Space-based Interferometer Research Articles

Observational model for microarcsecond astrometry with the Space Interferometry Mission

The Space Interferometry Mission (SIM) is a space-based long-baseline optical interferometer for precision astrometry. One of the primary objectives of the SIM instrument is to accurately determine the directions to a grid of stars, together with their proper motions and parallaxes, improving a priori knowledge by nearly three orders of magnitude. The basic astrometric observable of the instrument is the pathlength delay, a measurement made by a combination of internal metrology measurements that determine the distance the starlight travels through the two arms of the interferometer and a measurement of the white light stellar fringe to find the point of equal pathlength. Because this operation requires a non--negligible integration time to accurately measure the stellar fringe position, the interferometer baseline vector is not stationary over this time period, as its absolute length and orientation are time--varying. This conflicts with the consistency condition necessary for extracting the astrometric parameters which requires a stationary baseline vector. This paper addresses how the time-varying baseline is ``regularized'' so that it may act as a single baseline vector for multiple stars, and thereby establishing the fundamental operation of the instrument.

Interferometric and spectroscopic monitoring of emission lines

We present a theoretical study of hot star wind variability in the presence of hypothetical large scale wind structures. Contrary to previous investigations that have focused on P-Cygni profiles, we investigate the impact on observable optical and near-infrared emission lines. Our working hypotheses assume that such extended wind structures are formed via a rotationally modulated stellar wind mass loss rate that gives rise to the so-called co-rotating interaction regions and that the modelled wind emissivity suffers from no opacity effect. Within this context, we find that the variability of emission lines traces an unambiguous S-shape in the frequency-time space, i.e. a spiralling pattern with positive and negative accelerations towards the line of sight over one stellar rotation period. Further, we demonstrate how lines forming at different heights can be used to provide dynamical and geometrical constraints on wind structures. Complementary to this spectroscopic approach, we also present theoretical expectations for VLT-AMBER interferometric observations of such a perturbed hot star outflow. For a fixed baseline orientation and length (space-based interferometer), the spectrally dispersed visibility and fringe phase output by the Differential Interferometry (DI) method show strong variable signatures, over a rotation period, of the same nature as those determined from spectroscopy. In the realistic case of both variable length and baseline orientation (ground-based interferometer), the DI method still yields a high detection sensitivity and geometrical characterisation of large scale wind structures.

Performance analysis of a space-based multiple-telescope nulling interferometer for DARWIN

The European Space Agency’s infrared space interferometer DARWIN is dedicated to the investigation of Earth-like extrasolar planets orbiting bright stars. A multi-aperture interferometer fed by free-flying telescopes allows spectroscopic analysis of the weak planet signal which could give hints on the possibility of the existence of Earth-like life. However, for a Sun/Earth-like constellation at an interstellar distance of some 50 light years, a star light suppression of about 60 dB is required to make the weak planet signal visible. In this paper we investigate the nulling capability of a space-based Robin Laurance interferometer in the case of stochastic disturbances of the array geometry and of stochastic alignment errors of the optical components, which both will be actively controlled. Mismatch of amplitude transmission, optical path length, and polarization transmission among the interferometer arms is taken into account.

Space missions to detect the cosmic gravitational-wave background

It is thought that a stochastic background of gravitational waves wasproduced during the formation of the universe. A great deal could belearned by measuring this cosmic gravitational-wave background (CGB),but detecting the CGB presents a significant technological challenge. Thesignal strength is expected to be extremely weak, and there will becompetition from unresolved astrophysical foregrounds such as whitedwarf binaries. Our goal is to identify the most promising approach todetecting the CGB. We study the sensitivities that can be reached using bothindividual, and cross-correlated pairs of space-based interferometers. Ourmain result is a general, coordinate-free formalism for calculating thedetector response that applies to arbitrary detector configurations. Weuse this general formalism to identify some promising designs for agravitational background interferometer mission. Our conclusion isthat detecting the CGB may not be out of reach.

Prospects for High Angular Resolution at Low Frequencies

High angular resolution imaging at low frequencies (below ∼ 10 MHz) requires observations from above the ionosphere. A radio interferometer array in space will be able to open new vistas in solar, terrestrial, galactic, and extragalactic astrophysics. A space-based interferometer could image and track transient disturbances in the solar corona and interplanetary medium - a new capability which is crucial for understanding many aspects of solar-terrestrial interaction and space weather. It could also produce the first sensitive, high-angular-resolution radio surveys of the entire sky at low frequencies. The radio sky will look entirely different below 10-30 MHz because new emission and absorption processes become dominant at these frequencies. As a result, low frequency surveys from space will provide a fundamentally new view of the universe and an extraordinarily large and varied science return.

Evolution of Differentially Rotating Supermassive Stars to the Onset of Bar Instability

Thermal emission from a rotating, supermassive star will cause the configuration to contract slowly and spin up. If internal viscosity and magnetic fields are sufficiently weak, the contracting star will rotate differentially. For each of the six initial angular momentum distributions considered, a cooling supermassive star will likely encounter the dynamical bar mode instability, which may trigger the growth of nonaxisymmetric bars. This scenario will lead to the generation of long-wavelength gravitational waves, which could be detectable by future space-based laser interferometers like the Laser Interferometer Space Antenna.

Synthetic Spectra of Simulated Terrestrial Atmospheres Containing Possible Biomarker Gases

NASA's proposed Terrestrial Planet Finder, a space-based interferometer, will eventually allow spectroscopic analyses of the atmospheres of extrasolar planets. Such analyses would provide information about the existence of life on these planets. One strategy in the search for life is to look for evidence of O 3 (and hence O 2) in a planet's atmosphere; another is to look for gases that might be present in an atmosphere analogous to that of the inhabited early Earth. In order to investigate these possibilities, we have calculated synthetic spectra for several hypothetical terrestrial-type atmospheres. The model atmospheres represent four different scenarios. The first two, representing inhabited terrestrial planets, are an Earth-like atmosphere containing variable amounts of oxygen and an early Earth-type atmosphere containing methane. In addition, two cases representing Mars-like and early Venus-like atmospheres were evaluated, to provide possible “false positive” spectra. The calculated spectra suggest that ozone could be detected by an instrument like Terrestrial Planet Finder if the O 2 concentration in the planet's atmosphere is ≥200 ppm, or 10 −3 times the present atmospheric level. Methane should be observable on an early-Earth type planet if it is present in concentrations of 100 ppm or more. Methane has both biogenic and abiogenic sources, but concentrations exceeding 1000 ppm, or 0.1% by volume, would be difficult to produce from abiogenic sources alone. High methane concentrations in a planet's atmosphere are therefore another potential indicator for extraterrestrial life.

Using binary stars to bound the mass of the graviton

Interacting white dwarf binary star systems, including helium cataclysmic variable (HeCV) systems, are expected to be strong sources of gravitational radiation, and should be detectable by proposed space-based laser interferometer gravitational wave observatories such as LISA. Several HeCV star systems are presently known and can be studied optically, which will allow electromagnetic and gravitational wave observations to be correlated. Comparisons of the phases of a gravitational wave signal and the orbital light curve from an interacting binary white dwarf star system can be used to bound the mass of the graviton. Observations of typical HeCV systems by LISA could potentially yield an upper bound on the inverse mass of the graviton as strong as $h/m_{g} = \lambda_{g} > 1 \times 10^{15}$ km ($m_{g} < 1 \times 10^{-24}$ eV), more than two orders of magnitude better than present solar system derived bounds.

The Effect of Companions on the SIM Reference Frame

AbstractThe Space Interferometry Mission (SIM) is a 10-m Michelson space-based optical interferometer designed for precision astrometry (4μas, 3μas/yr) with better accuracy hoped for over a narrow field of view. It is intended to search for planets and investigate a number of problems in Galactic and extra-galactic astronomy.The accuracy and stability of SIM’s celestial reference frame is subject to degradation over the 5-year mission from the reflex motion induced by massive companions of the objects used to construct the celestial reference frame. We present the results of simulations which show the sensitivity of reference frame accuracy to companions as a function of mass and period. We assume that pre-launch ground surveys will eliminate all objects with RMS radial velocity &gt; 10 m/s. We further assume that the standard astrometric parameters of position, parallax, and proper motion plus acceleration terms in right ascension and declination will be allowed to absorb reflex motion.

Integrated optics for astronomical interferometry

We propose a new instrumental concept for long-baseline optical single-mode interferometry using in- tegrated optics which were developed for telecommunica- tion. Visible and infrared multi-aperture interferometry requires many optical functions (spatial ltering, beam combination, photometric calibration, polarization con- trol) to detect astronomical signals at very high angular resolution. Since the 80's, integrated optics on planar sub- strate have become available for telecommunication appli- cations with multiple optical functions like power divid- ing, coupling, multiplexing, etc. We present the concept of an optical/infrared interferometric instrument based on this new technology. The main advantage is to provide an interferometric combination unit on a single optical chip. Integrated optics are compact, provide stability, low sensitivity to external constrains like temperature, pres- sure or mechanical stresses, no optical alignment except for coupling, simplicity and intrinsic polarization control. The integrated optics devices are inexpensive compared to devices that have the same functionalities in bulk op- tics. We think integrated optics will fundamentally change single-mode interferometry. Integrated optics devices are in particular well-suited for interferometric combination of numerous beams to achieve aperture synthesis imaging or for space-based interferometers where stability and a minimum of optical alignments are wished.

Projects of optical space-based stellar interferometers

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text N. V. Ryabova and D. N. Es'kov, "Projects of optical space-based stellar interferometers," J. Opt. Technol. 66, 83- (1999) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

The first space-based gravitational-wave detectors

Gravitational waves provide a laboratory for general relativity and a window to energetic astrophysical phenomena invisible with electromagnetic radiation. Several terrestrial detectors are currently under construction, and a space-based interferometer is envisioned for launch early next century to detect test-mass motions induced by waves of relatively short wavelength. Very-long-wavelength gravitational waves can be detected using the plasma in the early Universe as test masses; the motion induced in the plasma by a wave is imprinted onto the cosmic microwave background (CMB). While the signature of gravitational waves on the CMB temperature fluctuations is not unique, the polarization pattern can be used to unambiguously detect gravitational radiation. Thus, forthcoming CMB polarization experiments, such as the Microwave Anisotropy Probe and Planck, will be the first space-based gravitational-wave detectors.

Spacecraft to spacecraft coherent laser tracking as a xylophone interferometer detector of gravitational radiation

Searches for gravitational radiation can be performed in space with two spacecraft tracking each other with coherent laser light. Four observables can be measured: two one-way Doppler data (each spacecraft transmits a light beam and the other reads out the Doppler data with an onboard laser), and two two-way Doppler data (light is coherently transponded by the other spacecraft and the Doppler effect is extracted by comparing the phases of transmitted and transponded lights). One-way and two-way tracking data recorded on board the two spacecraft are time tagged and telemetered back to Earth for data analysis. By linearly combining the four data sets, we derive a method for reducing by several orders of magnitude, at selected Fourier components, the frequency fluctuations due to the lasers. The gravitational wave signal remaining at these frequencies makes this spacecraft to spacecraft coherent laser tracking technique the equivalent of a xylophone interferometer detector of gravitational radiation. Estimates for the strain sensitivities achievable with these experiments are presented for gravitational wave bursts, monochromatic signals, and a stochastic background of gravitational radiation. This experimental technique could be implemented with two spacecraft carrying an appropriate optical payload, or with the proposed broadband, space-based laser interferometer detectors of gravitational waves operated in this non-interferometric mode.

Angular resolution of the LISA gravitational wave detector

We calculate the angular resolution of the planned LISA detector, a space-based laser interferometer for measuring low-frequency gravitational waves from galactic and extragalactic sources. LISA is not a pointed instrument; it is an all-sky monitor with a quadrupolar beam pattern. LISA will measure simultaneously both polarization components of incoming gravitational waves, so the data will consist of two time series. All physical properties of the source, including its position, must be extracted from these time series. LISA's angular resolution is therefore not a fixed quantity, but rather depends on the type of signal and on how much other information must be extracted. Information about the source position will be encoded in the measured signal in three ways: 1) through the relative amplitudes and phases of the two polarization components, 2) through the periodic Doppler shift imposed on the signal by the detector's motion around the Sun, and 3) through the further modulation of the signal caused by the detector's time-varying orientation. We derive the basic formulae required to calculate the LISA's angular resolution $\Delta \Omega_S$ for a given source. We then evaluate $\Delta \Omega_S$ for two sources of particular interest: monchromatic sources and mergers of supermassive black holes. For these two types of sources, we calculate (in the high signal-to-noise approximation) the full variance-covariance matrix, which gives the accuracy to which all source parameters can be measured. Since our results on LISA's angular resolution depend mainly on gross features of the detector geometry, orbit, and noise curve, we expect these results to be fairly insensitive to modest changes in detector design that may occur between now and launch. We also expect that our calculations could be easily modified to apply to a modified design.

Measuring gravitational waves from binary black hole coalescences. II. The waves’ information and its extraction, with and without templates

We discuss the extraction of information from detected binary black hole (BBH) coalescence gravitational waves by the ground-based interferometers LIGO and VIRGO, and by the space-based interferometer LISA. We focus on the merger phase that occurs after the gradual inspiral and before the ringdown. Our results are (i) if numerical relativity simulations have not produced template merger waveforms before BBH events are detected, one can study the merger waves using simple band-pass filters. For BBHs smaller than about ${40M}_{\ensuremath{\bigodot}}$ detected via their inspiral waves, the band-pass filtering signal-to-noise ratio indicates that the merger waves should typically be just barely visible in the noise for initial and advanced LIGO interferometers. (ii) We derive an optimized maximum-likelihood method for extracting a best-fit merger waveform from the noisy detector output; one ``perpendicularly projects'' this output onto a function space (specified using wavelets) that incorporates our (possibly sketchy) prior knowledge of the waveforms. An extension of the method allows one to extract the BBH's two independent waveforms from outputs of several interferometers. (iii) We propose a computational strategy for numerical relativists to pursue, if they successfully produce computer codes for generating merger waveforms, but if running the codes is too expensive to permit an extensive survey of the merger parameter space. In this case, for LIGO-VIRGO data analysis purposes, it would be advantageous to do a coarse survey of the parameter space aimed at exploring several qualitative issues and at determining the ranges of the several key parameters which we describe. (iv) A complete set of templates could be used to test the nonlinear dynamics of general relativity and to measure some of the binary's parameters via matched filtering. We estimate the number of bits of information obtainable from the merger waves (about 10--60 for LIGO-VIRGO, up to 200 for LISA), estimate the information loss due to template numerical errors or sparseness in the template grid, and infer approximate requirements on template accuracy and spacing.

Measuring gravitational waves from binary black hole coalescences. I. Signal to noise for inspiral, merger, and ringdown

We estimate the expected signal-to-noise ratios (SNRs) from the three phases (inspiral,merger,ringdown) of coalescing binary black holes (BBHs) for initial and advanced ground-based interferometers (LIGO/VIRGO) and for space-based interferometers (LISA). LIGO/VIRGO can do moderate SNR (a few tens), moderate accuracy studies of BBH coalescences in the mass range of a few to about 2000 solar masses; LISA can do high SNR (of order 10^4) high accuracy studies in the mass range of about 10^5 to 10^8 solar masses. BBHs might well be the first sources detected by LIGO/VIRGO: they are visible to much larger distances (up to 500 Mpc by initial interferometers) than coalescing neutron star binaries (heretofore regarded as the "bread and butter" workhorse source for LIGO/VIRGO, visible to about 30 Mpc by initial interferometers). Low-mass BBHs (up to 50 solar masses for initial LIGO interferometers; 100 for advanced; 10^6 for LISA) are best searched for via their well-understood inspiral waves; higher mass BBHs must be searched for via their poorly understood merger waves and/or their well-understood ringdown waves. A matched filtering search for massive BBHs based on ringdown waves should be capable of finding BBHs in the mass range of about 100 to 700 solar masses out to 200 Mpc (initial LIGO interferometers), and 200 to 3000 solar masses out to about z=1 (advanced interferometers). The required number of templates is of order 6000 or less. Searches based on merger waves could increase the number of detected massive BBHs by a factor of order 10 or more over those found from inspiral and ringdown waves, without detailed knowledge of the waveform shapes, using a "noise monitoring" search algorithm. A full set of merger templates from numerical relativity could further increase the number of detected BBHs by an additional factor of up to 4.

On the antenna pattern of an orbiting interferometer

The response of an interferometer changing its orientation with respect to a fixed reference frame is analysed in terms of the beam-pattern factors and the polarization-averaged antenna power pattern. Given the motion of the antenna, the latter quantity describes the directionality of the antenna as a function of time. An interesting case is represented by the class of motions where the plane of the detector is constrained to move on the surface of a cone of constant aperture; at the same time, the two arms are rotating around a vertical axis. This picture describes, in particular, the motion of LISA, a proposed space-based laser interferometer, as well as of other planned missions. The overall sky coverage, and that of the Galactic plane in particular, is provided as a function of the aperture of the cone. Similarly, one can consider the case of an Earth-based interferometer. Using the same formalism, one can derive a simple expression for the antenna pattern, averaged over the time of arrival of the signal, as a function of the position and orientation on the Earth's surface. In particular, there turn out to be particular values for the terrestrial latitude and the inclination angle with respect to the local parallel which render the time-averaged antenna response perfectly isotropic. In the frequency domain, the general result is that the motion of the detector introduces in the instrumental response to a long-duration continuous signal a few harmonics of the orbital frequency, whose magnitude depends on the position of the source in the sky. In particular, we describe the response of LISA to sinusoidal waves coming from a few known binary systems in our Galaxy.

Infrared Interferometers for Observing Extra-Solar Planets

AbstractConcepts are outlined for ground - and space-based interferometers for observing nearby extra-solar earth-like planets (in the 8-20 μ range). The collecting elements are special coronagraphs, two of which constitute part of a Michelson interferometer. Performance characteristics are compared to those inferred for several competing systems. One conclusion is that one could construct a groundbased instrument that seems superior to the Keck interferometer system («superior» meaning up to a tenfold increase in angular resolution in each of two orthogonal directions, up to a tenfold increase in sensitivity, and a ten-fold lower cost). Space-based instruments of comparable/superior (?) performance to the Oases-1 and-2 systems might also be constructed.

Measuring black-hole parameters and testing general relativity using gravitational-wave data from space-based interferometers.

Among the expected sources of gravitational waves for the Laser Interferometer Space Antenna (LISA) is the capture of solar-mass compact stars by massive black holes residing in galactic centers. We construct a simple model for such a capture, in which the compact star moves freely on a circular orbit in the equatorial plane of the massive black hole. We consider the gravitational waves emitted during the late stages of orbital evolution, shortly before the orbiting mass reaches the innermost stable circular orbit. We construct a simple model for the gravitational-wave signal, in which the phasing of the waves plays the dominant role. The signal's behavior depends on a number of parameters, including $\ensuremath{\mu}$, the mass of the orbiting star, $M$, the mass of the central black hole, and $J$, the black hole's angular momentum. We calculate, using our simplified model, and in the limit of large signal-to-noise ratio, the accuracy with which these quantities can be estimated during a gravitational-wave measurement. For concreteness we consider a typical system consisting of a $10{M}_{\ensuremath{\bigodot}}$ black hole orbiting a nonrotating black hole of mass ${10}^{6}{M}_{\ensuremath{\bigodot}}$, whose gravitational waves are monitored during an entire year before the orbiting mass reaches the innermost stable circular orbit. Defining $\ensuremath{\chi}\ensuremath{\equiv}\frac{\mathrm{cJ}}{G{M}^{2}}$ and $\ensuremath{\eta}\ensuremath{\equiv}\frac{\ensuremath{\mu}}{M}$, we find $\ensuremath{\Delta}\ensuremath{\chi}\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}\frac{{10}^{\ensuremath{-}2}}{\ensuremath{\rho}}$, $\frac{\ensuremath{\Delta}\ensuremath{\eta}}{\ensuremath{\eta}}\ensuremath{\simeq}6\ifmmode\times\else\texttimes\fi{}\frac{{10}^{\ensuremath{-}2}}{\ensuremath{\rho}}$, and $\frac{\ensuremath{\Delta}M}{M}\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}\frac{{10}^{\ensuremath{-}3}}{\ensuremath{\rho}}$. Here, $\ensuremath{\rho}$ denotes the signal-to-noise ratio associated with the signal and its measurement. That these uncertainties are all much smaller than $\frac{1}{\ensuremath{\rho}}$, the signal-to-noise ratio level, is due to the large number of wave cycles received by the detector in the course of one year. These are the main results of this paper. Our simplified model also suggests a method for experimentally testing the strong-field predictions of general relativity.

Optical design, modelling and tolerancing of a Fizeau interferometer dedicated to astrometry

Space-based interferometers dedicated to wide-angle astrometry would dramatically increase the accuracy of angular measurements fundamental to a wide range of astrophysical problems. The proposed Global Astrometric Interferometer for Astrophysics (GAIA), a continuously rotating instrument comprising two or three interferometers, will reach the as level on more than 35 million objects. The necessary wide field-of-view for such a precision could be obtained with a Fizeau interferometer. We designed and modelled a 2.6 m baseline interferometer with two 40 cm apertures, and overall dimensions compatible with the size of the Ariane V payload shroud. It has a ~ 1 degree diffraction limited field-of-view. The response of the optical system to small perturbations on each optical element is given in terms of fringe visibility, which is shown to depend mainly on sub-aperture spot separation. The robustness of the design to thermal, mechanical and manufacturing errors is discussed.