Table of contents

In June 2006 the LISA International Science Team (LIST) accepted the bid presented by the Institut d'Estudis Espacials de Catalunya (IEEC) to host the 7th International LISA Symposium. This was during its 11th meeting at the University of Maryland, just before the 6th edition of the Symposium started in NASA's Goddard Space Flight Center.

The 7th International LISA Symposium took place at the city of Barcelona, Spain, from 16–20 June 2008, in the premises of CosmoCaixa, a modern Science Museum located in the hills near Tibidabo. Almost 240 delegates registered for the event, a record breaking figure compared to previous editions of the Symposium. Many of the most renowned world experts in LISA, Gravitational Wave Science, and Astronomy, as well as Engineers, attended LISA 7 and produced state-of-the-art presentations, while everybody benefited from the opportunity to have live discussions during the week in a friendly environment.

The programme included 31 invited plenary lectures in the mornings, and 8 parallel sessions in the afternoons. These were classified into 7 major areas of research: LISA Technology, LISA PathFinder, LISA PathFinder Data Analysis, LISA Data Analysis, Gravitational Wave sources, Cosmology and Fundamental Physics with LISA and Other Gravitational Wave Detectors. 138 abstracts for communications were received, of which a selection was made by the session convenors which would fit time constraints. Up to 63 posters completed the scientific programme. More details on the programme, including some of the talks, can be found at the Symposium website:http://www.ice.cat/research/LISA_Symposium. There was however a remarkable add-on: Professor Clifford Will delivered a startling presentation to the general public, who completely filled theAuditori—the main Conference Room, 320 seats—and were invited to ask questions to the speaker who had boldly guided them through the daunting world of Black Holes, Waves of Gravity, and other Warped Ideas of Dr Einstein.

The Proceedings of the 7th International LISA Symposium are jointly published by Classical and Quantum Gravity (CQG) andJournal of Physics: Conference Series (JPCS). This formula has a precedent in the last Amaldi Conference (Sydney 2007), and was motivated by the impossibility to fit all communications into a single CQG volume. Plenary speakers were invited to submit their contributions to CQG, and so were a number of parallel session authors chosen by the session convenors and the Science Organising Committee (SOC). Authors of the other parallel session presentations and posters were invited to submit to JPCS. All papers have been peer reviwed prior to being accepted for publication in either journal, and the whole set is well representative of the talks we heard during the Symposium.

Thanks are accordingly due to all authors for their collaborative attitude and, more generally, to all delegates who came to Barcelona and made of the Symposium a first class scientific event. The LISA community has been steadily growing since the Symposium took off in Chilton, near Oxford (UK) back in 1996. The support of such community strongly endorses a complex mission Project, whose short term future requires such support for a much longer term new era of Gravitational Wave Astronomy and Fundamental Physics. In this sense, the number of attendees and their active interest in the LISA mission sparks optimism.

The 7th International LISA Symposium sponsors are also sincerely acknowldged. They are: the Albert Einstein Institute (Hannover), the Spanish Ministry of Science and Innovation, the Generalitat de Catalunya (AGAUR), the Barcelona Institute of High Energy Physics (IFAE), the University of Barcelona (UB), the Polytechnique University of Catalunya (UPC), the Spanish Society of General Relativity and Gravitation (SEGRE), CosmoCaixa, NASA and the European Space Agency (ESA). The latter provided the LISA PathFinder model, a 1:4 scale model whose primer display we enjoyed during the Symposium.

Finally, the Local Organising Committee and the IEEC staff have given their enthusiastic support to the organisation in every detail, and have efficiently worked for months to make the Symposium happen. Many thanks to all of them, and congratulations.

Alberto Lobo and Carlos F Sopuerta Institut de Ciències de l'Espai (CSIC-IEEC) Guest Editors

This is a co-publication with Classical and Quantum Gravity. The bulk of the papers, after peer review, are published in Journal of Physics: Conference Series. However a selection of papers, are published separately in a special issue of Classical and Quantum Gravity.

Temperature sensors and heaters belong in the diagnostics subsystem of the LISA Technology Package (LTP) on board LISA Pathfinder, the technology demonstrator for LISA. A number of these diagnostics items are placed at short distances from the LTP proof masses, and are negative temperature coefficient (NTC) thermistors. By design, these devices have tiny amounts of ferromagnetic materials which therefore constitute a potential source of disturbance to the performance of the LTP. We present a detailed magnetic characterisation of the NTC's, and use the data to evaluate their impact on the acceleration noise budget of the LTP. The effect is seen to be small, and can be further reduced if the NTC's are submitted to a demagnetisation process before they are attached. Re-magnetisation is unlikely, as rather strong fields (mili-Tesla) are required to re-magnetise the NTC's

We investigate the potential of conducting interesting gravitational science experiments with LISA Pathfinder, by executing well defined de-orbiting manoeuvres following the nominal mission. Preliminary work suggests that the residual control authority of the micropropulsion system is sufficient to follow trajectories that cross the region surrounding the Sun-Earth saddle point, and also include one or multiple Earth flybys. Crossing the saddle point region may allow tests of Modified Newtonian Dynamics (MOND), while the flybys may potentially shed some light on the so-called flyby anomaly. We present some sample trajectories and discuss the limitations of the current model. Finally, we discuss the work required to take these ideas from the proof of principle presented here, to a concrete proposal for an extended mission.

Aside from LISA Pathfinder's top-level acceleration requirement, there is a stringent independent requirement for the accuracy of the optical metrology system. In case of a perfectly aligned metrology system (optical bench and test masses) it should rather be independent of residual displacement jitter due to control. However, this ideal case will not be achieved as mechanical tolerances and uncertainties lead to a direct impact of test mass and spacecraft displacement jitter on the optical measurement accuracy.

