Robin T. Stebbins

Low-frequency gravitational-wave science with eLISA/NGO

We review the expected science performance of the New Gravitational-Wave Observatory (NGO, a.k.a. eLISA), a mission under study by the European Space Agency for launch in the early 2020s. eLISA will survey the low-frequency gravitational-wave sky (from 0.1 mHz to 1 Hz), detecting and characterizing a broad variety of systems and events throughout the Universe, including the coalescences of massive black holes brought together by galaxy mergers; the inspirals of stellar-mass black holes and compact stars into central galactic black holes; several millions of ultra-compact binaries, both detached and mass transferring, in the Galaxy; and possibly unforeseen sources such as the relic gravitational-wave radiation from the early Universe. eLISAs high signal-to-noise measurements will provide new insight into the structure and history of the Universe, and they will test general relativity in its strong-field dynamical regime.

The Solar Acoustic Spectrum and Eigenmode Parameters

The Global Oscillation Network Group (GONG) project estimates the frequencies, amplitudes, and linewidths of more than 250,000 acoustic resonances of the sun from data sets lasting 36 days. The frequency resolution of a single data set is 0.321 microhertz. For frequencies averaged over the azimuthal order m, the median formal error is 0.044 microhertz, and the associated median fractional error is 1.6 × 10−5. For a 3-year data set, the fractional error is expected to be 3 × 10−6. The GONG m-averaged frequency measurements differ from other helioseismic data sets by 0.03 to 0.08 microhertz. The differences arise from a combination of systematic errors, random errors, and possible changes in solar structure.

The inverted pendulum as a probe of anelasticity

We discuss the rich dynamics of an inverted pendulum kept from falling over by an anelastic flexure. The anelastic character of the flexure is dramatically emphasized by the pendulum. We report measurements showing internal friction in one flexure to be independent of frequency over a band of three to four decades. This anelastic behavior in flexures can limit the sensitivity in gravitational wave detectors and in other precision measurements.

Waves in the solar photosphere

Time-sequences of line profile data have been subjected to a unique analysis which produces an amplitude and phase of the velocity and intensity at several line depths for each time sample and spatial point on the Sun. The data have been filtered to pass only the frequencies and spatial wavenumbers of the 5-min band. Yet, a secondary oscillation emerges, the phase of which propagates downward. Empirical eigenfunctions for velocity and intensity are given, and the kinetic energy flux is computed.

LISA and its in-flight test precursor SMART-2

LISA will be the first space-home gravitational wave observatory. It aims to detect gravitational waves in the 0.1 MHz+1 Hz range from sources including galactic binaries, super-massive black-hole binaries, capture of objects by super-massive black-holes and stochastic background. LISA is an ESA approved Cornerstone Mission foreseen as a joint ESA-NASA endeavour to be launched in 2010-11. The principle of operation of LISA is based on laser ranging of test-masses under pure geodesic motion. Achieving pure geodesic motion at the level requested for LISA, 3×10^(−15) ms^(−2)/√Hz at 0.1 mHz, is considered a challenging technological objective. To reduce the risk, both ESA and NASA are pursuing an in-flight test of the relevant technology. The goal of the test is to demonstrate geodetic motion within one order of magnitude from the LISA performance. ESA has given this test as the primary goal of its technology dedicated mission SMART-2 with a launch in 2006. This paper describes the basics of LISA, its key technologies, and its in-flight precursor test on SMART-2.

On the Origin of Solar Oscillations

We have made high-resolution observations of the Sun in which we identify individual sunquakes and see power from these seismic events being pumped into the resonant modes of vibration of the Sun. A typical event lasts about 5 minutes. We report the physical properties of the events and relate them to theories of the excitation of solar oscillations. We also discuss the local seismic potential of these events.

An ultra-low-noise, low-frequency, six degrees of freedom active vibration isolator

To extend the operating frequency band of earth-based interferometric gravitational wave (GW) detectors down to 1 Hz, an unconventional system is required that provides approximately a factor of one million vibration isolation for the horizontal and more for the vertical, starting at 1 Hz. To this end, a three-stage ultra-low-noise, high-performance active and passive isolation system is being designed and constructed in order to demonstrate that a high degree of reduction can be achieved for vibrations at the support points of a GW detector’s final pendulum system. The first stage of this system has been fully characterized and is the main subject of this article. It is an equilateral triangular platform, 1.1 m on a side, with a total mass of 460 kg, including the vacuum system that will contain the other two stages. Active isolation is achieved by six servocontrol loops, using signals from low-frequency displacement sensors to feed back to noncontacting force transducers. The first stage has been active...

Current error estimates for LISA spurious accelerations

The performance of the LISA gravitational wave detector depends critically on limiting spurious accelerations of the fiducial masses. Consequently, the requirements on allowable acceleration levels must be carefully allocated based on estimates of the achievable limits on spurious accelerations from all disturbances. Changes in the allocation of requirements are being considered, and are proposed here. The total spurious acceleration error requirement would remain unchanged, but a few new error sources would be added, and the allocations for some specific error sources would be changed. In support of the recommended revisions in the requirements budget, estimates of plausible acceleration levels for 17 of the main error sources are discussed. In most cases, the formula for calculating the size of the effect is known, but there may be questions about the values of various parameters to use in the estimates. Different possible parameter values have been discussed, and a representative set is presented. Improvements in our knowledge of the various experimental parameters will come from planned experimental and modelling studies, supported by further theoretical work.

The global oscillation network group site survey

The Global Oscillation Network Group (GONG) Project is planning to place a set of instruments around the world to observe solar oscillations as continuously as possible for at last three years. The Project has now chosen the sites that will comprise the network. This paper describes the methods of data collection and analysis that were used to make this decision.Solar irradiance data were collected with a one-minute cadence at fifteen sites around the world and analyzed to produce statistics of cloud cover, atmospheric extinction, and transparency power spectra at the individual sites. Nearly 200 reasonable six-site networks were assembled from the individual stations, and a set of statistical measures of the performance of the networks was analyzed using a principal component analysis. An accompanying paper presents the results of the survey.

Multistage active vibration isolation system

The major obstacle to the detection of low-frequency gravitational waves with an earth-based interferometer is seismic noise. The current design of the initial Laser Interferometer Gravitational-Wave Observatory (LIGO) receiver, now under construction, projects that ground noise will limit the operating band to frequencies above 40 Hz. In this article, we describe recent progress on the JILA active vibration isolation system. This device is being constructed to demonstrate the technology needed for useful reduction of low-frequency seismic noise in a gravitational wave interferometer. It consists of three spring-mounted stages, each of which provides both active and passive isolation. To date, all of the control loops on the first two of the three stages have been closed. Together they can reduce large vibrations by at least 70 dB in both vertical and horizontal directions at 1.5 Hz and above.