Estimation of an atomic clock's frequency stability, separate from its reference, is often done using a three-cornered hat procedure. A major requirement for the success of this method is that clocks be uncorrelated. If this requirement is not satisfied, the three-cornered hat procedure can lead to misleading or even negative variance estimates. Others have considered this problem and developed an analysis that allows for the possibility of cross correlation between clocks. We have extended and applied these ideas to obtain mathematically consistent frequency stability estimates on atomic clock data from the U.S. Naval Observatory. In addition, we derived an expression for the clock weights that produce a minimum variance combination of clocks in the presence of correlations.

We present a method for estimating the absolute frequency stability of iliV clocks separate from a reference. The method introduced is a modification of the one proposed by Tauella and Premoli (1993). After developing the theory we apply the method to atomic clock data gathered from the USNO.

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 less than 0.11 h-2 times the closure density of the universe.

We propose a statistical measure suitable for com- paring the rotational stabilities of pulsars with one another and with terrestrial time scales. By a simple extension of notation long used in the clock community, we call the statisticz(�). Defined in terms of third-order polynomials fitted to sequences of measured time offsets, �z(�) is sensitive to variations in the frequency drift rate of the clock or pulsar. We apply the new sta- tistical formalism to real pulsar data and terrestrial time scales, and show that over the past 10 years these two kinds of data have comparable stabilities.

We have organized a system to provide pulsar profiles and time-of-arrivals (TOAs) via anonymous ftp and the World Wide Web (WWW) at the address http://orion.usno.navy.mil/archive.html.

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.

Recent improvements in cesium and hydrogen terrestrial frequency standards have brought the frequency precision of International Atomic Time (TAI) to a value of 2.5E-15 s/s over an averaging time of a month. In this paper we illustrate the improvement graphically, and discuss the state of the art for frame ties between the radio, dynamical, and optical frames. In a larger paper, available via the World Wide Web, we illustrate the measured accuracy curves of the frequency standards, show their effect on the ensemble time scales, explain the reasons for the confusing array of available time scales, and discuss the inverse problem of using pulsar data to correct the terrestrial time scale.

The best current approximation to an inertial reference frame is provided by Very Long Baseline Interferometry (VLBI) observations of extragalactic radio sources with red shifts (z) up to 3.8. The stability of the resulting reference frame directly depends on the amount of any secular changes in the observed source positions.Two types of potentially observable secular motions should be present in extragalactic source positions. Gravitational accelerations of the solar system will cause secular motions through aberration, amounting to, e.g., about 4 microarcsec (μas) year–1 due to the mass of the galaxy. Extragalactic mass concentrations will cause gravitational deflections in the apparent positions of more distant radio sources, and these will change with time as the mass concentrations evolve. This effect could easily cause secular motions of order 1 μas year–1 in some, or even most, radio sources with z ≥ 1.The present astrometric VLBI data set contains about one million observations over a 15 year period, with current source proper motion formal errors being as small as 2.5 μas year–1. Proper motion estimates from these data reveal many sources with statistically significant proper motion estimates of order 30 μas year–1, about an order of magnitude larger than expected. Work continues to determine if the observed motions are due to systematic errors or reflect true secular changes in source positions. The results from a continued proper motion analysis of the complete astrometric VLBI data set will be presented.

AbstractThe U.S. Naval Observatory Navnet program monitors changes in the rotation of the Earth on a regular basis using radio interferometric observations acquired with telescopes in Alaska, Hawaii, Florida, West Virginia and, in the past, Maryland; other radio telescopes have also participated occasionally. These observations have been used to derive a radio interferometric celestial reference system, Navy 1990-5, using two years of dual frequency measurements from 24-hour-duration observing sessions. A total of 84 extragalactic radio sources, mostly quasars, have been observed by the Navnet program to date, of which 70 currently have source position formal errors of one milli second of arc or less. The root mean square of the difference between source position estimates from the Navnet data and an independently derived catalog using completely different data is less than one milli second of arc in both right ascension and declination after the adjustment of an arbitrary rotational offset between the two celestial reference frames.

We present 2.7 and 8.1 GHz radio light curves for 46 compact radio sources observed with the Green Bank interferometer during the period 1979-1987. Flux densities for the period 1979-1985 were reported in a previous paper. The flux densities reported here extend the daily monitoring an additional 2 years. This marks the end of the dual use of the interferometer by the US Naval Observatory to measure Earth rotation parameters and by the Naval Research Laboratory to study compact source variability. The interferometer is currently dedicated to searching for extreme scattering events (ESEs). ESEs may be found in the radio light curves of 0300 + 470, 0333 + 321, 0954 + 658, 1502 + 106, 1611 + 343, 1821 + 107, and 2352 + 495 during the period 1979-1987, including three events observed in 0300 + 470 and 0333 + 321 during the additional 2 years reported here.