In this paper, we present a strategy how to cover these effects for a systematic requirement breakdown. We use a simplified nonlinear geometrical model for the differential distance measurement of the test masses which is linearized and linked to the equations of motion for both the spacecraft and the two test masses. This leads from test mass relative displacement to a formulation in terms of applied force/torque and thus allows to distinguish the absolute motion of each of the three bodies. It further shows how motions in each degree of freedom couple linearly into the optical measurement via DC misalignments of the laser beam and the test masses. This finally allows for deriving requirements on the alignment accuracy of components and on permittable closed-loop acceleration noise.

In the last part a budget for the expected measurement performance is compiled from simulations as no measurement data is available yet.

The Diagnostic Subsytem in the LISA Technology Package (LTP) on board the LISA Pathfinder mission (LPF) will characterise those external disturbances with a potential impact on the performance of the experiment coming from either thermal, magnetic or charged particles perturbations. A correct design of the experiments to measure these effects in flight requires a closed loop analysis that takes into account the dynamics of the test masses, the force applied by the controllers and those noisy terms (coming from sensing or force noise) that enters into the loop. We describe this analysis in the thermal case and we give a first numerical example of the instrument response to controlled thermal inputs.

The current design, and material implementation of the magnetic field sensing in the LISA Technology Package (LTP) on board LISA Pathfinder (LPF), is based on a set of 4 high-precision 3-axis fluxgate magnetometers. In order to avoid magnetic disturbances on the LTP proof masses (TM 's), originated by the sensors themselves, these are placed somewhat far from the TM's, which results in partial field information losses. We are currently investigating alternative magnetic sensing techniques, based on AMR (Anisotropic Magnetoresistive) devices. These are much smaller in size than fluxgates, therefore a more numerous array can be thought of for flight. In addition, there is a chance that they may be attached closer to the TM's, thereby enhancing magnetic field sensing spacial resolution. Several issues need to be addressed, such as real sensitivity (including electronics noise) and set/reset trigger procedures. A brief overview about the stability of the magnetic fields and gradients generated in the LTP by means of the coil will also be given. This paper show the latest results of our research.

Two key components of LISA's inter-spacecraft clock tone transfer chain are electro-optic modulators (EOMs) and high-frequency (HF) cable assemblies. At modulation frequencies of 2GHz, we characterized the excess phase noise of these components in the LISA frequency range (0.1 mHz to 1 Hz). The upper phase noise limit was found to be almost an order of magnitude better than required. In addition, phase dependencies on temperature were determined. The measured coefficients are within a few milliradians per Kelvin and thereby negligible due to the specified on-board temperature stability.

The LISA Technology Package (LTP) is the main payload onboard the LISA Pathfinder Spacecraft. The LTP Instrument together with the Drag-Free Attitude Control System (DFACS) and the respective LTP and DFACS operational software forms the LTP Experiment. It is completed by the FEEPs of the LPF spacecraft that are controlled by DFACS in order to control the spacecraft's attitude along with the experiment's needs. This article concentrates on aspects of the Industrial development of the LTP Instrument items and on essential performance issues of LTP. Examples of investigations on specific issue will highlight the kind of special problems to be solved for LTP in close cooperation with the Scientific Community.

A comprehensive electrostatic finite-element (FE) analysis of the LISA Pathfinder Inertial Sensor (IS) has been carried out at Astrium GmbH. Starting with a detailed geometrical model of the IS housing and test mass (TM) flight units, FE results were derived from multiple analyses runs applying the Maxwell 3D field simulation software. The electrostatic forces and torques on the TM in 6DoF, as well as all non-negligible capacitances between the TM, the 18 electrodes, and the housing, have been extracted for different TM translations and rotations. The results of the FE analyses were expected to confirm the existing IS electrostatic model predictions used for performance analysis, simulations, and on-board algorithms. Major discrepancies were found, however, between the results and the model used so far. In general, FE results give considerably larger capacitance values than the equivalent infinite non-parallel plate estimates. In contrast, the FE derived forces and torques are in general significantly lower compared to the analytic IS electrostatic model predictions. In this paper, these results are discussed in detail and the reasons for the deviations are elaborated. Based on these results, an adapted analytic IS electrostatic model is proposed that reflects the electrostatic forces, torques, and stiffness values in the LISA Pathfinder IS significantly more accurate.

The test masses of LISA Pathfinder are free flying and therefore not grounded to the spacecraft by a wire. Because of galactic cosmic rays, solar energetic particles, and unknown microscopic surface effects during initial test mass release, an unacceptable level of absolute charge might be present on the test masses. A charged test mass can endanger transition to high accuracy control modes which are required for science experiments. Furthermore, charged test masses introduce unwanted disturbance accelerations for example due to Coulomb interactions with surrounding conducting surfaces. The charge management system is designed to discharge the test masses up to a tolerable level of absolute charge such that the mission goal can be achieved. It is therefore an essential part of the experiments to be performed with the LISA Technology Package. The paper describes charge management tasks to be performed on board the spacecraft and summarizes the principles of charge measurement and discharge control. An overview of the experiment operations is given where the interconnection of operational charge management system modes and operational modes of the drag-free, suspension and attitude control system is considered. Simulated performance results are presented.

