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Following initial experimental attempts in the 1960s to detect gravitational radiation through the change of length of a test bar, a number of prototype detectors were set up in the 1970s using laser interferometers to measure the separation of suspended test masses. These experiments with arm lengths of 3 to 40 m developed into schemes to build large gravitational wave observatories in several countries. At the same time the ideas for carrying out such interferometer experiments in space began to be developed. A space experiment offers the possibility of escaping from the low frequency background noise that is present on Earth due to seismic and atmospheric processes. It also offers the possibility of a very long path length which relaxes the requirements on position measurement noise. A space experiment thus opens up the possibility of searching for signals at high strain sensitivity, <10^-30Hz^-1/2, in the low frequency range of 10^-4 - 1 Hz. It is thought that this range will contain sources that will be detected with certainty and at a high signal-to-noise ratio, enabling gravitational theory to be tested in the strong gravitational fields that surround compact objects as well as providing astrophysical data on the sources themselves.

In 1992 the ESA's science programme commenced a study of the Satellite Test of the Equivalence Principle (STEP). This heralded an upsurge of interest in fundamental physics in ESA programmes which was strengthened by a proposal for LISA in 1993. After a number of assessment studies, LISA was included in 1995 as a cornerstone mission in ESA's Horizons 2000 programme.

The purpose of the First International LISA Symposium was to discuss the LISA mission and the astrophysical observations which it can make together with the relevant gravitational theory. Included in the programme were papers on the technology needed for LISA and also reports on recent progress and developments in ground based detectors of gravitational radiation.

Following the launch failure of ESA's cluster mission in June 1996 pressures on the Horizons 2000 programme have increased which may result in cornerstone missions such as LISA being delayed. It is unlikely that the high-frequency instruments being built on Earth will address to a significant degree the astrophysics and gravitational theory which will be studied with LISA. So it is important that every effort is made to launch LISA and thus open the low frequency window for gravitational radiation as soon as possible. A new study of LISA is in progress which takes advantage of new technology to reduce the number and cost of the space craft and this will be dealt with fully at the second International LISA Symposium which will be held in Hannover, 5 - 9 October 1998.

This special issue contains the papers from the 1996 Symposium that were submitted for publication. The papers have been refereed in accordance with the policy of the Classical and Quantum Gravity editorial board.

The Symposium was organised by the Space Science department at CLRC's Rutherford Appleton Laboratory. Sponsorship for the meeting and this publication was provided by Matra Marconi Space and the Gravitational Research Groups at the Universities of Glasgow, Hannover and Potsdam. Thanks are due to the members of the local Organizing Committee, the Science Programme Committee and others who acted as referees.

M C W Sandford

Scientific Programme Committee P Bender (JILA, USA) A Brillet (Orsay, France) K Danzmann (Hannover, Germany) J Hough (Glasgow, UK) Y Jafry (ESTEC, The Netherlands) M Sandford (RAL, UK) B Schutz (Cardiff, UK/Potsdam, Germany) D Shoemaker (MIT, USA) K Thorne (CalTech, USA) R Weiss (MIT, USA)

Local Organizing Committee M Caldwell S Clelland M C W Sandford N D Valentine

The European Space Agency has selected LISA, a gravitational wave observatory, as a cornerstone mission in its future science program Horizons 2000. This observatory will complement the development of ground-based gravitational wave detectors currently under construction. A spaceborne detector will enable the observation of low-frequency gravitational waves in a frequency range from to which is totally inaccessible to ground-based experiments. This frequency range is unique in that it is expected to contain signals from massive black holes, galactive binary stars, as well as the most violent events in the Universe.

LISA will attain this low-frequency sensitivity by employing laser interferometric distance measurements over a very long baseline of . Three of these baselines form an equilateral triangle with spacecraft at each vertex. The cluster of spacecraft is in an Earth-like orbit around the Sun trailing the Earth by .

The spacecraft contain infrared light-emitting Nd:YAG lasers and freely floating test masses made from a special platinum - gold alloy with vanishing magnetic susceptibility. The spacecraft are being kept centred on their test masses by using drag-free technology and field-emission electric propulsion, thus letting the test masses follow purely inertial orbits.

The Laser Interferometer Space Antenna (LISA) is a space mission designed to detect gravitational waves in the frequency range from below 0.0001 Hz to 1 Hz by measuring changes in the distance between spacecraft separated by several million kilometres. The spacecraft orbit in a triangular formation forming three (not independent) interferometers with arm lengths determined by the distances between the vertices. The nominal orbit configuration is described and contrasted with an alternative configuration. Changes in the distances between the vertices cause a Doppler shift in the laser signals between spacecraft. The size of the measurement error introduced by this Doppler shift is dependent on the stability of the spacecraft formation.

