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), of the double neutron star (DNS) PSR J1756−2251 (Faulkner et al. 2005) and of the relativistic binary PSR J1141−6545 (Kaspi et al. 2000). The aim is to provide the strongest tests to date for general relativity and to measure 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 three targets of this project have been discovered in our surveys at Parkes and have resulted already in more than 40 publications. In particular, we note the large number of citations (almost 600) of our papers on the Double Pulsar and the large number of publications (more than 300) written in response. 

While PSR J1141−6545 is in a relativistic 4.5-hr orbit about a heavy white dwarf companion, PSR J1756−2251 is a 28-ms pulsar in a DNS with a 7.7-hr eccentric orbit, showing many similarities with the original binary pulsar PSR B1913+16. 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 these systems 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 and PSR J1756−2251 are 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). Interestingly, in the last term, B has become rather weak and is essentially undetectable. This behaviour is not unexpected and mostly likely due to a combination of orbital and geodetic precession. We will continue the monitoring of B, which should become stronger again eventually.

PSR J1756−2251. After our discovery, Parkes observations resulted in a measurement of an advance of periastron of dω = 2.58244(16), implying a total system mass of 2.57008(24)M⊙, the gravitational redshift parameter and a Shapiro delay (Faulkner et al. 2005). Continued timing and combination with GBT data, determined the masses to Mp = 1.312(17)M⊙ and Mc = 1.258(18)M⊙ (Ferdman 2008). The light companion mass adds further support to the idea that most young NS in DNSs are formed via a white dwarf e-capture supernova (see van den Heuvel 2007) which would result in a very narrow mass range of 1.25M⊙ (as the difference between the Chandrasekhar mass and the NS binding energy). Moreover, our result provide an independent test of GR at the 5% level. Geodetic precession has not been detected yet (Ferdman 2008).

PSR J1141−6545. In the last years, we have collected excellent polarization data (lately with the Pulsar Digital Filterbanks), while we are also continuing to take data with the initial filterbank system in parallel, providing us with a very long time baseline for consistency checks and giving us information ranging further back in time than those of Hotan et al. (2005). We find that an extrapolation back in time indeed suggests that the pulsar was invisible during the 70cm survey. Secondly, the obvious structure in the evolution of the pulse width data is a reflection of the internal beam structure which appears to be patchy rather than circular. After carefully calibrating all our polarization data, a combined modeling of the polarization and pulse width data has recently been completed, resulting in constraints on the absolute orientation of the pulsar spin axis (Manchester et al. to be submitted). The determined geometry suggests that we’ll see a “replay” of the previously observed profiles in reverse order, which we would like to confirm with further observations.

We have also studied all sources to detect the expected aberration-induced changes of the pulse profile on orbital timescales. This poses an observational challenge, requiring the best available data. Unfortunately, we have so far not succeeded for the Double Pulsar or PSR J1756−2251 (Ferdman 2008, Ferdman et al. 2008)), suggesting that more sensitivity is needed.

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 both the Double Pulsar and PSR J1756−2251, ii) measure new relativistic parameters, including new effects that can be uniquely addressed by these systems and iii) look for and quantify the effects of both orbital aberration and long-term geodetic precession of the pulsars’ spin axes in all three systems:

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 tentative 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. • NS masses: With continuing measurements of PSR J1756−2251 we will be able to further improve on our measurement of the young NS’s mass. A comparison with the light mass of PSR J1141−4565, born under different conditions, will allows us to test the proposed theory that the young pulsars in DNSs are not born in the standard iron-core collapse.
   

4. •Geodetic precession: Continued monitoring the pulse shape and polarization properties of the pulsars in all three systems, 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 all four pulsars in the three systems, we will have a sample of recycled and non-recycled pulsars where we can compare their actual beam structure for the first time. We  are now able to model that the movement of our line-of-sight through the beam of PSR J1141-6546 has started to reverse. We should therefore be able to see a reversed repeat of the previous profile evolution, providing tight and unique constraints on the underlying geometry and beam shape.