MILLISECOND radio pulsars, which are old (∼109yr), rapidly rotating neutron stars believed to be spun up by accretion of matter from their stellar companions, are usually found in binary systems with other degenerate stars1. Using the 305-m Arecibo radiotelescope to make precise timing measurements of pulses from the recently discovered 6.2-ms pulsar PSR1257 +12 (ref. 2), we demonstrate that, rather than being associated with a stellar object, the pulsar is orbited by two or more planet-sized bodies. The planets detected so far have masses of at least 2.8 M⊕ and 3.4 M⊕ where M⊕ is the mass of the Earth. Their respective distances from the pulsar are 0.47 AU and 0.36 AU, and they move in almost circular orbits with periods of 98.2 and 66.6 days. Observations indicate that at least one more planet may be present in this system. The detection of a planetary system around a nearby (∼500 pc), old neutron star, together with the recent report on a planetary companion to the pulsar PSR1829–10 (ref. 3) raises the tantalizing possibility that a non-negligible fraction of neutron stars observable as radio pulsars may be orbited by planet-like bodies. The utility of the clock-like pulses emitted by pulsars [see Nature 217, 709-713 (1968)] did not end with the insights they have yielded into the properties of neutron stars, and hence of high-density matter. In 1992, Wolszczan and Frail reported precise pulsar timing measurements which exhibited periodic variations. Unlike similar observations reported in the previous year (cited as ref. 3 in the paper), these variations were not an artefact, but revealed the presence of two or more planet-sized objects orbiting the neutron star — the first such objects to be detected outside the Solar System.

A planetary system around the millisecond pulsar PSR1257 + 12

We present and analyse a catalogue of 233 pulsars with proper motion measurements. The sample contains a wide variety of pulsars including recycled objects and those associated with globular clusters or supernova remnants. After taking the most precise proper motions for those pulsars for which multiple measurements are available, the majority of the proper motions (58 per cent) are derived from pulsar timing methods, 41 per cent using interferometers and the remaining 1 per cent using optical telescopes. Many of the one-dimensional (1D) and two-dimensional (2D) speeds (referring to speeds measured in one coordinate only and the magnitudes of the transverse velocities, respectively) derived from these measurements are somewhat lower than earlier estimates because of the use of the most recent electron density model in determining pulsar distances. The mean 1D speeds for the normal and recycled pulsars are 152(10) and 54(6) km s −1 , respectively. The corresponding mean 2D speeds are 246(22) and 87(13) km s −1 . PSRs B2011+38 and B2224+64 have the highest inferred 2D speeds of ∼1600 km s −1 .W estudy the mean speeds for different subsamples and find that, in general, they agree with previous results. Applying a novel deconvolution technique to the sample of 73 pulsars with characteristic ages less than 3 Myr, we find the mean threedimensional (3D) pulsar birth velocity to be 400(40) km s −1 . The distribution of velocities is well described by a Maxwellian distribution with 1D rms σ = 265 km s −1 . There is no evidence for a bimodal velocity distribution. The proper motions for PSRs B1830−08 and B2334+61 are consistent with their proposed associations with the supernova remnants W41 and G114.3+0.3, respectively. Ke yw ords: stars: kinematics ‐ pulsars: general.

A statistical study of 233 pulsar proper motions

Formation and evolution of binary and millisecond radio pulsars

Neutron Stars 1: Equation of State and Structure

limiting flux density of the survey is about 0.2 mJy. At shorter or longer periods or higher dispersions, the sensitivity is reduced. Timing observations are carried out for pulsars discovered in the survey for 12‐18 months after confirmation to obtain accurate positions, spin parameters, dispersion measures, pulse shapes and mean flux densities. The survey is proving to be extremely successful, with more than 600 pulsars discovered so far. We expect that, when complete, this one survey will come close to finding as many pulsars as all previous pulsar surveys put together. The newly discovered pulsars tend to be young, distant and of high radio luminosity. They will form a valuable sample for studies of pulsar emission properties, the Galactic distribution and evolution of pulsars, and as probes of interstellar medium properties. This paper reports the timing and pulse shape parameters for the first 100 pulsars timed at Parkes, including three pulsars with periods of less than 100 ms which are members of binary systems. These results are briefly compared with the parameters of the previously known population.

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The Parkes multi-beam pulsar survey - I. Observing and data analysis systems, discovery and timing of 100 pulsars

We report the discovery of a remarkable pulsar with period 1.6 ms, moving in a nearly circular 9.17 hour orbit around a low mass companion star. At an observing frequency of 430 MHz, the pulsar, PSR 1957+20, is eclipsed once each orbit for about 50 minutes. For a few minutes before an eclipse becomes complete, and for more than 20 minutes after the signal reappears, the pulses are delayed by as much as several hundred microseconds — presumably as a result of propagation through plasma surrounding the companion. The pulsar’s orbit about the system barycenter has a radius of 0.089 light seconds projected onto the line of sight. The observed orbital period and size, together with the fact that eclipses occur, imply a surprisingly low companion mass, only a few percent the mass of the sun. The eclipsing mass extends well beyond the Roche lobe of the companion, and appears to possess a comet-like tail, strongly uggesting that the pulsar is evaporating its companion.