Young galaxies probably acquire central black holes which (during and soon after their formation) manifest themselves as quasars and may be reactivated later. It seems unlikely that the holes form suddenly enough to be intense sources of gravitational radiation. Their formation and growth may, however, yield lower level emission. There is a guarantee of intense bursts from later mergers of supermassive holes and of weak nearly periodic waves from stars captured into relativistic orbits around massive holes.

This paper gives an overview of binaries with ultra-short periods, discusses possible ways in which such binaries are formed and estimates the number of such binaries and their relative importance for the space-based gravitational wave detector LISA.

The capture and subsequent inspiral of stellar mass black holes on eccentric orbits by central massive black holes is one of the more interesting likely sources of gravitational radiation detectable by LISA. We estimate the rate of observable events and the associated uncertainties. A moderately favourable mass function could provide many detectable bursts each year, and a detection of at least one burst per year is very likely given our current understanding of the populations in cores of normal spiral galaxies.

Massive black hole binaries are one of the most interesting classes of sources that could be detectable by LISA, as they allow tests of general relativity in strong-field conditions and to probe into the very centre of galactic nuclei. We review our present knowledge about the scenario that leads to the formation of these systems and the event rate of detectable gravitational wave signals. We also discuss the use of a LISA-observation catalogue to gain information on the mass - redshift distribution of sources.

We have revised our earlier rough estimate of the combined galactic and extragalactic binary confusion noise level curve for gravitational waves. This was done to correct some numerical errors and to allow for roughly three frequency bins worth of information about weaker sources being lost for each galactic binary signal that is removed from the data. The results are still based on the spectral amplitude estimates for different types of galactic binaries reported by Hils et al in 1990, and assume that the gravitational wave power spectral densities for other galaxies are proportional to the optical luminosities. The estimated confusion noise level drops to the LISA instrumental noise level at between roughly 3 and 8 mHz.

The observed microwave background anisotropies in combination with the theory of quantum mechanically generated cosmological perturbations predict a measurable amount of relic gravitational waves in the frequency intervals tested by LISA and ground-based laser interferometers.

Asymmetric core-collapse supernova are promising sources of gravitational radiation. Asymmetries can arise because of both convective instabilities and rotation.

Convective instabilities occur in two distinct, spatially well separated regions during the first second of the explosion: (i) inside the proto-neutron star immediately below the neutrinosphere and (ii) in the neutrino-heated `hot-bubble' region interior to the outward propagating revived shock wave. The gravitational wave signature of both convective instabilities has recently been computed. One finds that for a supernova located at a distance of 10 kpc the maximum dimensionless gravitational wave amplitudes range from about to in two-dimensional models. Anisotropic emission of neutrinos resulting from the non-spherical stratification of the proto-neutron star also generates gravitational waves. The amplitudes can be larger than the wave amplitudes due to mass motions by a factor of 5 - 10. In three-dimensional simulations the gravitational wave amplitudes due to mass motion and anisotropic neutrino emission are reduced by an order of magnitude. Most of the gravitational radiation from convection inside the proto-neutron star is emitted in the frequency band 100 - 1000 Hz, while convective motions in the hot-bubble region generate waves from several 100 Hz down to a few Hz.

Pre-supernova models of rotating stars have not yet been calculated. Thus, in order to predict the gravitational wave signature of rotational core collapse one has to rely on parameter studies of the collapse of likely initial models. A recent comprehensive study shows that the dimensionless gravitational wave amplitudes are in the range for a source at a distance of 10 Mpc. The spectra cover a frequency range of 50 Hz to 3 kHz, with most of the power being emitted between 500 Hz and 1 kHz.

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa to be commissioned in the year 2000. Virgo has been designed to achieve a strain sensitivity of a few times at 200 Hz. A large effort has gone into the conception of the mirror suspension system, which is expected to reduce noise to the level of at 10 Hz. The expected signals and main sources of noise are briefly discussed; the choices made are illustrated together with the present status of the experiment.

