning at a period of only
several milliseconds and with a magnetic field of 10 8 –10 10 G, is again above the
death-line (or, more precisely, the death-valley) and its radio emission switches on. 2
The typical evolutionary path of a NS, leading to a recycled pulsar, is reported in
the period-magnetic field diagram of Fig. 2.8.
2.3.1.1 Pulsars with a Neutron Star Companion
If the progenitor of the NS companion is a massive star, i.e. more massive than
8-10 M , the transfer of its swollen external layers towards the compact object is
done mainly via wind; accretion can occur also in a short Roche lobe overflow phase
or in a common envelope phase, during which the orbit shrinks. The evolution of
such a big star is relatively fast and the amount of accreted mass, hence angular
momentum, is small. The companion star ends its life in a supernova explosion and,
if the binary is not disrupted by the event, a double neutron star (DNS) system is
created. The spun-up NS is now a mildly recycled pulsar, rotating with a period of
tens of milliseconds and with a surface magnetic field, which decreased, during the
short accretion phase, by only a couple orders of magnitude (B s ∼ 10 10 G). Since
it possesses a much higher magnetic field (hence ˙
P ) than its recycled companion,
the new pulsar born in the second supernova explosion slows down and reaches the
death-line much faster than the other pulsar in the system. A graphical description
of the evolutionary steps leading to the formation of a double neutron star system
(DNS) is reported in Fig. 2.9 for the specific case of a binary system starting with
two main sequence stars of about 13 M and 10 M .
We note that, in order to form a DNS, the original binary must survive both of the
supernova explosions that give birth to the two NSs. If the first supernova disrupts
the system, no accretion phase occurs and the outcome of the evolution will be two
isolated ordinary (i.e. non recycled) pulsars that will later end their life, on a time
scale of ∼10 8 yrs since birth, in the so-called pulsar graveyard, below the death line
(see Fig. 2.8). If, on the other hand, the system is disrupted by the second supernova
explosion, one of the two stars will end as an isolated mildly-recycled pulsar (known
as a disrupted recycled pulsar, DRP [36]), and the other star as an isolated ordinary
pulsar.
2 In recent years a small number of transitional millisecond pulsars, binary systems switching
from an accretion powered X-ray pulsar state to a rotation-powered radio pulsar state, have been
discovered [30–32]. Their existence is a direct observational proof that, at least for a sub-class of
binaries, the recycling model is correct.
M. Burgay et al.
(accretion-driven decay; [28]), or because of the variation in the rotational regime
(spin-driven decay; [29]), hence making the NS move downwards in the diagram.
As a result, when the accretion stops, the neutron star, spinning at a period of only
several milliseconds and with a magnetic field of 10 8 –10 10 G, is again above the
death-line (or, more precisely, the death-valley) and its radio emission switches on. 2
The typical evolutionary path of a NS, leading to a recycled pulsar, is reported in
the period-magnetic field diagram of Fig. 2.8.
2.3.1.1 Pulsars with a Neutron Star Companion
If the progenitor of the NS companion is a massive star, i.e. more massive than
8-10 M , the transfer of its swollen external layers towards the compact object is
done mainly via wind; accretion can occur also in a short Roche lobe overflow phase
or in a common envelope phase, during which the orbit shrinks. The evolution of
such a big star is relatively fast and the amount of accreted mass, hence angular
momentum, is small. The companion star ends its life in a supernova explosion and,
if the binary is not disrupted by the event, a double neutron star (DNS) system is
created. The spun-up NS is now a mildly recycled pulsar, rotating with a period of
tens of milliseconds and with a surface magnetic field, which decreased, during the
short accretion phase, by only a couple orders of magnitude (B s ∼ 10 10 G). Since
it possesses a much higher magnetic field (hence ˙
P ) than its recycled companion,
the new pulsar born in the second supernova explosion slows down and reaches the
death-line much faster than the other pulsar in the system. A graphical description
of the evolutionary steps leading to the formation of a double neutron star system
(DNS) is reported in Fig. 2.9 for the specific case of a binary system starting with
two main sequence stars of about 13 M and 10 M .
We note that, in order to form a DNS, the original binary must survive both of the
supernova explosions that give birth to the two NSs. If the first supernova disrupts
the system, no accretion phase occurs and the outcome of the evolution will be two
isolated ordinary (i.e. non recycled) pulsars that will later end their life, on a time
scale of ∼10 8 yrs since birth, in the so-called pulsar graveyard, below the death line
(see Fig. 2.8). If, on the other hand, the system is disrupted by the second supernova
explosion, one of the two stars will end as an isolated mildly-recycled pulsar (known
as a disrupted recycled pulsar, DRP [36]), and the other star as an isolated ordinary
pulsar.
2 In recent years a small number of transitional millisecond pulsars, binary systems switching
from an accretion powered X-ray pulsar state to a rotation-powered radio pulsar state, have been
discovered [30–32]. Their existence is a direct observational proof that, at least for a sub-class of
binaries, the recycling model is correct.