CNRS Institute for research in astrophysics and planetology
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Margerin L.,CNRS Institute for research in astrophysics and planetology
European Physical Journal: Special Topics | Year: 2017

Equipartition is a central concept in the analysis of random wavefields which stipulates that in an infinite scattering medium all modes and propagation directions become equally probable at long lapse time in the coda. The objective of this work is to examine quantitatively how this conclusion is affected in an open waveguide geometry, with a particular emphasis on seismological applications. To carry our this task, the problem is recast as a spectral analysis of the radiative transfer equation. Using a discrete ordinate approach, the smallest eigenvalue and associated eigenfunction of the transfer equation, which control the asymptotic intensity distribution in the waveguide, are determined numerically with the aid of a shooting algorithm. The inverse of this eigenvalue may be interpreted as the leakage time of the diffuse waves out of the waveguide. The associated eigenfunction provides the depth and angular distribution of the specific intensity. The effect of boundary conditions and scattering anisotropy is investigated in a series of numerical experiments. Two propagation regimes are identified, depending on the ratio H∗ between the thickness of the waveguide and the transport mean path in the layer. The thick layer regime H∗ > 1 has been thoroughly studied in the literature in the framework of diffusion theory and is briefly considered. In the thin layer regime H∗ < 1, we find that both boundary conditions and scattering anisotropy leave a strong imprint on the leakage effect. A parametric study reveals that in the presence of a flat free surface, the leakage time is essentially controlled by the mean free time of the waves in the layer in the limit H∗ → 0. By contrast, when the free surface is rough, the travel time of ballistic waves propagating through the crust becomes the limiting factor. For fixed H∗, the efficacy of leakage, as quantified by the inverse coda quality factor, increases with scattering anisotropy. For sufficiently thin layers H∗≈ 1/5, the energy flux is predominantly directed parallel to the surface and equipartition breaks down. Qualitatively, the anisotropy of the intensity field is found to increase with the inverse non-dimensional leakage time, with the scattering mean free time as time scale. Because it enhances leakage, a rough free surface may result in stronger anisotropy of the intensity field than a flat surface, for the same bulk scattering properties. Our work identifies leakage as a potential explanation for the large deviation from isotropy observed in the coda of body waves. © 2017, The Author(s).

Anterrieu E.,CNRS Institute for research in astrophysics and planetology | Anterrieu E.,French National Center for Scientific Research
IEEE Transactions on Geoscience and Remote Sensing | Year: 2011

The SMOS mission is a European Space Agency project aimed at global monitoring of surface Soil Moisture and Ocean Salinity from radiometric L-band observations. This paper is concerned with the contamination of the data collected by SMOS by radio-frequency interferences (RFIs) which degrade the performance of the mission. RFI events are evidenced on both reference radiometer measurements and interferometric ones. It is explained why well-known standard RFI detection methods cannot be used. A specific method for the SMOS mission is presented and illustrated with data acquired during the commissioning phase. © 2011 IEEE.

Wiens R.C.,Los Alamos National Laboratory | Maurice S.,CNRS Institute for research in astrophysics and planetology
Elements | Year: 2015

The ChemCam laser-induced breakdown spectrometer on the rover Curiosity has provided more than 200,000 spectra from over 5000 different locations on Mars. This instrument is the first chemical microprobe on Mars and has an analytical footprint 0.3-0.6 mm in diameter. ChemCam has observed a measure of hydration in all the sedimentary materials encountered along the rover traverse in Gale Crater, indicating the ubiquity of phyllosilicates as a constituent of the analyzed sandstones, mudstones, and conglomerates. Diagenetic features, including calcium sulfate veins, millimeter-thick magnesium-rich diagenetic ridges, and manganese-rich rock surfaces, provide clues to water-rock interactions. Float clasts of coarsegrained igneous rocks are rich in alkali feldspars and some are enriched in fl uorine, indicating greater magmatic evolution than expected on Mars. The identifi cation of individual soil components has contributed to our understanding of the evolution of Martian soil. These observations have broadened our understanding of Mars as an active and once habitable planet.

Barret D.,CNRS Institute for research in astrophysics and planetology | Vaughan S.,University of Leicester
Astrophysical Journal | Year: 2012

High-frequency quasi-periodic oscillations (QPOs) from weakly magnetized neutron stars display rapid frequency variability (second timescales) and high coherence with quality factors up to at least 200 at frequencies about 800-850Hz. Their parameters have been estimated so far from standard min(χ2) fitting techniques, after combining a large number of power density spectra (PDS), to have the powers normally distributed (the so-called Gaussian regime). Before combining PDS, different methods to minimize the effects of the frequency drift to the estimates of the QPO parameters have been proposed, but none of them relied on fitting the individual PDS. Accounting for the statistical properties of PDS, we apply a maximum likelihood method to derive the QPO parameters in the non-Gaussian regime. The method presented is general, easy to implement, and can be applied to fitting individual PDS, several PDS simultaneously, or their average, and is obviously not specific to the analysis of kHz QPO data. It applies to the analysis of any PDS optimized in frequency resolution and for low-frequency variability or PDS containing features whose parameters vary on short timescales, as is the case for kHz QPOs. It is equivalent to the standard χ2 minimization fitting when the number of PDS fitted is large. The accuracy, reliability, and superiority of the method is demonstrated with simulations of synthetic PDS, containing Lorentzian QPOs of known parameters. Accounting for the broadening of the QPO profile, due to the leakage of power inherent to windowed Fourier transforms, the maximum likelihood estimates of the QPO parameters are asymptotically unbiased and have negligible bias when the QPO is reasonably well detected. By contrast, we show that the standard min(χ2) fitting method gives biased parameters with larger uncertainties. The maximum likelihood fitting method is applied to a subset of archival Rossi X-ray Timing Explorer data of the neutron star X-ray binary 4U1608-522, for which we show that the lower kHz QPO parameters can be measured on timescales as short as 8 s. To demonstrate the potential use of the results of the maximum likelihood method, we show that in the observation analyzed the time evolution of the frequency is consistent with a random walk. We then show that the broadening of the QPO due to the frequency drift scales as √T, as expected from a random walk (T is the integration time of the PDS). This enables us to estimate the intrinsic quality factor of the QPO to be 260, whereas previous analysis indicated a maximum value around 200. © 2012. The American Astronomical Society. All rights reserved.

Ballmoos P.V.,CNRS Institute for research in astrophysics and planetology
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2013

Gamma-ray astronomy presents an extraordinary scientific potential for the study of the most powerful sources and the most violent events in the Universe. Those extreme conditions occur generally at the endpoints of stellar lives, when the comparatively calm thermal evolution gives way to more violent nonthermal processes. Present telescopes in nuclear astrophysics make use of inelastic interaction processes based on geometrical optics or quantum optics, i.e. shadowcasting in modulating aperture systems, and particle tracking detectors respectively. After reviewing the above instrument concepts, we focus on recent developments in crystal diffraction optics. For the first time in gamma-ray astronomy, this type of optics permits to concentrate photons from a large collector onto a small detector, dramatically improving the sensitivity of next generation space telescopes. © 2013 Elsevier B.V. All rights reserved.

Barret D.,CNRS Institute for research in astrophysics and planetology
Astrophysical Journal | Year: 2012

EXO1745-248 is a transient neutron star low-mass X-ray binary located in the globular cluster Terzan 5. It was in outburst in 2000 and displayed during one Rossi X-ray Timing Explorer observation a highly coherent quasi-periodic oscillation (QPO) at frequencies between 670 and 715Hz. Applying a maximum likelihood method to fit the X-ray power density spectrum, we show that the QPO can be detected on segments as short as T = 48 s. We find that its width is consistent with being constant, while previous analysis based on longer segment duration (200s) found it variable. If the QPO frequency variations in EXO1745-248 follow a random walk (i.e., the contribution of the drift to the measured width increases like ), we derive an intrinsic width of 2.3 Hz. This corresponds to an intrinsic quality factor of Q 297 ± 50 at 691Hz. We also show that Q is consistent with being constant between 2.5 and 25keV. IGR J17480-2446 is another X-ray transient located in Terzan 5. It is a very interesting object showing accretion-powered pulsations and burst oscillations at 11Hz. We report on the properties of its kHz QPOs detected between October 18 and October 23, soon after the source had moved from the so-called Atoll state to the Z state. Its QPOs are typical of persistent Z sources; in the sense that they have low Q factors (30) and low rms amplitudes (5%). The highest frequency (at 870 Hz), if orbital, sets a lower limit on the inner disk radius of 18.5 km and an upper limit to the dipole moment of the magnetic field μ ≤ 5 × 1026 G cm3. © 2012 The American Astronomical Society. All rights reserved.

Barret D.,Toulouse 1 University Capitole | Barret D.,CNRS Institute for research in astrophysics and planetology
Astrophysical Journal | Year: 2013

High frequency soft reverberation lags have now been detected from stellar mass and supermassive black holes. Their interpretation involves reflection of a hard source of photons onto an accretion disk, producing a delayed reflected emission, with a time lag consistent with the light travel time between the irradiating source and the disk. Independently of the location of the clock, the kHz quasi-periodic oscillation (QPO) emission is thought to arise from the neutron star boundary layer. Here, we search for the signature of reverberation of the kHz QPO emission, by measuring the soft lags and the lag energy spectrum of the lower kHz QPOs from 4U1608-522. Soft lags, ranging from 15 to 40 μs, between the 3-8 keV and 8-30 keV modulated emissions are detected between 565 and 890 Hz. The soft lags are not constant with frequency and show a smooth decrease between 680 Hz and 890 Hz. The broad band X-ray spectrum is modeled as the sum of a disk and a thermal Comptonized component, plus a broad iron line, expected from reflection. The spectral parameters follow a smooth relationship with the QPO frequency, in particular the fitted inner disk radius decreases steadily with frequency. Both the bump around the iron line in the lag energy spectrum and the consistency between the lag changes and the inferred changes of the inner disk radius, from either spectral fitting or the QPO frequency, suggest that the soft lags may indeed involve reverberation of the hard pulsating QPO source on the disk. © 2013. The American Astronomical Society. All rights reserved.

Calvet M.,CNRS Institute for research in astrophysics and planetology | Margerin L.,CNRS Institute for research in astrophysics and planetology
Bulletin of the Seismological Society of America | Year: 2013

The coda quality factor of short-period S waves (Qc) excited by local earthquakes in the Pyrenees has been measured as a function of the length of the coda window (LW) for different choices of the onset time of the coda (tW). In the 2-16 Hz frequency band, we observe a transient regime characterized by an increase of Qc with LW, followed by a stabilization around a plateau the value of which depends on the central frequency of the signal. Using Monte Carlo simulations of wave transport in a variety of random media (≈1200 models), we demonstrate that the lapsetime dependence of Qc in the Pyrenees may be modeled by multiple anisotropic scattering of seismic waves, without invoking any depth dependence of the attenuation properties in the crust. In our model, anisotropic scattering is quantified by the ratio between the transport mean free path and the mean path (l*=l). At 6 Hz, the data require an anisotropy factor l*=l ≥ 5, a transport mean free path l ≈ 400 km, and an intrinsic quality factor Qi ≈ 800. From the frequency-dependent plateau of Qc at large lapse time, we infer an intrinsic quality factor of the form Qi ≈ 400f0:4 in the Pyrenees. We also show how the rapid increase of the lapse-time dependence of Qc with frequency may be exploited to put constraints on the power spectrum of heterogeneities in the crust.

Margerin L.,CNRS Institute for research in astrophysics and planetology
Geophysical Journal International | Year: 2013

This paper presents an analytical study of the multiple scattering of seismic waves by a collection of randomly distributed point scatterers. The theory assumes that the energy envelopes are smooth, but does not require perturbations to be small, thereby allowing the modelling of strong, resonant scattering. The correlation tensor of seismic coda waves recorded at a threecomponent sensor is decomposed into a sum of eigenmodes of the elastodynamic multiple scattering (Bethe-Salpeter) equation. For a general moment tensor excitation, a total number of four modes is necessary to describe the transport of seismic waves polarization. Their spatiotemporal dependence is given in closed analytical form. Two additional modes transporting exclusively shear polarizations may be excited by antisymmetric moment tensor sources only. The general solution converges towards an equipartition mixture of diffusing P and S waves which allows the retrieval of the local Green's function from coda waves. The equipartition time is obtained analytically and the impact of absorption on Green's function reconstruction is discussed. The process of depolarization of multiply scattered waves and the resulting loss of information is illustrated for various seismic sources. It is shown that coda waves may be used to characterize the source mechanism up to lapse times of the order of a few mean free times only. In the case of resonant scatterers, a formula for the diffusivity of seismic waves incorporating the effect of energy entrapment inside the scatterers is obtained. Application of the theory to high-contrast media demonstrates that coda waves are more sensitive to slow rather than fast velocity anomalies by several orders of magnitude. Resonant scattering appears as an attractive physical phenomenon to explain the small values of the diffusion constant of seismic waves reported in volcanic areas. © The Author 2012 Published by Oxford University Press on behalf of The Royal Astronomical Society.

Deville Y.,CNRS Institute for research in astrophysics and planetology
Signal Processing | Year: 2012

Relatively few results have been reported about the separability of given classes of nonlinear mixtures by means of statistical criteria such as ICA. We here first prove the ICA separability of a wide class of nonlinear global (i.e. mixingseparating) models involving reference signals, i.e. unmixed signals. We also show the second-order separability of sub-classes of the above class of models. This work therefore concerns nonlinear extensions of (linear) adaptive noise cancellation. We illustrate the usefulness of our general results by applying them to a quantum information processing problem, which involves a model of Heisenberg-coupled quantum states (i.e. qubits). This paper opens the way to practical ICA-based and second-order blind source separation (BSS) methods for nonlinear mixtures encountered in various applications. These BSS methods are also outlined in this paper. © 2011 Elsevier B.V. All rights reserved.

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