Gravitational coupling between the free-falling test masses and the surrounding spacecraft is one of the dominant noise sources for both LISA Pathfinder and LISA. At present, there are no plans to verify any of the self-gravity requirements by test, on the ground. Here, we explore the possibilities of conducting such tests, using a customised torsion balance. We discuss the main sources of systematic and statistical uncertainty present in such a set-up. Our preliminary assessment indicates that the sensitivity is sufficient to carry out meaningful self-gravity tests.

This paper presents the current state of the LISA Technology Package (LTP) fibre injector qualification project in terms of vibration and shock tests. The fibre injector is a custom built part and therefore must undergo a full space qualification process. The mounting structure and method for sinusoidal vibration and random vibration tests as well as shock tests will be presented. Furthermore a proposal will be presented to use the fibre injector pair qualification model to build an optical prototype bench. The optical prototype bench is a full-scale model of the flight model. It will be used for development and rehearsal of all the assembly stages of the flight model and will provide an on-ground simulator for investigation as an updated engineering model.

In the last few years the Lisa group in Napoli has developed an Optical Read-Out (ORO) system based on optical levers as an auxiliary and backup readout for the Gravitational Reference Sensor (GRS) of LISA. Bench-top measurements, with a rigid set-up have successfully proven that the ORO fits the requirements for sensitivity both in translational and rotational DOFs, exceeding the capacitive sensor performance in a wide range of frequencies. Last year an ORO system designed in Napoli in collaboration with the Trento LISA group, has been installed, as an auxiliary readout system, on the four mass torsion pendulum developed in Trento. In this paper we report on the testing of this ORO device and its performances in comparison with the capacitive one; we also outline further improvements and their advantages for the torsion pendulum facility performances.

In the context of the LISA Mission Formulation Study, the LISA System was studied in detail and a new baseline architecture for the whole mission was established. This new baseline is the result of trade-offs on both, mission and system level. The paper gives an overview of the different mission scenarios and configurations that were studied in connection with their corresponding advantages and disadvantages as well as performance estimates. Differences in the required technologies and their influence on the overall performance budgets are highlighted for all configurations. For the selected baseline concept, a more detailed description of the configuration is given and open issues in the technologies involved are discussed.

The objective of the constellation acquisition phase for the LISA mission is to establish the three laser links between the three spacecraft of the LISA constellation so that the interferometric measurements for the science experiment can commence. The laser beam acquisition for LISA is extremely challenging given the 5 million km distance between the spacecraft, the inherent limits of the attitude sensors accuracy, the orbit determination accuracy issues and the time required to phase-lock the incoming and outgoing laser signals. This paper presents the design of the control system for the acquisition phase of the LISA constellation: the acquisition operational procedure is outlined, guidance laws are defined together with the Gyro Mode attitude control principle, which implements a Kalman filter for disturbances rejection purposes. Constellation-wide non-linear simulations demonstrate that the LISA constellation acquisition phase is feasible by means of the proposed control strategy.

We present formulae for the amplitude modulation of the X,Y and Z TDI combinations for monochromatic gravitational waves and use these to study the LISA angular resolution in the case of large SNR. The angular resolution, Δθ/(0.1 * SNR), is found to lie between 2° and 5° and is somewhat dependent on the ecliptic colatitude angle (β), on the polarisation (h+,h_x) and much less on the ecliptic longitude angle (λ). Comparisons with other studies are presented. Future studies will treat the case of small SNR.

We have investigated two alternative laser systems for the Laser Interferometer Space Antenna (LISA). One consisted of the laser of LISA's technology precursor LISA Pathfinder and a fiber amplifier originally designed for a laser communication terminal onboard TerraSar-X. The other consisted of a commercial fiber distributed feedback (DFB) laser seeding a fiber amplifier. We have shown that the TerraSar-X amplifier can emit more than 1W without the onset of stimulated Brillouin scattering as required by LISA. We have measured power noise and frequency noise of the LISA Pathfinder laser (LPL) and the fiber laser. The fiber laser shows comparable or even lower power noise than the LPL. LISA will use electro-optical modulators (EOMs) between seed laser and amplifier for clock noise comparison between spacecraft. This scheme requires that the excess noise added by the amplifiers be negligible. We have investigated the phase characteristics of two fiber amplifiers emitting 1 W and found them to be compatible with the LISA requirement on amplifier differential phase noise.

We present the status of our investigations on the LISA Phasemeter. The new prototype is based on a custom-designed breadboard with four high-speed ADC and two DAC channels, extended readout capabilities and a large FPGA (field programmable gate array). The required main functionalities and performance of the prototype have been demonstrated in laboratory conditions.

Among the many interesting possibilities for types of future missions that would benefit strongly from LISA and LISA Pathfinder technology development, three will be discussed. They are in the fields of fundamental physics, Earth science, and gravitational wave astronomy. The first is a mission to measure the gravitational time delay due to the Sun from a spacecraft near the L-1 point of the Earth-Sun system. It would require gravitational reference sensors (GRSs) with roughly 10-¹³ [10-⁶ Hz/f] m/s² /vHz performance at frequencies down to about 0.3 microHz. The second type of mission is future drag-free missions to measure time variations in the Earth's gravitational field. One example of such a mission will be described, with two satellites in the same polar orbit at about 300 km altitude. Changes in the roughly 50 km satellite separation would be measured with 10-¹⁴ or better accuracy, and spurious accelerations of the test masses in the GRS on each satellite would be the other main measurement accuracy limitation. The third mission is a possible moderately improved LISA follow-on mission aimed at being able to detect mergers of 10 solar mass black holes with IMBHs out to redshifts of about 10 in order to investigate the formation and growth of IMBHs in more detail than LISA will be able to achieve.

