We report observational upper limits on the mass-energy of the cosmological gravitational-wave background, from limits on proper motions of quasars.Gravitational waves with periods longer than the time span of observations produce a simple pattern of apparent proper motions over the sky, composed primarily of second-order transverse vector spherical harmonics. A fit of such harmonics to measured motions yields a 95%-confidence limit on the mass-energy of gravitational waves with frequencies ν < 2 × 10 −9 Hz, of < 0.11h −2 times the closure density of the universe.

Gravitational waves affect the observed direction of light from distant sources. At telescopes, this change in direction appears as periodic variations in the apparent positions of these sources on the sky; that is, as proper motion. A wave of a given phase, traveling in a given direction, produces a characteristic pattern of proper motions over the sky. Comparison of observed proper motions with this pattern serves to test for the presence of gravitational waves. A stochastic background of waves induces apparent proper motions with specific statistical properties, and so, may also be sought. In this paper we consider the effects of a cosmological background of gravitational radiation on astrometric observations. We derive an equation for the time delay measured by two antennae observing the same source in an Einstein-de Sitter spacetime containing gravitational radiation. We also show how to obtain similar expressions for curved Friedmann-Robertson-Walker spacetimes.Comment: 31 pages plus 3 separate figures, plain TeX, submitted to Ap

Two-way satellite time and frequency transfer (TWSTFT) has become an important technical component in the process of the realization of International Atomic Time. To employ the full potential of the technique, especially for true time transfer, a dedicated calibration is necessary. This consists of the calibration either of the operational link at large, including every component involved, or of the involved ground stations' internal delays only. Both modes were successfully employed by circulating and operating a portable reference station between the sites involved. In this paper, we summarize the theoretical background for the different calibration modes applied and report examples of results from the 13 calibration campaigns performed up to now in Europe and between Europe and the United States. In all of these exercises, estimated uncertainties around 1 ns were achieved. Consecutive campaigns showed a very good reproducibility at the nanosecond level. Additionally, we address and briefly discuss sources that possibly limit the uncertainty for true time transfer employing TWSTFT.

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We have constructed a new timescale, TT(IPTA16), based on observations of radio pulsars presented in the first data release from the International Pulsar Timing Array (IPTA). We used two analysis techniques with independent estimates of the noise models for the pulsar observations and different algorithms for obtaining the pulsar timescale. The two analyses agree within the estimated uncertainties and both agree with TT(BIPM17), a post-corrected timescale produced by the Bureau International des Poids et Mesures (BIPM). We show that both methods could detect significant errors in TT(BIPM17) if they were present. We estimate the stability of the atomic clocks from which TT(BIPM17) is derived using observations of four rubidium fountain clocks at the US Naval Observatory. Comparing the power spectrum of TT(IPTA16) with that of these fountain clocks suggests that pulsar-based timescales are unlikely to contribute to the stability of the best timescales over the next decade, but they will remain a valuable independent check on atomic timescales. We also find that the stability of the pulsar-based timescale is likely to be limited by our knowledge of solar-system dynamics, and that errors in TT(BIPM17) will not be a limiting factor for the primary goal of the IPTA, which is to search for the signatures of nano-Hertz gravitational waves.