Lense-Thirring frame dragging induced by a fast-rotating white dwarf in a binary pulsar system.

Radio pulsars in short-period eccentric binary orbits can be used to study both gravitational dynamics and binary evolution. The binary system containing PSR J1141-6545 includes a massive white dwarf (WD) companion that formed before the gravitationally bound young radio pulsar. We observed a temporal evolution of the orbital inclination of this pulsar that we infer is caused by a combination of a Newtonian quadrupole moment and Lense-Thirring (LT) precession of the orbit resulting from rapid rotation of the WD. LT precession, an effect of relativistic frame dragging, is a prediction of general relativity. This detection is consistent with an evolutionary scenario in which the WD accreted matter from the pulsar progenitor, spinning up the WD to a period of <200 seconds.

The host galaxy of a fast radio burst.

In recent years, millisecond-duration radio signals originating in distant galaxies appear to have been discovered in the so-called fast radio bursts. These signals are dispersed according to a precise physical law and this dispersion is a key observable quantity, which, in tandem with a redshift measurement, can be used for fundamental physical investigations. Every fast radio burst has a dispersion measurement, but none before now have had a redshift measurement, because of the difficulty in pinpointing their celestial coordinates. Here we report the discovery of a fast radio burst and the identification of a fading radio transient lasting ~6 days after the event, which we use to identify the host galaxy; we measure the galaxy's redshift to be z = 0.492 ± 0.008. The dispersion measure and redshift, in combination, provide a direct measurement of the cosmic density of ionized baryons in the intergalactic medium of ΩIGM = 4.9 ± 1.3 per cent, in agreement with the expectation from the Wilkinson Microwave Anisotropy Probe, and including all of the so-called 'missing baryons'. The ~6-day radio transient is largely consistent with the radio afterglow of a short γ-ray burst, and its existence and timescale do not support progenitor models such as giant pulses from pulsars, and supernovae. This contrasts with the interpretation of another recently discovered fast radio burst, suggesting that there are at least two classes of bursts.

Gravitational waves from binary supermassive black holes missing in pulsar observations.

Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems would modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrained the characteristic amplitude of this background, A(c,yr), to be <1.0 × 10(-15) with 95% confidence. This limit excludes predicted ranges for A(c,yr) from current models with 91 to 99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments and that higher-cadence and shorter-wavelength observations would be more sensitive to gravitational waves.

Gravitational-wave limits from pulsar timing constrain supermassive black hole evolution.

The formation and growth processes of supermassive black holes (SMBHs) are not well constrained. SMBH population models, however, provide specific predictions for the properties of the gravitational-wave background (GWB) from binary SMBHs in merging galaxies throughout the universe. Using observations from the Parkes Pulsar Timing Array, we constrain the fractional GWB energy density (Ω(GW)) with 95% confidence to be Ω(GW)(H0/73 kilometers per second per megaparsec)(2) < 1.3 × 10(-9) (where H0 is the Hubble constant) at a frequency of 2.8 nanohertz, which is approximately a factor of 6 more stringent than previous limits. We compare our limit to models of the SMBH population and find inconsistencies at confidence levels between 46 and 91%. For example, the standard galaxy formation model implemented in the Millennium Simulation Project is inconsistent with our limit with 50% probability.

A population of fast radio bursts at cosmological distances.

Searches for transient astrophysical sources often reveal unexpected classes of objects that are useful physical laboratories. In a recent survey for pulsars and fast transients, we have uncovered four millisecond-duration radio transients all more than 40° from the Galactic plane. The bursts' properties indicate that they are of celestial rather than terrestrial origin. Host galaxy and intergalactic medium models suggest that they have cosmological redshifts of 0.5 to 1 and distances of up to 3 gigaparsecs. No temporally coincident x- or gamma-ray signature was identified in association with the bursts. Characterization of the source population and identification of host galaxies offers an opportunity to determine the baryonic content of the universe.

Transformation of a star into a planet in a millisecond pulsar binary.

Millisecond pulsars are thought to be neutron stars that have been spun-up by accretion of matter from a binary companion. Although most are in binary systems, some 30% are solitary, and their origin is therefore mysterious. PSR J1719-1438, a 5.7-millisecond pulsar, was detected in a recent survey with the Parkes 64-meter radio telescope. We show that this pulsar is in a binary system with an orbital period of 2.2 hours. The mass of its companion is near that of Jupiter, but its minimum density of 23 grams per cubic centimeter suggests that it may be an ultralow-mass carbon white dwarf. This system may thus have once been an ultracompact low-mass x-ray binary, where the companion narrowly avoided complete destruction.

A test of general relativity from the three-dimensional orbital geometry of a binary pulsar.

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 to be consistent with the emission of gravitational waves as predicted by general relativity, but independent verification was not possible. Such verification 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.