66
M. Burgay et al.
2.3.1.2 Pulsars with a White Dwarf Companion
If the companion star was born with a mass below ∼8 M , its evolution is
slower. When the star enters the Red Giant phase, it fills its Roche lobe and starts
transferring mass towards the NS. This phase can last up to ∼10 8 yrs during which
the matter accreted onto the NS surface at a rate of
<
∼ 10 −8 M yr −1 is able to spin it
up to millisecond periods. At the same time, its magnetic field gets down to ∼10 8 or
10 9 G. At the end of the accretion phase, the NS parameters are such that its radio
emission can turn on again. The NS turns, hence, into a fully recycled millisecond
pulsar, while its companion evolves into a white dwarf (see Fig. 2.10).
2.3.1.3 Additional Reading
In the sections above, we have only depicted general features of the evolutionary
paths leading to the formation of relativistic binary pulsars. Actually, at every step,
the evolution can take various alternate branches, according to the initial masses of
both stars, to their relative distances and to the details of the supernova explosion(s).
Therefore, the final outcomes of the evolution can also cover a wide range of
possibilities, whose discussion goes beyond the scope of this chapter. For a more
complete picture of (some of) the different possible evolutionary paths, we refer,
e.g., to the following reviews: [37–39].
2.3.2 The Current Sample
Here we present the sample of binary pulsars for which at least one post-Keplerian
parameter of the orbit has been measured (see Sect. 2.5). Of course this is not a
completely exhaustive definition of a relativistic binary, since, prolonging the timespan of the timing observations or increasing the precision of single measurements
can lead to the inclusion of already known but not yet completely studied pulsars
in the sample; on the other hand, some of the measured parameters may not be
related (or not exclusively related) to relativistic gravity but, for instance, to tidal
effects (if the size of the companion is not negligible with respect to the size of the
orbit, as in the case of non-compact objects) or to the effects of the gravitational
potential well of a globular cluster (for a pulsar inside the cluster) or of the Galaxy
Fig. 2.9 (continued) main sequence; non conservative mass transfer from primary to secondary
star; helium core of the primary—accreted secondary; primary supernova explosion and formation
of an ordinary pulsar; non conservative mass transfer from the secondary towards the NS and
possible common envelope (CE) phase; further evolution of helium core of the secondary and
second mass transfer phase with CE; second supernova explosion; formation of a double NS system
with a young and a mildly recycled pulsar
M. Burgay et al.
2.3.1.2 Pulsars with a White Dwarf Companion
If the companion star was born with a mass below ∼8 M , its evolution is
slower. When the star enters the Red Giant phase, it fills its Roche lobe and starts
transferring mass towards the NS. This phase can last up to ∼10 8 yrs during which
the matter accreted onto the NS surface at a rate of
<
∼ 10 −8 M yr −1 is able to spin it
up to millisecond periods. At the same time, its magnetic field gets down to ∼10 8 or
10 9 G. At the end of the accretion phase, the NS parameters are such that its radio
emission can turn on again. The NS turns, hence, into a fully recycled millisecond
pulsar, while its companion evolves into a white dwarf (see Fig. 2.10).
2.3.1.3 Additional Reading
In the sections above, we have only depicted general features of the evolutionary
paths leading to the formation of relativistic binary pulsars. Actually, at every step,
the evolution can take various alternate branches, according to the initial masses of
both stars, to their relative distances and to the details of the supernova explosion(s).
Therefore, the final outcomes of the evolution can also cover a wide range of
possibilities, whose discussion goes beyond the scope of this chapter. For a more
complete picture of (some of) the different possible evolutionary paths, we refer,
e.g., to the following reviews: [37–39].
2.3.2 The Current Sample
Here we present the sample of binary pulsars for which at least one post-Keplerian
parameter of the orbit has been measured (see Sect. 2.5). Of course this is not a
completely exhaustive definition of a relativistic binary, since, prolonging the timespan of the timing observations or increasing the precision of single measurements
can lead to the inclusion of already known but not yet completely studied pulsars
in the sample; on the other hand, some of the measured parameters may not be
related (or not exclusively related) to relativistic gravity but, for instance, to tidal
effects (if the size of the companion is not negligible with respect to the size of the
orbit, as in the case of non-compact objects) or to the effects of the gravitational
potential well of a globular cluster (for a pulsar inside the cluster) or of the Galaxy
Fig. 2.9 (continued) main sequence; non conservative mass transfer from primary to secondary
star; helium core of the primary—accreted secondary; primary supernova explosion and formation
of an ordinary pulsar; non conservative mass transfer from the secondary towards the NS and
possible common envelope (CE) phase; further evolution of helium core of the secondary and
second mass transfer phase with CE; second supernova explosion; formation of a double NS system
with a young and a mildly recycled pulsar