Timing & geodetic precession
in the Double Pulsar
Timing & geodetic precession
in the Double Pulsar
P455
This project is to continue
the exploitation of our discovery of the ﬁrst Double
Pulsar, PSR J0737−3039A/B (Burgay et al. 2003, Lyne
et al. 2004) with the aim of providing the strongest
tests to date for general relativity and of measuring
for the first time the moment-of-inertia of a neutron
star. Additionally, we will determine the system
geometries and map the pulsar beams via geodetic
precession.
The targets of this
project have been discovered in our survey at Parkes
and have resulted already almost 70 publications that
received a total of 1800 citations. The two pulsars
of the double pulsar system, a recycled, old 22-ms
pulsar and a young 2.8-s pulsar, are in a
highly-relativistic, eccentric 2.4-hour orbit,
allowing us unprecedented tests of gravitational
physics which are fundamentally different from what
has been possible before. The purpose of this project
is the further exploitation of this system with
continued timing and the study of precession effects
caused by relativistic spin-orbit coupling
(e.g. Kramer 1998, Stairs et al. 2004). We stress that
for both purposes the Parkes observations continue to
be extremely suitable, valuable and important. While
the Double Pulsar is also monitored by us with the
Green Bank Telescope (GBT), it is Parkes which
provides us with the unique high-quality, long data
sets which are recorded with the same consistent
hardware set-up. It is this consistency and
reliability of the Parkes data that are the key to our
success, complemented by the usually more precise GBT
data.
Double Pulsar. While having
already measured as many relativistic corrections (so
called “Post-Keplerian” (PK) parameters) as for
the previously best test-beds for gravitational
physics (i.e. PSRs B1913+16 and B1534+12), both
observed orbits give us access to the mass ratio of
the two neutron stars (NSs) which is independent of
the theory of gravity and in particular of
strong-ﬁeld effects. Our tests of gravitational
theories are therefore not only the most precise tests
of GR ever performed in the strong ﬁeld limit
(confirming GR at the 0.05% level, Kramer et
al. 2006), but they are also qualitatively
different. Investigating alternative theories of
gravity, we developed a timing model which implements
a unique method to investigate and pose limits on the
existence of preferred frames in the Universe, as
proposed by classes of theories in which a violation
of gravitational Lorentz invariance can occur (Wex
& Kramer 2007).
While geodetic precession
causes the pulsar axes to precess about the total
angular momentum vector with unprecedentedly large
rates of ∼5◦/yr, we have still not detected the
expected changes in the pulse profile of pulsar
A. This provides interesting clues about the
system’s geometry (Manchester et al. 2005, Ferdman
2008, Ferdman et al. 2008) and is perfectly consistent
with our study of the evolution of the system which
indicates a low velocity kick and, surprisingly, a
very low progenitor mass for pulsar B of less than
2M⊙ (Stairs et al. 2006). In contrast, our Parkes
observations reveal profile changes for B as expected
(Burgay et al. 2005) which can be combined with our
recent GBT studies (using ephemerides derived with the
help of Parkes) of the eclipse of A by B which we use
for the first theory-independent test of relativistic
spin precession in strong gravitational fields (Breton
et al., 2008, Science). The geodetic precession of
pulsar B has recently led to a disappearance of its
emission from our view (Perera et al., 2010). Such a
behaviour was expected eventually, and we are able to
model the beam pattern and the viewing geometry in a
way that is consistent with the results from Breton et
al. (2008). The current model suggests that the pulsar
may re-appear either in about 12 years, or, if the
shape of the beam is symmetric, already in 2014. Of
course these predictions depends on details of the
actual beam shape and a continuous monitoring of
pulsar B to catch it as soon as it reappears will be
crucial to accurately establish the shape and geometry
of the beams.
Ongoing Observations
The ongoing observations
performed at the Parkes radio telescope in summary are
crucial to i) further improve the precision on the
currently measured relativistic parameters of the
Double Pulsar ii) measure new
relativistic parameters, including new effects that
can be uniquely addressed by this system and iii)
look for and quantify the effects of both orbital
aberration and long-term geodetic precession of the
pulsars’ spin axes:
•Best GR tests ever: While
we have already obtained the most stringent tests
of GR, we would like to improve the limits even
further. Our significant measurement of a rather
small proper motion, corresponding to a transverse
speed of only 10km/s, let us predict that our
tests will even surpass in precision the various
solar system measurements of PPN parameters
(Kramer et al. 2006). This result is supported by
a recent detection of a timing parallax,
indicating a distance that is consistent with the
estimate based on the dispersion measure, and the
analysis of our currently available data (Kramer
et al. in prep.). Continued observations will also
improve our tests of alternative theories of
gravity.
•New relativistic
parameters: Soon we expect to measure δθ,
a never-before measured PK parameter that
describes a relativistic deformation of the orbit
(Damour & Taylor 1992). This parameter becomes
measurable as soon as geodetic precession causes
changing aberration to be visible in the timing
data. In the longer term, the data will allow us
to measure the moment of inertia I of a NS via the
effects of relativistic spin-orbit coupling on our
timing data. Measuring I and relating it to the
high precision mass and rotational spin
measurements, has to the potential to rule whole
classes of equation-of-state for nuclear
matter.
•Geodetic precession:
Continued monitoring the pulse shape and
polarization properties of the pulsar(s) in the
double pulsar system, we obtain independent information
about the system geometry, enabling tests of GR
based on pulse structure parameters (Damour &
Taylor 1992). Reconstructing the beam shapes for
both pulsars in the system, we will
have a recycled and a non-recycled pulsar
where we can compare their actual beam structure.
Scientific case
The aim of the project is to perform timing and profile monitoring of the first Double Pulsar in order to get the most precise tests of GR to date and to measure for the first time the moment of inertia of a NS.