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 clock-like properties of pulsars moving in the gravitational fields of their unseen neutron-star companions have allowed unique tests of general relativity and provided evidence for gravitational radiation. We report here the detection of the 2.8-sec pulsar J0737−3039B as the companion to the 23-ms 1

Anomalous X-ray pulsars (AXPs) are slowly rotating neutron stars with very bright and highly variable X-ray emission that are believed to be powered by ultra-strong magnetic fields of > 10 14 G, according to the 'magnetar' model. 1 The radio pulsations that have been observed from more than 1,700 neutron stars with weaker magnetic fields have never been detected from any of the dozen known magnetars. The X-ray pulsar XTE J1810-197 was revealed (in 2003) as the first AXP with transient emission when its luminosity increased 100-fold from the quiescent level 2 ; a coincident radio source of unknown origin was detected one year later. 3 Here we show that XTE J1810-197 emits bright, narrow, highly linearly polarized radio pulses, observed at every rotation, thereby establishing that magnetars can be radio pulsars. There is no evidence of radio emission before the 2003 X-ray outburst (unlike ordinary pulsars, which emit radio pulses all the time), and the flux varies from day to day. The flux at all radio frequencies is approximately equal -and at > 20 GHz XTE J1810-197 is currently the brightest neutron star known. These observations link magnetars to ordinary radio pulsars, rule out alternative accretion models for AXPs, and provide a new window into the coronae of magnetars. Pulsations with period P = 5.54 s were easily detected, with period-averaged flux density S 1.4 = 6 mJy and a narrow average profile with full-width at half-maximum of 0.15 s (Fig. 1). We detected individual pulses from virtually every rotation of the neutron star (see Fig. 2). These are composed of < ∼ 10-ms-wide sub-pulses with peak flux densities up to > ∼ 10 Jy and follow a differential flux distribution approximated by d log N = −d log S, with no giant pulses like those observed from the Crab pulsar. 6 A timing model accounting for every turn of the neutron star during the period 17 March-7 May yields barycentric P = 5.54024870 s± 20 ns on MJD 53855.0 anḋ P = (1.016 ± 0.001) × 10 −11 , with root-mean-square residual σ = 5 ms. We use this to set constraints on any putative companion to the AXP by requiring the light-traveltime across the projected orbital semi-major axis to be less than σ. From Kepler's third law, with assumed neutron star mass 1.4 M ⊙ , the upper limits on the minimum companion mass lie in the range ∼ 0.003-0.03 M ⊙ for orbital periods in the range 2 h-5 min, effectively ruling out the existence of any Roche lobe-filling star orbiting this AXP. The delay in pulse arrival times measured between 2.9 and 0.7 GHz implies an integrated column density of free electrons between the Earth and XTE J1810-197 of 178 ± 5 cm −3 pc. Together with a model for the Galactic distribution of free electrons, 7 the distance to XTE J1810-197 is D ≈ 3.3 kpc (here we use ≈ to indicate a quantity known to within about a factor of two or better), consistent with X-ray-and optically-derived estimates of 2.5-5 kpc (refs 8-10

The International Pulsar Timing Array project combines observations of pulsars from both Northern and Southern hemisphere observatories with the main aim of detecting ultra-low frequency (∼ 10 −9 − 10 −8 Hz) gravitational waves. Here we introduce the project, review the methods used to search for gravitational waves emitted from coalescing supermassive binary black-hole systems in the centres of merging galaxies and discuss the status of the project.

We present the first VLBI maps of H 2 O maser emission (1.3 cm) in the nucleus of the Circinus galaxy, constructed from data obtained with the Australia Telescope Long Baseline Array. The maser emission traces a warped, edge-on accretion disk between radii of 0:11 AE 0:02 and $0.40 pc, as well as a wide-angle outflow that extends up to $1 pc from the estimated disk center. The disk rotation is close to Keplerian (v / r À0:5 ), the maximum detected rotation speed is 260 km s À1 , and the inferred central mass is ð1:7 AE 0:3Þ Â 10 6 M . The outflowing masers are irregularly distributed above and below the disk, with relative outflow velocities up to $AE160 km s À1 , projected along the line of sight. The flow probably originates closer than 0.1 pc to the central engine, possibly in an inward extension of the accretion disk, although there is only weak evidence of rotation in the outward-moving material. We observe that the warp of the disk appears to collimate the outflow and to fix the extent of the ionization cone observed on larger angular scales. This study provides the first direct evidence (i.e., through imaging) of dusty, high-density, molecular material in a nuclear outflow less than 1 pc from the central engine of a Seyfert galaxy, as well as the first graphic evidence that warped accretion disks can channel outflows and illumination patterns in active galactic nuclei. We speculate that the same arrangement, which in some ways obviates the need for a geometrically thick, dusty torus, may apply to other type 2 active galactic nuclei.

The variable X-ray source 1E 1547

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