Space-borne gravitational wave observatories like the Laser Interferometer Space Antenna (LISA) and those beyond, which may utilize a Modular Gravitational Reference Sensor (MGRS), greatly benefit from precise knowledge of the mass center location and moment of inertia tensor of the test mass prior to launch. The motion of the mass center of a drag-free test mass, which follows a pure geodesic, must be inferred from measurements of the surface. Therefore, knowledge of the mass center is critical for calibration of the cross-coupling between rotational and translational degrees of freedom. Together with the moment of inertia tensor, the mass center can also provide an estimate of the material density inhomogeneity to quadratic order, and the gravitational potential to second order, which improves modeling of self gravitation forces. These benefits, which are independent of the test mass shape, motivate the development of three new techniques for improving mass center and moment of inertia measurements beyond the current state of the art. A static pendulum is proposed to determine the mass center of a cubic test mass to ∼ 1 μm by measuring the equilibrium position with the cube in up to 24 different orientations relative to the pendulum platform. Measuring the natural frequency of a dynamic torsion pendulum can determine both the mass center and moment of inertia tensor of arbitrarily shaped objects to ∼ 5 μm and 1 part in ∼ 10⁴ respectively. The velocity modulation technique for measuring the mass center of a sphere has raised the bar in precision to ∼ 150 nm, a factor of 20 improvement over the work presented at the LISA 6th symposium. This new technique involves rolling the sphere down a set of parallel rails to spectrally shift the mass center offset information to the rolling rate frequency, in order to avoid the 1/f noise that typically prevents other techniques from achieving precision below 1 μm.

This paper presents the implementation of an analog optical phase-locked-loop with an offset frequency of about 20MHz between two lasers, where the detected light powers were of the order of 31 pW and 200 μW. The goal of this setup was the design and characterization of a photodiode transimpedance amplifier for application in LISA. By application of a transimpedance amplifier designed to have low noise and low power consumption, the phase noise between the two lasers was a factor of two above the shot noise limit down to 60mHz. The achievable phase sensitivity depends ultimately on the available power of the highly attenuated master laser and on the input current noise of the transimpedance amplifier of the photodetector. The limiting noise source below 60mHz was the analog phase measurement system that was used in this experiment. A digital phase measurement system that is currently under development at the AEI will be used in the near future. Its application should improve the sensitivity.

The Laser Interferometer Space Antenna (LISA) mission, a space based gravitational wave detector, uses laser metrology to measure distance fluctuations between proof masses aboard three spacecraft. LISA is unique from a mission design perspective in that the three spacecraft and their associated operations form one distributed science instrument, unlike more conventional missions where an instrument is a component of an individual spacecraft. The design of the LISA spacecraft is also tightly coupled to the design and requirements of the scientific payload; for this reason it is often referred to as a "sciencecraft." Here we describe some of the unique features of the LISA spacecraft design that help create the quiet environment necessary for gravitational wave observations.

Polarization maintaining single-mode optical fibers are key components in the interferometry of the Laser Interferometer Space Antenna (LISA). LISA's measurement principle relies on the availability of space qualified fibers of this type which influence the phase of light with a wavelength of 1064 nm passing in opposite directions through them with differences smaller than 6 prad/. We present a measurement scheme suitable to sense these non-reciprocal phase changes, as well as results obtained using this setup on samples of commercially available fibers. The experimental setup for the fiber characterization consists of a quasi-monolithic interferometer which constitutes a representative cut-out of the local interferometry on-board LISA concerning the fiber. Several noise sources are identified and improvements to the setup are presented to overcome them. The noise level achieved using this setup is between approximately 40 prad/ and 400 prad/ in the frequency range between 1 mHz and 1 Hz. It is also verified that this noise level is limited by the setup and not introduced by the fiber.

The Gravity Recovery and Climate Experiment (GRACE) is one of the present missions to map the Earth's gravity field. The aim of a GRACE follow-on mission is to map the gravitational field of the Earth with higher resolution over at least 6 years. This should lead to a deeper insight into geophysical processes of the Earth's system. One suggested detector for this purpose consists of two identical spacecraft carrying drag-free test masses in a low Earth orbit at an altitude of the order of 300 km, following each other with a distance on the order of 50 to 100 km. Changes in the Earth's gravity field will induce distance fluctuations between two test masses on separate spacecraft. These variations in the frequency range 1 to 100 mHz are to be monitored by a laser interferometer with nanometer precision. We present preliminary results of a heterodyne interferometer configuration using polarising optics, demonstrating the required phase sensitivity.

LISA relies on several techniques to reduce the initial laser frequency noise in order to achieve an interferometric length measurement with an accuracy of ≈ 10pm/. LISA will use ultra-stable reference cavities as a first step to reduce the laser frequency noise. In a second step the frequency will be stabilized to the LISA arms which provide a better reference in the frequency band of interest. We present experimental results demonstrating Arm locking with LISA-like light travel times and Doppler shifts. We also integrated this system with a LISA-like pre-stabilization system using our ultra-stable cavities. The addition of realistic Doppler shifts led to further refinements of the arm locking controllers compared to the controller architecture discussed in the past. A first experimental result of the new controller is also presented.

In order to implement optical inter-spacecraft ranging and data transfer for LISA, the carrier of the laser link must be phase modulated with pseudo-randon noise sidebands. The data acquisition and delay estimation are then implemented in the phasemeter as back end processing. This work presents a proposed demodulation scheme with submeter ranging accuracy and several kilobytes data rate. Its functionality is demonstrated both in a software simulation and in a FPGA-based hardware implementation.