GEO600, an interferometric gravitational-wave detector with an arm length of 600 m, is currently being built in northern Germany close to Hannover. GEO600 incorporates an externally modulated fourfold delay-line Michelson interferometer giving a round-trip optical length of 2400 m. A master - slave combination of a monolithic diode-pumped Nd:YAG ring laser and an injection-locked amplifier will give a light power of about 10 W at a wavelength of 1064 nm. Power recycling increases the light power inside the interferometer to a level of about 10 kW. The use of both power and signal recycling will yield a sensitivity of the same order of magnitude as the first stages of the other large-scale gravitational-wave detectors LIGO and VIRGO currently under construction. High signal recycling factors allow the sensitivity to be increased at a chosen frequency while reducing the bandwidth of the detector. This gives an advantage over broad-band detectors in detecting narrow-band periodic sources such as pulsars. The 25 cm diameter mirrors will be suspended as double pendulums from a platform supported by vibration-reduction systems. The passive filtering properties of this system sufficiently reduce the seismic noise in the frequency range of interest, i.e. 50 - 1000 Hz. The detector will start taking data in the year 2000.

TAMA is a Japanese joint project to build a 300 m baseline interferometer called TAMA300 which is now under construction at Mitaka, Tokyo. The motivation for this project is to develop techniques that are necessary for a future kilometre-sized interferometric detector and to catch any rare events occurring in nearby galaxies. The project, the interferometer and the construction status are reported in this paper.

An overview of the experiments for the search for gravitational waves by means of resonant detectors is given. Since 1990 cryogenic resonant antennas have been in operation and data have been recorded by Explorer, Allegro, Niobe and Nautilus. The sensitivity for pulse detection with SNR = 1 is now (corresponding to a total energy of less than 0.001 solar masses for a source in the Galactic Center). The sensitivity for monochromatic waves is for one year of integration and about for stochastic background detection.

Large resonant detectors operating at 1 kHz might reach, in the near future, a spectral amplitude sensitivity of the order of and, for pulse and monochromatic wave sensitivity, respectively, and . Cross-correlating two such large antennas for one year can give a sensitivity for stochastic background detection of the order of , corresponding to a ratio between the gravitational wave energy density to that needed for a closed Universe of .

A linearized version of the standard Wiener filter theory is used to calculate the sensitivity to an isotropic stochastic gravitational-wave background of the AURIGA - NAUTILUS pair of ultra-low-temperature bar detectors, near to operating in coincidence in Italy, obtaining a sensitivity of . The addition of the VIRGO interferometric detector under construction in Italy improves the figure to . We also consider the pair formed by VIRGO and one large mass spherical detector located in a nearby available site in Italy and a pair of spherical detectors located at the AURIGA and NAUTILUS sites that promise to achieve sensitivities of .

The first run of the ultracryogenic resonant bar detector AURIGA is in progress. Diagnostics on the cryogenics, the data acquisition system and on the noise characteristics have been performed, with results in accord with the design. The bar reached 140 mK. In tests down to 2 K the detector noise was very close to `Brownian'.

This paper is a brief summary of the most relevant features of the solution to the general problem of the coupled motion of a set of resonant transducers and a solid sphere when acted upon by a GW excitation, as recently investigated by us. A remarkably elegant theory emerges out of the analysis, which fully displays the system dynamics for arbitrary resonator configurations. The power of the method can be used to consider alternative layouts to the TIGA proposal, the virtues and/or drawbacks of which can then be assessed. A specific new resonator distribution will be presented which takes advantage of the significant cross section of a spherical GW detector at its second quadrupole resonance and which is also useful for eventual monopole radiation sensing.

Gravitational wave signals from a large number of astrophysical sources will be present in the LISA data. Information about as many sources as possible must be estimated from time series of strain measurements. Several types of signals are expected to be present: simple periodic signals from relatively stable binary systems, chirped signals from coalescing binary systems, complex waveforms from highly relativistic binary systems, stochastic backgrounds from galactic and extragalactic binary systems and possibly stochastic backgrounds from the early Universe. The orbital motion of the LISA antenna will modulate the phase and amplitude of all these signals, except the isotropic backgrounds and thereby give information on the directions of sources.

Here we describe a candidate process for disentangling the gravitational wave signals and estimating the relevant astrophysical parameters from one year of LISA data. Nearly all of the sources will be identified by searching with templates based on source parameters and directions.

LISA is a space-borne, laser-interferometric gravitational wave detector currently under study by the European Space Agency. We give a brief introduction about the main features of the detector, concentrating on its one-year orbital motion around the Sun. We show that the amplitude as well as the phase of a gravitational wave is modulated due to that motion, allowing us to extract information from the signal. The most common way to estimate the parameters which characterize a signal present in a noisy data stream is to use the matched filtering technique. A brief review of the theory of parameter estimation, based on the work of Finn and Cutler, will be given. We carried out a simulation of the detection of a monochromatic gravitational wave based on that theory and focusing on estimating the angular parameters of the source. The results of the semi-analytic calculations are presented in detail and interpreted to determine the angular resolution of LISA.

