The double pulsar system, PSR J0737-3039A/B, is unique in that both neutron stars are detectable as radio pulsars. This, combined with significantly higher mean orbital velocities and accelerations when compared to other binary pulsars, suggested that the system would become the best available testbed for general relativity and alternative theories of gravity in the strong-field regime. 1Here we report on precision timing observations taken over the 2.5 years since its discovery and present four independent strong-field tests of general relativity. Use of the theory-independent mass ratio of the two stars makes these tests uniquely different from earlier studies. By measuring relativistic corrections to the Keplerian discription of the orbital motion, we find that the "postKeplerian" parameter s agrees with the value predicted by Einstein's theory of general relativity within an uncertainty of 0.05%, the most precise test yet obtained. We also show that the transverse velocity of the system's center of mass is extremely small. Combined with the system's location near the Sun, this result suggests that future tests of gravitational theories with the double pulsar will supersede the best current Solar-system tests. It also implies that the second-born pulsar may have formed differently to the usually assumed core-collapse of a helium star.

The merger 1 of close binary systems containing two neutron stars should produce a burst of gravitational waves, as predicted by the theory of general relativity 2 . A reliable estimate of the double-neutron-star merger rate in the Galaxy is crucial in order to predict whether current gravity wave detectors will be successful in detecting such bursts. Present estimates of this rate are rather low 3−7 , because we know of only a few doubleneutron-star binaries with merger times less than the age of the Universe. Here we report the discovery of a 22-ms pulsar, PSR J0737−3039, which is a member of a highly relativistic double-neutron-star binary with an orbital period of 2.4 hours. This system will merge in about 85 Myr, a time much shorter than for any other known neutron-star binary. Together with the relatively low radio luminosity of PSR J0737−3039, this timescale implies an order-of-magnitude increase in the predicted merger rate for double-neutron-star systems in our Galaxy (and in the rest of the Universe). PSR J0737−3039 was discovered during a pulsar search carried out using a multibeam receiver 8 on the Parkes 64-m radio telescope in New South Whales, Australia. The original detection showed a large change in apparent pulsar period during the 4-min observation time, suggesting that the pulsar is a member of a tight binary system. Follow-up observations undertaken at Parkes consisting of continuous ∼ 5-hour observations showed that the orbit has a very short period (2.4 hrs) and a significant eccentricity (0.088). The derived orbital parameters implied that the system is relatively massive, probably consisting of two neutron stars, and predicted a huge rate of periastron advanceω due to effects of general relativity. Indeed, after only a few days of pulse-timing observations we were able to detect a significant value ofω.Interferometric observations made using the Australia Telescope Compact Array (ATCA) in the 20-cm band gave an improved position and flux density for the pulsar. Knowledge of the pulsar position with subarcsecond precision allowed determination of the rotational period derivative,Ṗ , and other parameters from the available data span. Table 1 reports results derived from a coherent phase fit to data taken over about five months. The measured value ofω = 16.88 • yr −1 is about four times that of PSR B1913+16 (ref. 9), previously the highestknown. If the observedω is entirely due to general relativity, it indicates a total system mass M = 2.58 ± 0.02 M , where M is the mass of the Sun. Figure 1 shows the constraints on the masses of the pulsar and its companion resulting from the observations so far and the mean pulse profile as an inset. The shaded region indicates values that are ruled out by the mass function M f and the observeḋ ω constrains the system to lie between the two diagonal lines. Together, these constraints imply that the pulsar mass m p is less than 1.35 M and that the companion mass m c is greater than 1.24 M . The derived upper limit on m p is consistent with the

Abstract:A new set of software applications and libraries for use in the archival and analysis of pulsar astronomical data is introduced. Known collectively as the psrchive scheme, the code was developed in parallel with a new data storage format called psrfits, which is based on the Flexible Image Transport System (FITS). Both of these projects utilise a modular, object-oriented design philosophy. psrchive is an open source development environment that incorporates an extensive range of c++ object classes and pre-built command line and graphical utilities. These deal transparently and simultaneously with multiple data storage formats, thereby enhancing data portability and facilitating the adoption of the psrfits file format. Here, data are stored in a series of modular header-data units that provide flexibility and scope for future expansion. As it is based on FITS, various standard libraries and applications may be used for data input, output, and visualisation. Both psrchive and psrfits are made publicly available to the academic community in the hope that this will promote their widespread use and acceptance.

