I. H. Stairs

A Massive Pulsar in a Compact Relativistic Binary

Introduction Neutron stars with masses above 1.8 solar masses (M☉), possess extreme gravitational fields, which may give rise to phenomena outside general relativity. These strong-field deviations lack experimental confrontation, because they become observable only in tight binaries containing a high-mass pulsar and where orbital decay resulting from emission of gravitational waves can be tested. Understanding the origin of such a system would also help to answer fundamental questions of close-binary evolution. Artist’s impression of the PSR J0348+0432 system. The compact pulsar (with beams of radio emission) produces a strong distortion of spacetime (illustrated by the green mesh). Conversely, spacetime around its white-dwarf companion (in light blue) is substantially less curved. According to relativistic theories of gravity, the binary system is subject to energy loss by gravitational waves. Methods We report on radio-timing observations of the pulsar J0348+0432 and phase-resolved optical spectroscopy of its white-dwarf companion, which is in a 2.46-hour orbit. We used these to derive the component masses and orbital parameters, infer the system’s motion, and constrain its age. Results We find that the white dwarf has a mass of 0.172 ± 0.003 M☉, which, combined with orbital velocity measurements, yields a pulsar mass of 2.01 ± 0.04 M☉. Additionally, over a span of 2 years, we observed a significant decrease in the orbital period, P ˙ b obs =−8.6±1.4 μs year−1 in our radio-timing data. Discussion Pulsar J0348+0432 is only the second neutron star with a precisely determined mass of 2 M☉ and independently confirms the existence of such massive neutron stars in nature. For these masses and orbital period, general relativity predicts a significant orbital decay, which matches the observed value, P ˙ b obs / P ˙ b GR =1.05±0.18 . The pulsar has a gravitational binding energy 60% higher than other known neutron stars in binaries where gravitational-wave damping has been detected. Because the magnitude of strong-field deviations generally depends nonlinearly on the binding energy, the measurement of orbital decay transforms the system into a gravitational laboratory for an as-yet untested gravity regime. The consistency of the observed orbital decay with general relativity therefore supports its validity, even for such extreme gravity-matter couplings, and rules out strong-field phenomena predicted by physically well-motivated alternatives. Moreover, our result supports the use of general relativity–based templates for the detection of gravitational waves from merger events with advanced ground-based detectors. Lastly, the system provides insight into pulsar-spin evolution after mass accretion. Because of its short merging time scale of 400 megayears, the system is a direct channel for the formation of an ultracompact x-ray binary, possibly leading to a pulsar-planet system or the formation of a black hole. Pulsar Tests Gravity Because of their extremely high densities, massive neutron stars can be used to test gravity. Based on spectroscopy of its white dwarf companion, Antoniadis et al. (p. 448) identified a millisecond pulsar as a neutron star twice as heavy as the Sun. The observed binarys orbital decay is consistent with that predicted by general relativity, ruling out previously untested strong-field phenomena predicted by alternative theories. The binary system has a peculiar combination of properties and poses a challenge to our understanding of stellar evolution. Observations of a pulsar confirm general relativity in the strong-field regime and reveal a perplexing stellar binary. Many physically motivated extensions to general relativity (GR) predict substantial deviations in the properties of spacetime surrounding massive neutron stars. We report the measurement of a 2.01 ± 0.04 solar mass (M☉) pulsar in a 2.46-hour orbit with a 0.172 ± 0.003 M☉ white dwarf. The high pulsar mass and the compact orbit make this system a sensitive laboratory of a previously untested strong-field gravity regime. Thus far, the observed orbital decay agrees with GR, supporting its validity even for the extreme conditions present in the system. The resulting constraints on deviations support the use of GR-based templates for ground-based gravitational wave detectors. Additionally, the system strengthens recent constraints on the properties of dense matter and provides insight to binary stellar astrophysics and pulsar recycling.

Tests of general relativity from timing the double pulsar

The double pulsar system PSR J0737-3039A/B is unique in that both neutron stars are detectable as radio pulsars. They are also known to have much higher mean orbital velocities and accelerations than those of other binary pulsars. The system is therefore a good candidate for testing Einsteins theory of general relativity and alternative theories of gravity in the strong-field regime. 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. These tests use the theory-independent mass ratio of the two stars. By measuring relativistic corrections to the Keplerian description of the orbital motion, we find that the “post-Keplerian” parameter s agrees with the value predicted by general relativity within an uncertainty of 0.05%, the most precise test yet obtained. We also show that the transverse velocity of the systems center of mass is extremely small. Combined with the systems 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 not have formed through the core collapse of a helium star, as is usually assumed.

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

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.