The sensitivity of LISA at frequencies above about 10 mHz is determined by the shot noise of the light together with the decreasing antenna transfer function, finally being limited by mechanical resonances above 1 Hz. In this paper we will investigate the transfer function of laser-interferometric gravitational-wave antennas in the upper frequency range, where the physical arm length of the antenna becomes comparable to or even greater than the wavelength of the gravitational waves. This analysis reveals results which are also interesting for ground-based detectors. Furthermore, we calculate the dependence of the LISA transfer function on orbit azimuth, source azimuth, source declination and polarization, and finally we form the average over these angles in order to arrive at a mean LISA sensitivity curve. It turns out that the resulting response is even better than estimated previously.

We address the problem of detecting a stochastic background originated by an anisotropic population of sources, like the galactic binaries, by exploiting the amplitude modulation of the signal as the antenna changes its orientation with respect to fixed stars. This modulation, if observed, could help in discriminating the background generated by galactic binaries from the isotropic gravitational wave background generated in the early Universe.

We discuss LISA's ability to resolve different polarizational states of a gravitational wave with fixed frequency and amplitude. Assuming a binary as the source of the gravitational wave, its orientation is connected with the polarization of the gravitational wave emitted. Using methods of signal processing, we calculate the 1- uncertainty range for measuring the orientation of the source.

We propose a multistep procedure for the on-line detection and analysis of gravitational wave signals emitted during the coalescence of compact binaries. This procedure, based on a hierarchical strategy, consists of a rough analysis of the gravitational wave signal using adaptive line enhancers (ALE) filters and the controlled random search (CRS) optimization algorithm followed by a refined analysis using the classic matched-filtering technique. The results of simulations for the rough analysis are quite promising both for the relatively small computational power needed and for the robustness of the algorithms used, so that it could be very helpful for gravitational wave detection with very large baseline interferometric detectors like LIGO and VIRGO.

This paper describes some investigations into the construction of a monolithic fused silica test mass suspension for use in interferometric gravitational wave detectors. We summarize results showing that the material Q factor of standard fused quartz in the form of ribbons is of a level which makes it suitable for use as a suspension material for the test masses of long baseline gravitational wave detectors and then present measurements of the Q factor of the pendulum mode of a non-conducting mass suspended on cylindrical fibres of fused quartz. Our results show that electrostatic charging of the mass can result in a significant decrease in the pendulum mode Q factor.

We report on the development of the frequency stabilization of diode-pumped Nd:YAG ring lasers for use on spaceborne gravitational wave detectors and give results on preliminary investigations of the frequency noise in the low-frequency domain.

A short review of the technology employed in the drag-free flights to date, on the Transit programme and planned missions is presented. The sensing, actuation, spacecraft design and control laws are discussed. The requirements for several quite different missions are presented and their comparison helps illustrate some aspects of drag-free design.

We report the key results of a study of the optimization of capacitive sensing for LISA. This work was a component of an ESA study into drag-free satellite control. We discuss the problems associated with the capacitive sensing and control of a proof mass which is electrically isolated and develop models for candidate non-resonant and resonant detection schemes. We calculate acceleration noise assuming an ideal control system whose only noise sources are the capacitive sensors and actuators. On the basis of this model it appears that the design goal of acceleration noise for the LISA mission is achievable using both resonant and non-resonant detection schemes. The residual acceleration noise on the proof mass is dominated by oscillator noise which produces force and displacement noise due to asymmetries in the electrode capacitance values and bridge components.

The build-up of electrical charge on the proof masses is an important disturbance for LISA. The charging is due to penetrating particle radiation (primarily protons) from cosmic rays, and from the Sun during solar flares. Estimates of charging rates have been computed using the GEANT Monte Carlo particle-transport code in combination with realistic proton flux models. The consequences of the charging are discussed.

We present one of the potentially significant limits to the sensitivity of the LISA interferometer - that of beam pointing noise - and describe one possible sensor whose fundamental sensitivity is significantly better than that required.

The shape of a wavefront in the far field is normally described by solving integral equations in the Fraunhofer formalism. In this paper the far field of a truncated Gaussian beam is treated as a superposition of Gauss - Hermitian modes.