Timing & geodetic precession

in the Double Pulsar



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:

  1. 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.

  2. 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.

  3. 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.