88
M. Burgay et al.
In the pulsar binaries where one or two PK parameters have been determined,
one can infer the mass of the companion but not test gravity theories. A number of
NS–WD binaries have led to the estimation of more PK parameters (see Table 2.1).
The most interesting NS–WD binaries for constraining tensor-scalar theories are
J0348+0432 and J1738+0333.
PSR J1738+0333, discovered in 2011 [60, 61] is a pulsar orbiting a low-mass WD
companion (0.18 solar masses) in a 8.5 h orbit and a low eccentricity (3 × 10 −7 ). It
currently provides the best limit on tensor-scalar theories thanks to a very precise
determination of the PK parameter ˙
P b , as well as proper motion and parallax.
The strong agreement with GR leads to the current best upper limit on dipolar
gravitational wave emission predicted by tensor-scalar theories.
PSR J0348+0432, discovered in 2013 [113, 114], is the pulsar with the highest
mass discovered so far. This provides a strong constraint on the orbital decay ˙
P b
and in turn a strong constraint on dipolar gravitational radiation. While the upper
limit on dipolar radiation is not as stringent as for J1738+0333, this pulsar rules out
an important part of the α 0 —β 0 parameter space, especially at negative β 0 ’s, since
massive systems predict an exceptionally large amount of dipolar radiation.
Constraints on tensor-scalar theories from pulsar binary observations and solar
system tests are shown in Fig. 2.15.
2.5.3 Future Prospects
Our constraints on general relativity and alternative theories of gravity will improve
thanks to longer datasets for interesting pulsars (such as the Double Pulsar and
NS–WD binaries) at current telescopes and at new telescopes such as the Large
European Array for Pulsars (LEAP) [116], the Five-hundred meter A-spherical
Single-aperture (FAST) radio telescope (China) [117] and the Square Kilometre
Array (SKA) and precursosr, in particular MeerKAT [118]. These new telescopes
will also allow the discovery of new binary systems (DNS binaries or NS–WD
binaries), in particular at the SKA [119]. The most interesting type of binaries—
that are yet to be discovered—are pulsar—black hole (PSR–BH) binaries. They are
expected to be rare: we could find a pulsar in orbit around the supermassive black
hole at the Galactic Center [120], or around a stellar-mass black hole [121]. We
could also possibly find pulsars in globular clusters and which orbit an intermediate
mass black hole [122].
As we have seen in the previous section (Fig. 2.15), PSR–BH binaries are
expected to provide the best constraints on tensor-scalar theories. But that’s not it.
Using a pulsar (as a clock) around a spinning black hole will give us unprecedented
knowledge about black holes, such as their mass M BH , angular momentum S BH and
quadrupole moment Q BH , allowing us to test the Cosmic Censorship Conjecture
(which excludes the existence of naked singularities) and the No Hair theorem
(which states that a black hole is only described by its mass, spin and charge)
[123, 124].
M. Burgay et al.
In the pulsar binaries where one or two PK parameters have been determined,
one can infer the mass of the companion but not test gravity theories. A number of
NS–WD binaries have led to the estimation of more PK parameters (see Table 2.1).
The most interesting NS–WD binaries for constraining tensor-scalar theories are
J0348+0432 and J1738+0333.
PSR J1738+0333, discovered in 2011 [60, 61] is a pulsar orbiting a low-mass WD
companion (0.18 solar masses) in a 8.5 h orbit and a low eccentricity (3 × 10 −7 ). It
currently provides the best limit on tensor-scalar theories thanks to a very precise
determination of the PK parameter ˙
P b , as well as proper motion and parallax.
The strong agreement with GR leads to the current best upper limit on dipolar
gravitational wave emission predicted by tensor-scalar theories.
PSR J0348+0432, discovered in 2013 [113, 114], is the pulsar with the highest
mass discovered so far. This provides a strong constraint on the orbital decay ˙
P b
and in turn a strong constraint on dipolar gravitational radiation. While the upper
limit on dipolar radiation is not as stringent as for J1738+0333, this pulsar rules out
an important part of the α 0 —β 0 parameter space, especially at negative β 0 ’s, since
massive systems predict an exceptionally large amount of dipolar radiation.
Constraints on tensor-scalar theories from pulsar binary observations and solar
system tests are shown in Fig. 2.15.
2.5.3 Future Prospects
Our constraints on general relativity and alternative theories of gravity will improve
thanks to longer datasets for interesting pulsars (such as the Double Pulsar and
NS–WD binaries) at current telescopes and at new telescopes such as the Large
European Array for Pulsars (LEAP) [116], the Five-hundred meter A-spherical
Single-aperture (FAST) radio telescope (China) [117] and the Square Kilometre
Array (SKA) and precursosr, in particular MeerKAT [118]. These new telescopes
will also allow the discovery of new binary systems (DNS binaries or NS–WD
binaries), in particular at the SKA [119]. The most interesting type of binaries—
that are yet to be discovered—are pulsar—black hole (PSR–BH) binaries. They are
expected to be rare: we could find a pulsar in orbit around the supermassive black
hole at the Galactic Center [120], or around a stellar-mass black hole [121]. We
could also possibly find pulsars in globular clusters and which orbit an intermediate
mass black hole [122].
As we have seen in the previous section (Fig. 2.15), PSR–BH binaries are
expected to provide the best constraints on tensor-scalar theories. But that’s not it.
Using a pulsar (as a clock) around a spinning black hole will give us unprecedented
knowledge about black holes, such as their mass M BH , angular momentum S BH and
quadrupole moment Q BH , allowing us to test the Cosmic Censorship Conjecture
(which excludes the existence of naked singularities) and the No Hair theorem
(which states that a black hole is only described by its mass, spin and charge)
[123, 124].