Binary pulsars provide an excellent system for testing general relativity because of their intrinsic rotational stability and the precision with which radio observations can be used to determine their orbital dynamics. Measurements of the rate of orbital decay of two pulsars have been shown 1,2 to be consistent with the emission of gravitational waves as predicted by general relativity, but independent veri®cation was not possible. Such veri®cation can in principle be obtained by determining the orbital inclination in a binary pulsar system using only classical geometrical constraints. This would permit a measurement of the expected retardation of the pulse signal arising from the general relativistic curvature of space-time in the vicinity of the companion object (the`Shapiro delay'). Here we report high-precision radio observations of the binary millisecond pulsar PSR J0437-4715, which establish the three-dimensional structure of its orbit. We see the Shapiro delay predicted by general relativity, and we determine the mass of the neutron star and its white dwarf companion. The determination of such masses is necessary in order to understand the origin and evolution of neutron stars , PSR J0437-4715 remains the closest and brightest millisecond pulsar known. It is bound to a low-mass helium white dwarf companion 5,6 in a nearly circular orbit. Owing to its proximity, relative motion between the binary system and the Earth signi®cantly alters the line-of-sight direction to the pulsar and, consequently, the orientation of the basis vectors used in the timing model (see Fig. 1). Although the physical orientation of the orbit in space remains constant, its parameters are measured with respect to this time-dependent basis and therefore also vary with time. Variations of the inclination angle, i, change the projection of the semi-major axis along the line-ofsight, x [ a p sini=c, where a p is the semi-major axis of the pulsar orbit.The heliocentric motion of the Earth induces a periodic variation of x, known as the annual-orbital parallax 7 :The superscripts`obs' and`int' refer to the observed and intrinsic values, respectively, r ! t is the position vector of the Earth with respect to the barycentre of the Solar System as a function of time, d is the distance to the pulsar, and Q9 sinI 0 2 cosJ 0 (see Fig. 1). Similarly, the proper motion of the binary system induces secular evolution of the projected semi-major axis 8,9 , such that:where m m a I 0 m d J 0 is the proper motion vector with components in right ascension, m a , and declination, m d . An apparent transverse quadratic Doppler effect (known as the Shklovskii effect) also arises from the system's proper motion and contributes to the observed orbital period derivative 10 :where b m 2 d=c, and m jmj. Observations of PSR J0437-4715 were conducted from 11 July 1997 to 13 December 2000, using the Parkes 64 m radio telescope. Over 50 terabytes of base-band data have been recorded with the S2 Recorder 11 and the Caltech Parkes Swinburne Recorder (CPSR) 12 , follo...

We have conducted a survey of intermediate Galactic latitudes using the 13‐beam 21‐cm multibeam receiver of the Parkes 64‐m radio telescope. The survey covered the region enclosed by 5°<|b|<15° and −100°

We present photometric, astrometric and spectroscopic observations of the nearby (9.8 pc) low-mass binary Gl 569Bab (in turn being a companion to the early-M star Gl 569A), made with the Keck adaptive optics facility. Having observed Gl 569Bab since August 1999, we are able to see orbital motion and to determine the orbital parameters of the pair. We find the orbital period to be 892 ± 25 days, the semi-major axis to be 0.90 ± 0.02 AU, the eccentricity to be 0.32 ± 0.02 and the inclination of the system to be 34 ± 3 degrees (1-σ). The total mass is found to be 0.123 +0.027 −0.022 M ⊙ (3-σ). In addition, we have obtained low resolution (R = 1500-1700) near-infrared spectra of each of the components in the J-and K-bands. We determine the spectral types of the objects to be M8.5V (Gl 569Ba) and M9V (Gl 569Bb) with an uncertainty of half a subclass. We also present new J-and K-band photometry which allows us to accurately place the objects in the HR diagram. Most likely the binary system is comprised of two brown dwarfs with a mass ratio of 0.89 and with an age of approximately 300 Myr.

We present improved timing parameters for 13 millisecond pulsars (MSPs), including nine new proper motion measurements. These new proper motions bring to 23 the number of MSPs with measured transverse velocities. In light of these new results we present and compare the kinematic properties of MSPs with those of ordinary pulsars. The mean transverse velocity of MSPs was found to be 85^13 km s 21 Y a value consistent with most models for the origin and evolution of MSPs and approximately a factor of 4 lower than that of ordinary pulsars. We also find that, in contrast to young ordinary pulsars, the vast majority of which are moving away from the Galactic plane, almost half of the MSPs are moving towards the plane. This near-isotropy would be expected of a population that has reached dynamic equilibrium. Accurate measurements of MSP velocities have allowed us to correct their measured spin-down rates for Doppler acceleration effects, and thereby derive their intrinsic magnetic field strengths and characteristic ages. We find that close to half of our sample of MSPs have a characteristic age comparable to or greater than the age of the Galactic disc.

In radio astronomy the polarimetric properties of radiation are often modified during propagation and reception. Effects such as Faraday rotation, receiver cross-talk, and differential amplification act to change the state of polarized radiation. A general description of such transformations is useful for the investigation of these effects and for the interpretation and calibration of polarimetric observations. Such a description is provided by the Lorentz group, which is intimately related to the transformation properties of polarized radiation. In this paper the transformations that commonly arise in radio astronomy are analyzed in the context of this group. This analysis is then used to construct a model for the propagation and reception of radio waves. The implications of this model for radio astronomical polarimetry are discussed.Comment: 10 pages, accepted for publication in Astrophysical Journa