A millisecond pulsar in an eclipsing binary

Recent observations1,2 at the Arecibo Observatory have resulted in the discovery of PSR1855+09, a pulsar with period P=5.362 ms, moving in a nearly circular orbit of period 12.3 days. The pulsar is only the third one known with P < 10 ms, and the sixth known radio pulsar in a binary system. (Discovery of a seventh binary pulsar is announced in an accompanying paper3.) Three of the seven binaries are among the fastest five of more than 400 pulsars—a fact that provides strong support for the conclusion that fast pulsars are ‘recycled’ neutron stars, spun up during a phase of mass accretion from an evolving companion star. The pulsar has a small dispersion measure (13.3 cm−3 pc), suggesting a distance of only ∼350 pc. The proximity of this pulsar and its location within 25 pc of the galactic plane argue that millisecond pulsars form a significant fraction (∼10%) of the pulsar population, leaving many detectable ones undiscovered4,5. Its signal is strong enough to permit pulse-arrival-time measurements with single-day uncertainties of <3µs. Timing observations already suggest that PSR1855+09, like the 1.5-ms pulsar PS R1937+21, will prove to be a natural clock of extremely high stability6. The existence of a second pulsar with extremely small timing uncertainties will greatly aid the search for background gravitational waves using millisecond pulsars as detectors.

New millisecond pulsar in a binary system

Polarization data for PSR 1957 + 20, collected at 321, 430, and 606 MHz are presented. The integrated profile of the pulsar showed that the main pulse is narrower than that of any known pulsar, with the slight broadening at 321 MHz, which is ascribed to scattering in the interstellar medium. In addition, circular polarization observations of three eclipses at 430 MHz were made in an attempt to investigate the time delay noted previously by Fruchter (1988, 1989). In plots of magnetic induction vs orbital phase, average parallel magnetic fields of about -2.7 and 0.7 gauss were found before and after the eclipse, respectively.

PSR 1957 + 20 - Polarization and the nature of the eclipsing medium

We have observed the eclipsing millisecond pulsar PSR 1957+20 on 32 occasions during the interval of March-September 1988 at the Arecibo Observatory. We have made radio observations at 318 MHz and 430 MHz, primarily, but we also have obtained a small amount of data a t 606 MHz. We report on these observations here, concentrating on the frequency dependence of the eclipse duration and the posteclipse delay. The basic parameters of the system are presented in the discovery paper,' and polarization characteristics and differential Faraday delay observations are summarized by Thorsett et a1.' All of these data were obtained with the Princeton-Arecibo millisecond pulsar timing system' using coherent dispersion removal: Observations near 430 MHz were made with dual circularly polarized signals a t 425 MHz and 435 MHz with 400-kHz bandwidths a t each observing frequency. Observations with the 3 18-MHz feed were made with a single linear polarization and two 260-kHz-wide channels separated by up to 18 MHz. Synchronous pulse averages were recorded a t intervals as short as every 10 seconds, with 30 seconds being a more typical integration time. The diffractive scintillation time scale for this pulsar a t 430 MHz is about 120 seconds and the scintillation bandwidth is about 50 kHz,' so scintillation causes strong signal-strength variations in our data. Individual averages were recorded with a ratio of signal (at the pulse peak) to root-mean-square off-pulse noise of greater than loo:], but a 15:l ratio was more typical, and scintillation minima were encountered in which the signal disappeared entirely for up to five minutes. Timing observations obtained a t 430 MHz far from eclipse are useful for determining the orbital parameters of the system. The full results of our timing solution will be reported elsewhere, but a number of newly determined parameters are particularly important. The spin-down rate of the pulsar has been determined to be dP/dt = (1.2 * 0.1) x lo-*' s s-', implying a surface magnetic field of B = 1.4 x lo* gauss and a spin-down luminosity of L = 1 . 1 x lo3' ergs s-'. This is the lowest surface magnetic field determined for any of the 450 known pulsars. It is consistent with the standard spin-up scenario for the creation of millisecond pulsars (e.g., Alpar et aL6)

D. R. Stinebring

Papers

A millisecond pulsar in an eclipsing binary

New millisecond pulsar in a binary system

PSR 1957 + 20 - Polarization and the nature of the eclipsing medium

Eclipse Duration and Posteclipse Delay in PSR 1957 + 20