The Modular Gravitational Reference Sensor (MGRS) is targeted as a next generation core instrument for both space gravitational wave detection and an array of other precision gravitational experiments in space. The objectives of the NASA funded program are to gain a system perspective of the MGRS, to develop key component technologies, and to establish important test platforms. Our original program was very aggressive in proposing ten areas of research and development. Significant advancements have been made in these areas, and we have met or exceeded the goals for the program set in 2007-2008. Additionally, we have initiated research projects for innovative technologies beyond the original plan. In this paper we will give a balanced overview of progress in MGRS technologies: the two layer sensing and control scheme, trade-off studies of GRS configurations, multiple optical sensor signal processing, optical displacement and angular sensors, differential optical shadow sensing, diffractive optics, proof mass center of mass and moment of inertia measurement, UV LED charge management, proof mass fabrication, thermal control and sensor development, characterization for various proof mass shapes, and alternative charge manage techniques.

We report on LISA experimental projects being pursued at JILA. Our focus is on the design and testing of a flight-compatible laser stabilization reference cavity. This is a dual cylinder ULE cavity, designed to provide high thermal and thermo-mechanical isolation in the millihertz frequency regime of interest to LISA. A modification of this hard-mounted design may allow for use in space without the need for clamping during launch. Progress so far consists of initial design, performance estimates, and construction. Simple thermal model calculations on the design indicate a thermal attenuation of 10⁶ at 1 mHz, corresponding to a cavity strain of 3*10-¹⁶ /rtHz for a 0.01 K/rtHz stability of the mounting surface. Finite element analysis indicates cavity strain attenuation of 5*10⁷ or better due to thermo-mechanical effects in the surrounding environment, and low sensitivity to vibration along the cavity axis. Setup and testing of two identical cavities and a laser-locking test system is ongoing. Another project was recently concluded, testing the low-frequency stability of commercial voltage references. Voltage reference performance is relevant to the stability of electrically applied forces on the LISA proof masses, and commercial references do not have well characterized noise in the sub-Hz regime. Our measurements confirmed that the best commercial reference was the AD587LN, with a typical noise of 2.1±0.6 ppm/rtHz at 0.1 mHz, in a temperature-stabilized environment of ∼10mK/rtHz. This agrees closely with prior work by other groups.

We report measurements of ultraviolet light emitting diode (UV LED) performance under conditions simulating operation in an orbiting satellite. UV LED light output maintained within less than 3% observational uncertainty, over more than 19,000 hours of operation in a nitrogen atmosphere, and over 8,000 hours operation at a pressure of less than 10^-7 torr vacuum. In addition, irradiation with 63 MeV protons to a total fluence of 2×10¹² protons/cm² does not degrade the UV light output. Spectrally, the emissive center-wavelength and spectral shape are unchanged after proton irradiation within the precision of our measurement. These results qualify the UV LED operation lifetime and radiation hardness for space flights

As part of the on-going LISA Mission Formulation study under ESA contract, EADS Astrium has recently suggested and investigated a variety of novel LISA payload architectures utilizing so-called "In-Field Pointing" for accommodation of seasonal constellation dynamics. Here, the annual variation in the angle between the interferometer arms of roughly ±1° is compensated by steering the lines of sight of the individual telescopes with a small actuated mirror located in an intermediate pupil plane inside the telescopes. This introduces a certain flexibility for the overall payload configuration and allows for very compact designs. In particular, it enables a "single active proof mass" mode with a true cold redundancy between a nominal and a backup GRS system on board each spacecraft, and thus enhances mission robustness.

The space-based gravitational wave detector LISA (Laser Interferometer Space Antenna) utilizes a high performance position sensor in order to measure the translation and tilt of the free flying proof mass with respect to the optical bench. Depending on the LISA optical bench design, this position sensor must have up to pm/ sensitivity for the translation measurement and up to nrad/ sensitivity for the tilt measurement. We developed a heterodyne interferometer, combined with differential wavefront sensing, for the tilt measurement. The interferometer design exhibits maximum symmetry where measurement and reference arm have the same frequency and polarization and the same optical path-lengths. The interferometer can be set up free of polarizing optical components preventing possible problems with thermal dependencies not suitable for the space environment. We developed a mechanically highly stable and compact setup which is located in a vacuum chamber. We measured initial noise levels below 10 pm/ (longitudinal measurement) for frequencies above 10 mHz and below 20 nrad/ (tilt measurement) for frequencies above 1 mHz. This setup can also be used for other applications, for example the measurement of the coefficient of thermal expansion (CTE) of structural materials, such as carbon fiber reinforced plastic (CFRP).

Reflective diffraction gratings enable novel optical configurations that simplify and improve laser interferometers. We have proposed an all-reflective grating interferometer that can be used in LISA type interferometers for space gravitational wave detection [1]. One configuration requires a highly polarization sensitive grating. We report on characterizations of a grating made atop high reflective dielectric layers. Using a direct measurement method, the diffraction efficiency at the Littrow angle for s-polarization is measured as 97.3% and for p-polarization 4.2%, leading to a s/p polarization diffraction ratio of 23.2. The depolarization from s- to p-polarization is measured to be ∼1.7×10^-4, and from p- to s-polarization 1.8×10-4. We derived a transfer matrix based on these measurements. Furthermore, we have developed a more accurate method for diffraction efficiency measurement using a grating cavity. These measurements are encouraging steps taken towards the requirements of an ideal grating interferometer.