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We present the discovery and follow‐up observations of 142 pulsars found in the Parkes 20‐cm multibeam pulsar survey of the Galactic plane. These new discoveries bring the total number of pulsars found by the survey to 742. In addition to tabulating spin and astrometric parameters, along with pulse width and flux density information, we present orbital characteristics for 13 binary pulsars which form part of the new sample. Combining these results from another recent Parkes multibeam survey at high Galactic latitudes, we have a sample of 1008 normal pulsars which we use to carry out a determination of their Galactic distribution and birth rate. We infer a total Galactic population of 30 000 ± 1100 potentially detectable pulsars (i.e. those beaming towards us) having 1.4‐GHz luminosities above 0.1 mJy kpc2. Adopting the Tauris & Manchester beaming model, this translates to a total of 155 000 ± 6000 active radio pulsars in the Galaxy above this luminosity limit. Using a pulsar current analysis, we derive the birth rate of this population to be 1.4 ± 0.2 pulsars per century. An important conclusion from our work is that the inferred radial density function of pulsars depends strongly on the assumed distribution of free electrons in the Galaxy. As a result, any analyses using the most recent electron model of Cordes & Lazio predict a dearth of pulsars in the inner Galaxy. We show that this model can also bias the inferred pulsar scaleheight with respect to the Galactic plane. Combining our results with other Parkes multibeam surveys we find that the population is best described by an exponential distribution with a scaleheight of 330 pc. Surveys underway at Parkes and Arecibo are expected to improve the knowledge of the radial distribution outside the solar circle, and to discover several hundred new pulsars in the inner Galaxy.

We have used a new observing system on the Parkes radio telescope to carry out a series of pulsar observations of the globular cluster 47 Tucanae at 20-cm wavelength. We detected all 11 previously known pulsars, and have discovered nine others, all of which are millisecond pulsars in binary systems. We have searched the data for relatively short orbital period systems, and found one pulsar with an orbital period of 96 min, the shortest of any known radio pulsar.The increased rate of detections with the new system resulted in improved estimates of the flux density of the previously known pulsars, determination of the orbital parameters of one of them, and a coherent timing solution for another one. Five of the pulsars now known in 47 Tucanae have orbital periods of a few hours and implied companion masses of only ∼ 0.03 M ⊙ . Two of these are eclipsed at some orbital phases, while three are seen at all phases at 20 cm but not always at lower frequencies. Four and possibly six of the other binary systems have longer orbital periods and companion masses ∼ 0.2 M ⊙ , with at least two of them having relatively large orbital eccentricities. All 20 pulsars have rotation periods in the range 2-8 ms.

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We report the discovery of two young isolated radio pulsars with very high inferred magnetic fields. PSR J1119−6127 has period P = 0.407 s, and the largest period derivative known among radio pulsars,Ṗ = 4.0×10 −12 . Under standard assumptions these parameters imply a characteristic spin-down age of only τ c = 1.6 kyr and a surface dipole magnetic field strength of B = 4.1 × 10 13 G. We have measured a stationary period-secondderivative for this pulsar, resulting in a braking index of n = 2.91 ± 0.05. We have also observed a glitch in the rotation of the pulsar, with fractional period change ∆P/P = −4.4 × 10 −9 . Archival radio imaging data suggest the presence of a previously uncataloged supernova remnant centered on the pulsar. The second pulsar, PSR J1814−1744, has P = 3.975 s andṖ = 7.4 × 10 −13 . These parameters imply τ c = 85 kyr, and B = 5.5 × 10 13 G, the largest of any known radio pulsar.Both PSR J1119−6127 and PSR J1814−1744 show apparently normal radio emission in a regime of magnetic field strength where some models predict that no emission should occur. Also, PSR J1814−1744 has spin parameters similar to the anomalous X-ray pulsar (AXP) 1E 2259+586, but shows no discernible X-ray emission. If AXPs are isolated, high magnetic field neutron stars ("magnetars"), these results suggest that their unusual attributes are unlikely to be merely a consequence of their very high inferred magnetic fields.