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 will modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrain 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 91-99.7% 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 result in an increased sensitivity to gravitational waves.Studies of the dynamics of stars and gas in nearby galaxies provide strong evidence for the ubiquity of supermassive (> 10 6 solar mass) black holes (SMBHs) (1). Observations of luminous quasars indicate that SMBHs are hosted by galaxies throughout the history of the universe (2) and affect global properties of the host galaxies (3). The prevailing dark energycold dark matter cosmological paradigm predicts that large galaxies are assembled through the hierarchical merging of smaller galaxies. The remnants of mergers can host gravitationally bound binary SMBHs with orbits decaying through the emission of gravitational waves (GWs) (4).Gravitational waves from binary SMBHs, with periods between ~ 0.1 and 30 yr (5), can be detected or constrained by monitoring, for years to decades, a set of rapidly rotating millisecond pulsars (MSPs) distributed throughout our galaxy. Radio emission beams from MSPs are observed as pulses that can be time-tagged with as small as 20 ns precision (6). When traveling across the pulsar-Earth line of sight, GWs induce variations in the arrival times of the pulses (7).The superposition of GWs from the binary SMBH population is a stochastic background (GWB), which is typically characterized by the strain-amplitude spectrum h c (f)=A c,yr [f/(1 yr -1 )] -2/3 , where f is the GW frequency, A c,yr is the characteristic amplitude of the GWB measured at f = 1 yr -1 , predicted to be A c,yr > 10 -15 (5,(8)(9)(10)(11)(12), and -2/3 is the predicted spectral index (5,(8)(9)(10)(11)(12). The GWB will add low-frequency perturbations to pulse arrival times. While the detection of the GWB would confirm the presence of a cosmological population of binary SMBHs, limits on its amplitude constrain models of galaxy and SMBH evolution (8).As part of the Parkes Pulsar Timing Array project to detect GWs (6), we have been monitoring 24 pulsars with the 64-m Parkes radio telescope. We have produced a new data set, using observations taken at a central wavelength of 10 cm and previously reported methods (6,8), that spans 11 yr, which is 3 yr longer than previous data sets analyzed at this wavelength. In addition to having greater sensitivity to the GWB because of the longer duration, the data set was improved by identifying and correc...

No abstract

We present timing models for 20 millisecond pulsars in the Parkes Pulsar Timing Array. The precision of the parameter measurements in these models has been improved over earlier results by using longer data sets and modelling the non-stationary noise. We describe a new noise modelling procedure and demonstrate its effectiveness using simulated data. Our methodology includes the addition of annual dispersion measure (DM) variations to the timing models of some pulsars. We present the first significant parallax measurements for PSRs J1024−0719, J1045−4509, J1600−3053, J1603−7202, and J1730−2304, as well as the first significant measurements of some post-Keplerian orbital parameters in six binary pulsars, caused by kinematic effects. Improved Shapiro delay measurements have resulted in much improved pulsar mass measurements, particularly for PSRs J0437−4715 and J1909−3744 with M p = 1.44 ± 0.07 M and M p = 1.47 ± 0.03 M respectively. The improved orbital period-derivative measurement for PSR J0437−4715 results in a derived distance measurement at the 0.16% level of precision, D = 156.79 ± 0.25 pc, one of the most fractionally precise distance measurements of any star to date.

We present detailed submillimeter-through centimeter-wave observations of the extraordinary extragalactic transient AT2018cow. The apparent characteristicsthe high radio luminosity, the rise and long-lived emission plateau at millimeter bands, and the sub-relativistic velocity -have no precedent. A basic interpretation of the data suggests E k 4 × 10 48 erg coupled to a fast but sub-relativistic (v ≈ 0.13c) shock in a dense (n e ≈ 3 × 10 5 cm −3 ) medium. We find that the X-ray emission is not naturally explained by an extension of the radio-submm synchrotron spectrum, nor by inverse Compton scattering of the dominant blackbody UVOIR photons by energetic electrons within the forward shock. By ∆t ≈ 20 days, the X-ray emission shows spectral softening and erratic inter-day variability. Taken together, we are led to invoke an additional source of X-ray emission: the central engine of the event. Regardless of the nature of this central engine, this source heralds a new class of