The LISA Test-Mass (TM) is sensitive to weak forces along all 6 Degrees of Freedom (DoFs). Extensive ground testing is required in order to evaluate the influence of crosstalks of various read-outs and actuators operating on different DoFs. To this purpose, and to better represent the flight conditions, we are developing a facility with 2 soft DoFs which consists of two stage roto-translational pendulum. This facility will measure the forces and stiffnesses simultaneously acting on the Test Mass along 2 different soft DoFs. The advantages with respect to a single DoF test bench are a more effective identification and debug of spurious effects between the TM and the capacitive position sensor surrounding it and the possibility to test actuation crosstalk with closed feedback loop. In particular, it allows us to measure the residual disturbance along one DoF when we close the control loop on the other one.

LISA requires high precision angular beam pointing and telescope steering. In this paper, we report recent results for an improved grating angular sensor. We have achieved better than 0.2 nrad/Hz^1/2 at 1 kHz with 14 mW of incident power, a factor of 5 improvement over our previously reported results. At 1 Hz we achieved 1-2 nrad/Hz^1/2. We realized these improvements by enclosing the grating angular sensor assembly in a vacuum chamber and mounting the optics components on a zerodur glass plate, thereby lowering the noise floor at low frequencies. Furthermore, by upgrading the electronics and thus the detector power handing capability, we also investigated sensitivity scaling versus incident laser power. The results will benefit the design of grating angular sensors.

LISA, the Laser Interferometer Space Antenna, is a proposed ESA/NASA space based gravitational wave detector. In order to help meet the many technological challenges of LISA, the ESA precursor mission LISA Pathfinder (LPF) will test some of the key enabling technologies for LISA. LPF however will only go so far, and much work is needed to take LPF technology to a state suitable for LISA. One such area is the use of polarising Mach-Zehnder interferometers. We report on the design and initial construction of an experiment to test the use of such interferometric techniques, as well as suitable component mounting mechanisms.

Gravitational inertial sensors will be placed on board the Laser Interferometer Space Antenna (LISA) and aboard its precursor mission LISA Pathfinder (LISA-PF) in order to detect low frequency gravitational waves in space. Free-floating test-masses (Au₇Pt₃ cubes) will be housed in inertial sensors for detecting possible laser signal variations induced by gravitational waves. Charging of the LISA test-masses due to exposure of the spacecraft to cosmic radiation and energetic solar particles will affect operation of gravitational inertial sensors. In this paper we report on the role of diffuse γ-rays in charging the LISA and LISA-PF test-masses with respect to protons and helium nuclei. The diffuse γ-ray flux in the Galaxy has been interpolated taking into account the outcomes of recent calculations. A comparison with γ-ray observations gathered by different experiments (COMPTEL and EGRET, Milagro, Whipple, HEGRA, TIBET) has been carried out. Simulations of the test-mass charging process have been performed by means of the FLUKA2006.3b package. Monte Carlo simulations of the interaction of cosmic particles with the LISA spacecraft indicate that the diffuse γ-ray contribution to the average steady-state test-mass charging rate and to the single-sided power spectrum of the charge rate noise is marginal with respect to that due to galactic cosmic-rays.

An electrostatic-controlled torsion pendulum was used to simulate the operation of the inertial sensor in flight. The twist motion of the proof mass was monitored by a capacitance transducer and was controlled by an electrostatic actuator. The influences of the parasitic coupling of the capacitance transducer, the magnetic field, and the translation motion coupling were measured. The torque noise of the controlled pendulum came to about 6×10^-13 N m Hz^-1/2 from 2 mHz to 0.1 Hz, mainly limited by the translation-rotation coupling and the back action of the capacitance sensor, which could be suppressed by extending the gap of the capacitance sensor.

We have designed combined passive and active thermal control system to achieve sub microkelvin temperature stability and uniformity over an optics bench size enclosure, which has an analogous structure to the LISA spacecraft. For the passive control, we have constructed a new thermal enclosure that has a multilayer structure with alternative conducting and insulating layers, which enables the temperature uniformity and ease the burden of the active control. The thermal enclosure becomes an important test facility for Modular Gravitational Reference Sensor (MGRS) development. For the active control, we have developed a model predictive control (MPC) algorithm, which will regulate temperature variations of the proof-mass (PM) down to sub-microkelvin over the LISA science band. The LISA mission requires extremely tight temperature control, which is as low as 30 μK/ over 0.1 mHz to 1 Hz. Both temporal stability and spatial uniformity in temperature must be achieved. Optical path length variations on optical bench must be kept below 40 pm/ over 0.1 mHz to 1 Hz. Temperature gradient across the proof mass housing also must be controlled to reduce differential thermal pressure. Thermal disturbances due to, for example, solar radiation and heat generation from electronics, are expected to be significant disturbance source to the LISA sensitivity requirements. The MGRS will alleviate the thermal requirement due to its wider gap between the proof-mass and the housing wall. However, a thermally stable and uniform environment is highly desirable to achieve more precise science measurement for future space science missions.

The performance of drag-free spacecraft will be ultimately limited by the uncompensated gravitational field and gravity field gradient of the spacecraft structure and payload. We identify sources of disturbance of gravitational origin and we explore a new concept for determining gravitational fields in the vicinity of the proof mass. The benefits of these diagnostic tools for future missions, like ASTROD, are discussed.

The direct detection of gravitational waves will provide valuable astrophysical information about many celestial objects. The most promising sources of gravitational waves are neutron stars and black holes. These objects emit waves in a very wide spectrum of frequencies determined by their quasi-normal modes oscillations. In this work we are concerned with the information we can extract from f and pI-modes when a candidate leaves its signature in the resonant mass detectors ALLEGRO, EXPLORER, NAUTILUS, MiniGrail and SCHENBERG. Using the empirical equations, that relate the gravitational wave frequency and damping time with the mass and radii of the source, we have calculated the radii of the stars for a given interval of masses M in the range of frequencies that include the bandwidth of all resonant mass detectors. With these values we obtain diagrams of mass-radii for different frequencies that have allowed to determine the better candidates to future detection taking in account the compactness of the source. Finally, to determine which are the models of compact stars that emit gravitational waves in the frequency band of the mass resonant detectors, we compare the mass-radii diagrams obtained by different neutron stars sequences from several relativistic hadronic equations of state (GM1, GM3, TM1, NL3) and quark matter equations of state (NJL, MTI bag model). We verify that quark stars obtained from MIT bag model with bag constant equal to 170 MeV and quark matter in color-superconductivity phase are the best candidates for mass resonant detectors.

DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the planned Japanese space gravitational wave antenna, aiming to detect gravitational waves from astrophysically and cosmologically significant sources mainly between 0.1 Hz and 10 Hz and thus to open a new window for gravitational wave astronomy and for the universe. DECIGO will consist of three drag-free spacecraft, 1000 km apart from each other, whose relative displacements are measured by a differential Fabry-Perot interferometer. We plan to launch DECIGO in middle of 2020s, after sequence of two precursor satellite missions, DECIGO pathfinder and Pre-DECIGO, for technology demonstration required to realize DECIGO and hopefully for detection of gravitational waves from our galaxy or nearby galaxies.

All modern routes leading to a quantum theory of gravity – i.e., perturbative quantum gravitational one-loop exact correction to the global chiral current in the standard model, string theory, and perhaps even loop quantum gravity – require supplementing the Einstein-Hilbert action with a parity-violating Chern-Simons term. Such a term leads to amplitude-birefringent gravitational wave propagation: i.e., one (circular) polarization state amplified with propagation while the other is attenuated. The proposed Laser Interferometer Space Antenna (LISA) is capable of observing gravitational wave sources at cosmological distances, suggesting the possibility that LISA observations may place a strong bound on this manifestation of quantum gravity. Here we report on a calculation of the effect that spacetime amplitude birefringence has on the signal LISA is capable of observing from inspiraling supermassive black hole binaries at large redshift. We find that the birefringence manifests itself in the observations as an anomalous precession of the binary's orbital angular momentum as it evolves toward coalescence, whose magnitude depends on the integrated history of the Chern-Simons coupling over the worldline of radiation wavefront. We estimate that LISA could place bounds on Chern-Simons modified gravity that are several orders of magnitude stronger than the present Solar System constraints, thus providing a probe of the quantum structure of spacetime.

LISA should be able to detect the gravitational waves from the QNM ringdown of supermassive black holes in the 10⁵ – 10⁸ solar mass range. On the other hand, it is reasonable to think that any quantum theory of gravitation should impose the quantization of the energy levels of these QNM. Here we discuss the possibility of distinguishing quantum aspects of gravity using LISA to observe QNM overtones of highly excited supermassive black holes.

PACS numbers: 04.80.Nn, 95.55.Ym

Gravitational-wave memory refers to the permanent displacement of the test masses in an idealized (freely-falling) gravitational-wave interferometer. Inspiraling binaries produce a particularly interesting form of memory—the Christodoulou memory. Although it originates from nonlinear interactions at 2.5 post-Newtonian order, the Christodoulou memory affects the gravitational-wave amplitude at leading (Newtonian) order. Previous calculations have computed this non-oscillatory amplitude correction during the inspiral phase of binary coalescence. Using an "effective-one-body" description calibrated with the results of numerical relativity simulations, the evolution of the memory during the inspiral, merger, and ringdown phases, as well as the memory's final saturation value, are calculated. Using this model for the memory, the prospects for its detection are examined, particularly for supermassive black hole binary coalescences that LISA will detect with high signal-to-noise ratios. Coalescing binary black holes also experience center-of-mass recoil due to the anisotropic emission of gravitational radiation. These recoils can manifest themselves in the gravitational-wave signal in the form of a "linear" memory and a Doppler shift of the quasi-normal-mode frequencies. The prospects for observing these effects are also discussed.

We estimate the probability of detecting a gravitational wave signal from coalescing compact binaries in simulated data from a ground-based interferometer detector of gravitational radiation using Bayesian model selection. The simulated waveform of the chirp signal is assumed to be a spin-less Post-Newtonian (PN) waveform of a given expansion order, while the searching template is taken to be either of the same PN family as the simulated signal or one level below its PN expansion order. Within the Bayesian framework we estimate the detection probabilities and the statistical uncertainties due to noise as a function of the signal-to-noise ratio (SNR), and the posterior distributions of the parameters characterizing the chirp signal. Our analysis indicates that the detection probabilities are not compromised when simplified models are used, while the accuracies in the determination of the parameters can be significantly worsened.

We present construction of the 3 and 4 dimensional grids in the parameter space for all sky-search of gravitational wave signals from white dwarf binaries with LISA data. The 3 dimensional grid is for search of frequency and the sky position of the source and the 4 dimensional grid includes the spin down parameter. The grid solves the covering problem in the parameter space with the constraint that nodes of the grid coincide with Fourier frequencies (multiples of the inverse of the observation time).This allows the use of the fast Fourier transform (FFT) in the evaluation of the optimal statistic and greatly speeds up the search.

A pilot study has been carried out to simulate stochastic gravitational wave background originating from first order phase transitions in the early universe. The space based gravitational wave detector LISA will be operational in the range of 10-4 to 0.1 Hz and could be sensitive to the red shifted gravitational waves from cosmological origin. In this study we have modeled the signals from first order phase transitions and we compared the signal both with the expected instrumental noise and realistic simulated foreground signals, originating from the white dwarf population in our galaxy.

In order to attain the requisite sensitivity for LISA – a joint space mission of the ESA and NASA- the laser frequency noise must be suppressed below the secondary noises such as the optical path noise, acceleration noise etc. By combining six appropriately time-delayed data streams containing fractional Doppler shifts – a technique called time delay interferometry (TDI) – the laser frequency noise may be adequately suppressed. Here we investigate the problem of TDI in the general case of unequal up-down links and also include the effect of the Earth on the spacecraft and the optical links. We show that there are symmetries in the physics which can be successfully used to simplify the algebra of the TDI. We finally give the example of the first generation modified Sagnac observable in which the laser frequency noise is suppressed because of the symmetries.

In the dense central regions of globular clusters close encounters of two white dwarfs are relatively frequent. The estimated frequency is one or more strong encounters per star in the lifetime of the cluster. Such encounters should be then potential sources of gravitational wave radiation. Thus, it is foreseeable that these collisions could be either individually detected by LISA or they could contribute significantly to the background noise of the detector. We compute the pattern of gravitational wave emission from these encounters for a sufficiently broad range of system parameters, namely the masses, the relative velocities and the distances of the two white dwarfs involved in the encounter.

We present preliminary results from self-consistent, high resolution direct N –body simulations of massive black hole binaries in mergers of galactic nuclei. The dynamics of the black hole binary includes the full Post-Newtonian corrections (up to 2.5PN) to its equations of motion. We show that massive black holes starting at separations of 100 pc can evolve down to gravitational-wave-induced coalescence in less than a Hubble time. The binaries, in our models, often form with very high eccentricity and, as a result, reach separations of 50 Schwarzschild radius with eccentricities which are clearly distinct from zero — even though gravitational wave emission damps the eccentricity during the inspiral. These deviations from exact circular orbits, at such small separations, may have important consequences for LISA data analysis.

We present a detailed descriptive analysis of the gravitational radiation from binary mergers of non-spinning black holes, based on numerical relativity simulations of systems varying from equal-mass to a 6:1 mass ratio. Our analysis covers amplitude and phase characteristics of the radiation, suggesting a unified picture of the waveforms' dominant features in terms of an implicit rotating source, applying uniformly to the full wavetrain, from inspiral through ringdown. We construct a model of the late-stage frequency evolution that fits the l = m modes, and identify late-time relationships between waveform frequency and amplitude. These relationships allow us to construct a predictive model for the late-time waveforms, an alternative to the common practice of modelling by a sum of quasinormal mode overtones. We demonstrate an application of this in a new effective-one-body-based analytic waveform model.

We discuss the nonlinear evolution of black hole ringdown in the framework of higher-order metric perturbation theory. By solving the initial-value problem of a simplified nonlinear field model analytically as well as numerically, we find that (i) second-order quasinormal modes (QNMs) are indeed excited at frequencies different from those of first-order QNMs, as predicted recently. We also find serendipitously that (ii) late-time evolution is dominated by a new type of power-law tail at late times. This "second-order power-law tail" decays more slowly than any late-time tails known in the first-order (i.e., linear) perturbation theory, and is generated at the wavefront of the first-order perturbation by an essentially nonlinear mechanism. These nonlinear components should be particularly significant for binary black hole coalescences, and could open a new precision science in gravitational wave studies.

Gravitational waves from extreme mass ratio inspirals are one of the important sources of LISA. We should calculate these waves so accurately that we can extract physical information of source by data analysis. Recently, we developed an efficient numerical method to compute gravitational waves from binary systems in which a point particle moves in circular orbits on the equatorial plane of the black hole. In this paper, we apply this method to compute gravitational waves from binary systems in which a point particle moves in general bound geodesic orbits of the black hole. We check the accuracy of our code using spherical symmetry of Schwarzschild black hole such that energy flux radiated by a point particle is independent of the inclination angle from the equatorial plane of black hole. We find that the accuracy of our code may be limited only by truncation of l, k and n –modes, where l is the degree of the spin-weighted spheroidal harmonics, and k and n are harmonics of the polar and radial motion, respectively. Then we evaluate the rate of change of three constants of motion, energy, angular momentum and the Carter constant, due to the emission of gravitational waves from a particle around Kerr black hole. This is the first time to compute the rate of change of the Carter constant using the adiabatic approximation. We also show that we can calculate gravitational waves accurately even in the case of high eccentric orbits. In this work, we truncate l mode up to 20 and estimated that relative accuracy of our numerical results are better than 10^-5 even in the high eccentric case, e = 0.9. Our numerical code may be useful to make templates of extreme mass ratio inspirals.

Extreme-mass-ratio inspirals (EMRIs), stellar-mass compact objects (SCOs) inspiralling into a massive black hole, are one of the main sources of gravitational waves expected for the Laser Interferometer Space Antenna (LISA). To extract the EMRI signals from the expected LISA data stream, which will also contain the instrumental noise as well as other signals, we need very accurate theoretical templates of the gravitational waves that they produce. In order to construct those templates we need to account for the gravitational backreaction, that is, how the gravitational field of the SCO affects its own trajectory. In general relativity, the backreaction can be described in terms of a local self-force, and the foundations to compute it have been laid recently. Due to its complexity, some parts of the calculation of the self-force have to be performed numerically. Here, we report on an ongoing effort towards the computation of the self-force based on time-domain multi-grid pseudospectral methods.