The authors present flux densities of SS 433 at 2695 and 8085 MHz spanning the period July 1979 - December 1985. The variations may be characterized as a clustering of flare events separated by periods of quiescent emission. A harmonic analysis reveals no significant periodicities in either the entire 7 yr data set or in subsets corresponding to active and quiescent time frames. There is, however, broadband power from roughly 50 to 200 days which fades during 1985. Moreover, the authors find that the temporal power distribution of SS 433 at both frequencies strongly resembles 1/f noise. Finally, a comparison with 408 MHz data (Bonsignori-Facondi et al., 1986) shows variations in the light curves to lag those at 2695 and 8085 MHz by approximately 4 days.

view Abstract Citations (13) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evidence for Refractive Scintillation of Two Extragalactic Sources Seen through the Galactic Disk Dennison, Brian ; Fiedler, R. ; Johnston, K. J. ; Spencer, J. H. ; Waltman, E. B. ; Angerhofer, P. E. ; Florkowski, D. R. ; Josties, F. J. ; Klepczynski, W. J. ; McCarthy, D. D. ; Matsakis, D. N. Abstract Extragalactic radio sources were qualitatively examined to determine if there are any cases in which apparent variations at 2.7 GHz are uncorrelated with variations at 8.1 GHz, as might be expected for extrinsically induced refractive scintillation. Two sources exhibiting this behavior were identified, NRAO 150 and 2013 + 370, both of which happen to lie within 2 deg of the Galactic plane. The calculated refractive scintillation properties, based on intrinsic VLBI sizes and scattering measurements in the Galactic disk, are in reasonable agreement with the observed fluctuations. The light curves of these sources are interpreted as significantly affected by refractive scintillation in the interstellar medium. The findings suggest that extragalactic sources seen through extended paths in the Galactic disk may be modulated by refractive scintillation even at gigahertz frequencies. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/164955 Bibcode: 1987ApJ...313..141D Keywords: Astronomical Spectroscopy; Extragalactic Radio Sources; Milky Way Galaxy; Refracted Waves; Scintillation; Interstellar Matter; Light Curve; Astrophysics; INTERSTELLAR: MATTER; RADIO SOURCES: GENERAL full text sources ADS | data products SIMBAD (2) NED (2)

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view Abstract Citations (61) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Daily Observations of Compact Extragalactic Radio Sources at 2695 and 8085 MHz, 1979--1985 Fiedler, R. L. ; Waltman, E. B. ; Spencer, J. H. ; Johnston, K. J. ; Angerhofer, P. E. ; Florkowski, D. R. ; Josties, F. J. ; Klepczynski, W. J. ; McCarthy, D. D. ; Matsakis, D. N. Abstract Radio observations of 33 compact extragalactic sources, obtained at 2.695 and 8.085 GHz using the Green Bank interferometer as part of a regular monitoring program during 1979-1985, are reported. The calibration and data-analysis procedures are described, and the results are presented in extensive tables and graphs. Although the light-curve variations are found to be dominated by intrinsic changes, some refractive interstellar scintillation effects and anomalous light-curve dropouts (tentatively attributed to occultation events) are also observed. An unexpected linear dependence of the rms variation at 2.695 GHz compared with that at 8.085 GHz was found, indicating that the onset and evolution of flares must be similar in different sources. Publication: The Astrophysical Journal Supplement Series Pub Date: November 1987 DOI: 10.1086/191228 Bibcode: 1987ApJS...65..319F Keywords: Extragalactic Radio Sources; Light Curve; Quasars; Radiant Flux Density; Variable Stars; Computational Astrophysics; Cross Correlation; Frequency Ranges; High Resolution; Astronomy; BL LACERTAE OBJECTS; QUASARS; RADIO SOURCES: VARIABLE full text sources ADS | data products SIMBAD (30) NED (26)

view Abstract Citations (33) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio Flux Density Variations of Cygnus X-3 Johnston, K. J. ; Spencer, J. H. ; Simon, R. S. ; Waltman, E. B. ; Pooley, G. G. ; Spencer, R. E. ; Swinney, R. W. ; Angerhofer, P. E. ; Florkowski, D. R. ; Josties, F. J. ; McCarthy, D. D. ; Matsakis, D. N. ; Reese, D. E. ; Hjellming, R. M. Abstract Variations in the flux density from Cyg X-3 are reported for the period October 1982 through March 1985 at 11 and 3.7 cm. Since 1972, Cyg X-3 has displayed 15 known large flaring events. Six of these events have taken place during the months of September and October. These radio flaring events are probably due to a variable accretion rate onto the collapsed star of the binary system. Based on this small sample of data, there is a possibility of a 120 day periodicity in these large flaring events. This may be due to the presence of a third body in the Cyg X-3 binary system which modulates the accretion rate. The recent spectacular event in September-October 1983 was also monitored at 6 cm. Publication: The Astrophysical Journal Pub Date: October 1986 DOI: 10.1086/164639 Bibcode: 1986ApJ...309..707J Keywords: Radiant Flux Density; Radio Sources (Astronomy); Stellar Radiation; X Ray Binaries; Radio Emission; Stellar Flares; Astrophysics; RADIO SOURCES: VARIABLE; STARS: RADIO RADIATION; X-RAYS: BINARIES full text sources ADS | data products SIMBAD (5) NED (1)