Transient radio bursts from rotating neutron stars

The radio sky is relatively unexplored for transient signals, although the potential of radio-transient searches is high. This was demonstrated recently by the discovery of a previously unknown type of source, varying on timescales of minutes to hours. Here we report a search for radio sources that vary on much shorter timescales. We found eleven objects characterized by single, dispersed bursts having durations between 2 and 30 ms. The average time intervals between bursts range from 4 min to 3 h with radio emission typically detectable for <1 s per day. From an analysis of the burst arrival times, we have identified periodicities in the range 0.4–7 s for ten of the eleven sources, suggesting origins in rotating neutron stars. Despite the small number of sources detected at present, their ephemeral nature implies a total Galactic population significantly exceeding that of the regularly pulsing radio pulsars. Five of the ten sources have periods >4 s, and the rate of change of the pulse period has been measured for three of them; for one source, we have inferred a high magnetic field strength of 5 × 1013 G. This suggests that the new population is related to other classes of isolated neutron stars observed at X-ray and γ-ray wavelengths.

A radio pulsar/x-ray binary link

From X-ray Binary to Pulsar Pulsars with millisecond rotational periods are thought to originate from neutron stars in low-mass x-ray binaries that had their spin frequencies increased by long-lasting mass transfer from their companion stars. Using data from a radio pulsar survey, Archibald et al. (p. 1411, published online 21 May; see the Perspective by Kramer) found a neutron star in a low-mass X-ray binary that is in the process of turning into a radio millisecond pulsar. The system, which consists of a solar-like star and a 1.69-millisecond radio pulsar, has gone through a recent accretion phase, characteristic of low-mass X-ray binaries, but it shows no accretion disk anymore, confirming the evolutionary connection between millisecond radio pulsars and low-mass X-ray binaries. Radio observations reveal a system undergoing the transition from a low-mass x-ray binary star to a millisecond radio pulsar. Radio pulsars with millisecond spin periods are thought to have been spun up by the transfer of matter and angular momentum from a low-mass companion star during an x-ray–emitting phase. The spin periods of the neutron stars in several such low-mass x-ray binary (LMXB) systems have been shown to be in the millisecond regime, but no radio pulsations have been detected. Here we report on detection and follow-up observations of a nearby radio millisecond pulsar (MSP) in a circular binary orbit with an optically identified companion star. Optical observations indicate that an accretion disk was present in this system within the past decade. Our optical data show no evidence that one exists today, suggesting that the radio MSP has turned on after a recent LMXB phase.

A Radio Pulsar Spinning at 716 Hz

We have discovered a 716-hertz eclipsing binary radio pulsar in the globular cluster Terzan 5 using the Green Bank Telescope. It is the fastest spinning neutron star found to date, breaking the 24-year record held by the 642-hertz pulsar B1937+21. The difficulty in detecting this pulsar, because of its very low flux density and high eclipse fraction (∼40% of the orbit), suggests that even faster spinning neutron stars exist. If the pulsar has a mass less than twice the mass of the Sun, then its radius must be constrained by the spin rate to be <16 kilometers. The short period of this pulsar also constrains models that suggest that gravitational radiation, through an r-mode (Rossby wave) instability, limits the maximum spin frequency of neutron stars.

The Parkes Multibeam Pulsar Survey - VI. Discovery and timing of 142 pulsars and a Galactic population analysis

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 kpc 2 . 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.

Switched Magnetospheric Regulation of Pulsar Spin-Down

Pulsar Clocks Pulsars are rotating neutron stars whose rotation rates can be extremely stable, sometimes rivaling the precision atomic clock. Unfortunately, not all pulsars are this precise—most show irregularities in their rotation rates. Using a large data set collected over many years at Jodrell Bank in the United Kingdom, Lyne et al. (p. 408, published online 24 June) show that the rotation of pulsars is not modulated by a single spin-down rate but typically by two, each accompanied by a unique pulse profile. The irregularities are linked to abrupt quasiperiodic changes in the pulsars magnetosphere, observed as changes in pulse shape and spin-down rate. Thus, it may be possible to use pulse-shape information to improve the precision of pulsars as stable clocks that can be used as probes of gravitational physics. Irregularities in pulsar rotation rates can be explained by quasi-periodic, abrupt changes in the pulsar magnetosphere. Pulsars are famed for their rotational clocklike stability and their highly repeatable pulse shapes. However, it has long been known that there are unexplained deviations (often termed timing noise) from the rate at which we predict these clocks should run. We show that timing behavior often results from two different spin-down rates. Pulsars switch abruptly between these states, often quasi-periodically, leading to the observed spin-down patterns. We show that for six pulsars the timing noise is correlated with changes in the pulse shape. Many pulsar phenomena, including mode changing, nulling, intermittency, pulse-shape variability, and timing noise, are therefore linked and are caused by changes in the pulsar’s magnetosphere. We consider the possibility that high-precision monitoring of pulse profiles could lead to the formation of highly stable pulsar clocks.

The International Pulsar Timing Array project: using pulsars as a gravitational wave detector

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 (similar to 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.

Swings between rotation and accretion power in a binary millisecond pulsar

It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star’s rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales.