Showing papers on "Point source published in 2007"

Chandra Multiwavelength Project X‐Ray Point Source Catalog

Abstract: We present the Chandra Multiwavelength Project (ChaMP) X-ray point source catalog with ~6800 X-ray sources detected in 149 Chandra observations covering ~10 deg2. The full ChaMP catalog sample is 7 times larger than the initial published ChaMP catalog. The exposure time of the fields in our sample ranges from 0.9 to 124 ks, corresponding to a deepest X-ray flux limit of f0.5-8.0 = 9 × 10-16 ergs cm-2 s-1. The ChaMP X-ray data have been uniformly reduced and analyzed with ChaMP-specific pipelines and then carefully validated by visual inspection. The ChaMP catalog includes X-ray photometric data in eight different energy bands as well as X-ray spectral hardness ratios and colors. To best utilize the ChaMP catalog, we also present the source reliability, detection probability, and positional uncertainty. To quantitatively assess those parameters, we performed extensive simulations. In particular, we present a set of empirical equations: the flux limit as a function of effective exposure time and the positional uncertainty as a function of source counts and off-axis angle. The false source detection rate is ~1% of all detected ChaMP sources, while the detection probability is better than ~95% for sources with counts 30 and off-axis angle <5'. The typical positional offset between ChaMP X-ray source and their SDSS optical counterparts is 0.7'' ± 0.4'', derived from ~900 matched sources.

Bayesian Methods of Astronomical Source Extraction

Abstract: We present two new source extraction methods, based on Bayesian model selection and using the Bayesian information criterion. The first is a source detection filter, which is able to simultaneously detect point sources and estimate the image background. The second is an advanced photometry technique that measures the flux, position (to subpixel accuracy), local background, and point-spread function. We apply the source detection filter to simulated Herschel SPIRE data and demonstrate the filter's ability to both detect point sources and simultaneously estimate the image background. We use the photometry method to analyze a simple simulated image containing a source of unknown flux, position, and point-spread function; we not only accurately measure these parameters but also determine their uncertainties (using Markov chain Monte Carlo sampling). The method also characterizes the nature of the source (distinguishing between a point source and an extended source). We demonstrate the effect of including additional prior knowledge. Prior knowledge of the point-spread function increases the precision of the flux measurement, while prior knowledge of the background has only a small impact. In the presence of higher noise levels, we show that prior positional knowledge (such as might arise from a strong detection in another wave band) allows us to accurately measure the source flux even when the source is too faint to be detected directly. These methods are incorporated in SUSSEXtractor, the source extraction pipeline for the forthcoming Akari Far-Infrared Surveyor all-sky survey. They are also implemented in a stand-alone, beta-version tool that is freely available.

The RMS Survey: Mid-Infrared Observations of Candidate Massive YSOs in the Southern Hemisphere ⋆

Abstract: Context.The Red MSX Source (RMS) survey is an ongoing effort to return a large, well-selected sample of massive young stellar objects (MYSOs) within our Galaxy. 2000 candidates have been colour-selected from the Mid-course Space Experiment (MSX) point source catalogue (PSC). A series of ground-based follow-up observations are being undertaken in order to remove contaminant objects (ultra-compact HII (UCHII) regions, planetary nebulae (PN), evolved stars), and to begin characterising these MYSOs. Aims.As a part of these follow-up observations, high resolution (~1$^{\prime}$$^{\prime}$) mid-IR imaging aids the identification of contaminant objects which are resolved (UCHII regions, PN) as opposed to those which are unresolved (YSOs, evolved stars) as well as identifying YSOs near UCHII regions and other multiple sources. Methods.We present 10.4u imaging observations for 346 candidate MYSOs in the RMS survey in the Southern Hemisphere, primarily outside the region covered by the GLIMPSE Spitzer Legacy Survey. These were obtained using TIMMI2 on the ESO 3.6 m telescope in La Silla, Chile. Our photometric accuracy is of order 0.05 Jy, and our astrometric accuracy is 0.8$^{\prime}$$^{\prime}$, which is an improvement over the nominal 2'' accuracy of the MSX PSC. Results.Point sources are detected in 64% of our observations, which are expected to be either YSOs or evolved stars. 24% contain only sources of extended emission, which are likely to be either UCHII regions or, in a few cases, PN. This is confirmed by comparison with radio continuum observations. We find that, as expected for a dusty HII region, the strength of 10.4 $\mu$m and radio continuum emission is related. The remaining targets (12%) result in non-detections. While for 63% of our targets we detect only one mid-infrared source, 25% show multiple sources. In these cases, our observations will allow the apportioning of the flux from larger beam measurements between the different sources. Within these multiple source targets, we find some point sources on or near UCHII regions. Our improved astrometric information will allow more accurate targeting of spectroscopy, which will be used to identify unresolved sources in cases where it is not clear whether they are YSOs or evolved stars.

Discovery of a bipolar X-ray jet from the T Tauri star DG Tau

Abstract: We have obtained and analyzed Chandra ACIS-S observations of the strongly accreting classical T Tauri star DG Tau. Our principal goals are to map the immediate environment of the star to characterize possible extended X-rays formed in the jet, and to re-visit the anomalous, doubly absorbed X-ray spectrum of DG Tau itself. We combine our new ACIS-S data with a data set previously obtained. The data are superimposed to obtain flux and hardness images. Separate X-ray spectra are extracted for DG Tau and areas outside its point spread function. We detect a prominent X-ray jet at a position angle of PA ~225 deg (tentatively suggested by Guedel et al. 2005), coincident with the optical jet axis. We also identify a counter jet at PA = 45 deg. The X-ray jets are detected out to a distance of ~5" from the star, their sources being extended at the ACIS-S resolution. The jet spectra are soft, with a best-fit electron temperature of 3.4 MK. We find evidence for excess absorption of the counter jet. The spectrum of the DG Tau point source shows two components with largely different temperatures and absorption column densities. The similar temperatures and small absorbing gas columns of the jet sources and the soft component of the "stellar" source suggest that these sources are related, produced either by shocks or by magnetic heating in the jets. Cooling estimates suggest that the pressure in the hot gas contributes to jet expansion. The hard "stellar" component, on the other hand, is associated with a stellar corona or magnetosphere. The excessive photoelectric absorption of this component suggests the presence of dust-depleted accretion streams above coronal magnetic fields.

On Localisation of a Radiological Point Source

Abstract: The problem is to localise a source of gamma radiation using dose rate measurements taken with a gamma probe at various points in space. A statistical model of dose-rate counts is developed using experimental data recorded in a laboratory. The problem is then studied using the theoretical Cramer-Rao bound (CRB) analysis, which quantifies the accuracy with which it is possible to localise the source and estimate its intensity. Three estimation algorithms are implemented and their performance investigated by simulations. The algorithms are then applied to field trial radiological survey data. The maximum likelihood algorithm performs the best of the considered algorithms.

A new algorithm for modelling photoionizing radiation in smoothed particle hydrodynamics

Abstract: We present a new fast algorithm which allows the simulation of ionizing radiation emitted from a point source to be included in high-resolution three-dimensional smoothed particle hydrodynamics simulations of star cluster formation. We employ a Stromgren volume technique in which we use the densities of particles near the line-of-sight between the source and a given target particle to locate the ionization front in the direction of the target. Along with one-dimensional tests, we present fully three-dimensional comparisons of our code with the three-dimensional Monte Carlo radiative transfer code, mocassin, and show that we achieve good agreement, even in the case of highly complex density fields.

Wind-tunnel Modelling of Dispersion from a Scalar Area Source in Urban-Like Roughness

Abstract: A wind-tunnel study was conducted to investigate ventilation of scalars from urban-like geometries at neighbourhood scale by exploring two different geometries a uniform height roughness and a non-uniform height roughness, both with an equal plan and frontal density of λ p = λ f = 25%. In both configurations a sub-unit of the idealized urban surface was coated with a thin layer of naphthalene to represent area sources. The naphthalene sublimation method was used to measure directly total area-averaged transport of scalars out of the complex geometries. At the same time, naphthalene vapour concentrations controlled by the turbulent fluxes were detected using a fast Flame Ionisation Detection (FID) technique. This paper describes the novel use of a naphthalene coated surface as an area source in dispersion studies. Particular emphasis was also given to testing whether the concentration measurements were independent of Reynolds number. For low wind speeds, transfer from the naphthalene surface is determined by a combination of forced and natural convection. Compared with a propane point source release, a 25% higher free stream velocity was needed for the naphthalene area source to yield Reynolds-number-independent concentration fields. Ventilation transfer coefficients w T /U derived from the naphthalene sublimation method showed that, whilst there was enhanced vertical momentum exchange due to obstacle height variability, advection was reduced and dispersion from the source area was not enhanced. Thus, the height variability of a canopy is an important parameter when generalising urban dispersion. Fine resolution concentration measurements in the canopy showed the effect of height variability on dispersion at street scale. Rapid vertical transport in the wake of individual high-rise obstacles was found to generate elevated point-like sources. A Gaussian plume model was used to analyse differences in the downstream plumes. Intensified lateral and vertical plume spread and plume dilution with height was found for the non-uniform height roughness.

A method of measurement of extragalactic magnetic fields by TeV gamma ray telescopes

Abstract: A method is proposed for measuring extragalactic magnetic fields in observations of TeV γ rays from distant sources. Multi-TeV γ rays from these sources interact with the infrared photon background producing secondary electrons and positrons, which can be just slightly deflected by extragalactic magnetic fields before they emit secondary γ rays via inverse Compton scattering of cosmic microwave background photons. Secondary γ rays emitted toward an observer on the Earth can be detected as an extended emission around an initially point source. The energy dependent angular profile of the extended emission is related to the strength of the extragalactic magnetic field along the line of sight. Small magnetic fields B ≤ 10−12 G in the voids of the large scale structure can be measured in this way.

Discovery of the Putative Pulsar and Wind Nebula Associated with the TeV Gamma-Ray Source HESS J1813–178

Abstract: We present a Chandra X-ray observation of G12.82-0.02, a shell-like radio supernova remnant coincident with the TeV gamma-ray source HESS J1813-178. We resolve the X-ray emission from the colocated Advanced Satellite for Cosmology and Astrophysics (ASCA) source into a point source surrounded by structured diffuse emission that fills the interior of the radio shell. The morphology of the diffuse emission strongly resembles that of a pulsar wind nebula. The spectrum of the compact source is well characterized by a power law with index Γ ≈ 1.3, typical of young and energetic rotation-powered pulsars. For a distance of 4.5 kpc, consistent with the X-ray absorption and an association with the nearby star formation region W33, the 2-10 keV X-ray luminosities of the putative pulsar and nebula are LPSR = 3.2 × 1033 ergs s-1 and LPWN = 1.4 × 1034 ergs s-1, respectively. Both the flux ratio of LPWN/LPSR = 4.3 and the total luminosity of this system predict a pulsar spin-down power of Ė > 1037 ergs s-1, placing it among the 10 most energetic young pulsars in the Galaxy. A deep search for radio pulsations using the Parkes telescope sets an upper limit of ≈0.07 mJy at 1.4 GHz for periods 50 ms. We discuss the energetics of this source and consider briefly the proximity of bright H II regions to this and several other High Energy Stereoscopic System (H.E.S.S.) sources, which may produce their TeV emission via inverse Compton scattering.

A theoretical model for horizontal convection at high Rayleigh number

Abstract: We present a simple flow model and solution to describe ‘horizontal convection’ driven by a gradient of temperature or heat flux along one horizontal boundary of a rectangular box. Following laboratory observations of the steady-state convection, the model is based on a localized vertical turbulent plume from a line or point source that is located anywhere within the area of the box and that maintains a stably stratified interior. In contrast to the ‘filling box’ process, the convective circulation involves vertical diffusion in the interior and a stabilizing buoyancy flux distributed over the horizontal boundary. The stabilizing flux forces the density distribution to reach a steady state. The model predictions compare well with previous laboratory data and numerical solutions. In the case of a point source for the plume (the case which best mimics the localized sinking in the large-scale ocean overturning) the thermal boundary layer is much thicker than that given by the two-dimensional boundary layer scaling of H. T. Rossby (Tellus, vol. 50, 1965, p. 242).

Experimental demonstration of directional acoustic radiation based on two-dimensional phononic crystal band edge states

Abstract: The authors have experimentally studied the radiation of a point acoustic source placed inside a two-dimensional phononic crystal of square lattice. They show that a highly directional radiation with a half-power angular width of 6° can be achieved when operating at the band edge frequency for the phononic crystal. Such combination of a point source and a phononic crystal may serve as highly directional acoustic source in applications.

South-West extension of the hard X-ray emission from the Coma cluster

Abstract: Aims. We explore the morphology of hard (18–30 keV) X-ray emission from the Coma cluster of galaxies. Methods. We analyse a deep (1.1 Ms) observation of the Coma cluster with the ISGRI imager on board the INTEGRAL satellite. Results. We show that the source extension in the North-East to South-West (SW) direction (∼17 � ) significantly exceeds the size of the point spread function of ISGRI, and that the centroid of the image of the source in the 18−30 keV band is displaced in the SW direction compared to the centroid in the 1−10 keV band. To test the nature of the SW extension we fit the data assuming different models of source morphology. The best fit is achieved with a diffuse source of elliptical shape, although an acceptable fit can be achieved assuming an additional point source SW of the cluster core. In the case of an elliptical source, the direction of extension of the source coincides with the direction toward the subcluster falling onto the Coma cluster. If the SW excess is due to the presence of a point source with a hard spectrum, we show that there is no obvious X-ray counterpart for this additional source, and that the closest X-ray source is the quasar EXO 1256+281, which is located 6.1 � from the centroid of the excess. Conclusions. The observed morphology of the hard X-ray emission clarifies the nature of the hard X-ray “excess” emission from the Coma cluster, which is due to the presence of an extended hard X-ray source SW of the cluster core.

Transmission of low-frequency sound through the water-to-air interface

Abstract: L.M. Brekhovskikh revealed and studied the important role played by inhomogeneous waves emitted by a point source when they pass through an interface with a medium in which the velocity of sound is lower, for example, from water to air. This paper studies the energy characteristics of sound emitted into air by an underwater point source. The energy transfer due to inhomogeneous waves is shown to cause the phenomenon of anomalous transparency of the interface for low-frequency sound. The anomalous transparency manifests itself in that the energy flux through the interface increases with decreasing frequency of sound and, at sufficiently low frequencies, almost all of the acoustic energy produced by the underwater source is emitted into air. Conversely, at high frequencies, when the contribution of the inhomogeneous waves becomes negligible, the water-to-air interface is similar to a perfectly reflecting surface and almost all of the acoustic energy produced by the source is emitted into water. The anomalous transparency phenomenon changes the conventional opinion on the possibility of acoustic coupling between points in water and air and on the role played by physical processes evolving in the water column in generating atmospheric acoustic noise.

Discovery of the Putative Pulsar and Wind Nebula Associated with the TeV Gamma-ray Source HESS J1813-178

Abstract: We present a Chandra X-ray observation of G12.82-0.02, a shell-like radio supernova remnant coincident with the TeV gamma-ray source HESS J1813-178. We resolve the X-ray emission from the co-located ASCA source into a point source surrounded by structured diffuse emission that fills the interior of the radio shell. The morphology of the diffuse emission strongly resembles that of a pulsar wind nebula. The spectrum of the compact source is well-characterized by a power-law with index Gamma approx 1.3, typical of young and energetic rotation-powered pulsars. For a distance of 4.5 kpc, consistent with the X-ray absorption and an association with the nearby star formation region W33, the 2-10 keV X-ray luminosities of the putative pulsar and nebula are L(PSR) = 3.2E33 ergs/s and L(PWN) = 1.4E34 ergs/s, respectively. Both the flux ratio of L(PWN)/L(PSR) = 4.3 and the total luminosity of this system predict a pulsar spin-down power of Edot > 1E37 ergs/s, placing it within the ten most energetic young pulsars in the Galaxy. A deep search for radio pulsations using the Parkes telescope sets an upper-limit of approx 0.07 mJy at 1.4 GHz for periods >~ 50 ms. We discuss the energetics of this source, and consider briefly the proximity of bright H2 regions to this and several other HESS sources, which may produce their TeV emission via inverse Compton scattering.

Accurate Determination of Underground GPR Wavefront and B-Scan Shape From Above-Ground Point Sources

Abstract: While the determination of the wavefront shape radiated by ground-penetrating radar (GPR) in contact with the ground is trivial, the refraction of rays at the ground surface from a point source in the air makes accurate determination of the transmitted wavefront challenging. Impulse GPRs usually have their emitters in air, transmitting waves through a quasi-planar interface into the soil. The waves radiated initially in air can be approximated as circular, but once they enter the soil half-space, they propagate with a wavefront shape that resembles a hyperbola. The exact shape is derived and shown to be well approximated by a hyperbola with specific parameters depending on the relative dielectric constant of the half-space epsiv' ( = n2), the height of the source h, and the propagation time from source to wavefront multiplied by the wave velocity in the half-space p. A correction formula is provided to reduce the error between the approximate hyperbola and the exact shape. In addition, an equation for the shape of the B-scan contour of a point scatterer is derived.

Motions Near a Shallow Rupturing Fault: Evaluation of Effects Due to the Free Surface

Abstract: Summary The displacement of the surface of a half space near a shallow rupturing fault is, generally, approximated poorly by doubling the amplitude calculated for the same source in an infinite space. To obtain this result, the motions of a half space were calculated using a Green's function which is a solution to Lamb's problem, and the motions of an infinite space were calculated using the formulae of Haskell. A good approximation to the half-space displacement caused by a P- or S-wave incident at most angles from a point source is numerical correction of the displacement resulting from the point source in an infinite space for the amplification and phase shift of a plane-wave incident at a free surface. This correction approximately doubles the amplitude of the infinite space displacement for SV-waves with angles of incidence within 30 degrees of vertical, for P-waves within 70 degrees of vertical, and for all SH-waves. The static offset of the free surface from a point source is not, in general, twice the offset calculated in the corresponding infinite space case. The displacement from an extended fault is calculated by superposition of point sources on the fault plane; when the infinite space amplitudes may be doubled for all (or most) of these point sources, it may also be doubled for the extended source.

Spitzer-IRAC GLIMPSE of high mass protostellar objects. I Infrared point sources and nebulae

Abstract: Aims. We conduct a statistical study of candidate massive protostellar objects in the 3.6–8.0 μ m bands of the Spitzer Space Telescope.Methods. The GLIMPSE archive was used to obtain 3.6–8.0 μ m point source photometry and images for 381 massive protostellar candidates lying in the Galactic midplane. The colours, magnitudes, and spectral indices of sources in each of the 381 target fields were analysed, and compared with the predictions of 2D radiative transfer model simulations. Results. Infrared point sources with intrinsic reddening were found associated with several massive protostars. Although no discernable embedded clusters were found in any targets, multiple sources or associations of reddened young stellar objects were found in many sources, indicating multiplicity at birth. The spectral index (α ) of these point sources in 3.6–8.0 μ m bands display high values of α = 2-5. A colour–magnitude analog plot was used to identify 79 infrared counterparts to the HMPOs that are bright at 8 μ m , centred on millimetre peaks, and that display α values in excess of 2. Compact nebulae are found in 75% of the detected sources with morphologies that can be described well by core-halo, cometary, shell-like, and bipolar geometries similar to those observed in ultra-compact HII regions.Conclusions. The IRAC band spectral energy distributions (SED) of the infrared counterparts of massive protostellar candidates are best described as representing YSOs with a mass range of 8–20 in their Class I evolutionary stages when compared with 2D radiative transfer models. They also suggest that the high α values represent reprocessed star/star+disk emission that is arising in the dense envelopes. Thus we are witnessing the luminous envelopes around the protostars rather than their photospheres or disks. We argue that the compact infrared nebulae very likely reflect the underlying physical structure of the dense cores and are found to imitate the morphologies of known UCHII regions. The observations are consistent with a scenario where massive protostars have formed inside dense cores and continue to accrete matter. Our results favour models of continuuing accretion involving both molecular and ionised accretion components to build the most massive stars rather than purely molecular, rapid accretion flows.

A low energy tunable pulsed X-ray source based on the pseudospark electron beam

Abstract: A low energy pulsed X-ray source with 17 J electrical input energy has been constructed based on the vacuum spark configuration triggered by the pseudospark electron beam. With such a low input electrical energy, the frequently studied X-ray emitting vacuum spark plasma is not obtained. Instead, this configuration gives rise to a point-like electron beam-target pulsed X-ray source at the tip of the anode. Emission of X-ray is observed during the pre-breakdown phase. The X-ray spectrum is dominated by the characteristic line emission of the anode material. The total X-ray energy emission from the source can be estimated by assuming it to be a monochromatic point source. It is found to have an X-ray production efficiency of 0.1%, giving an average X-ray dosage of 18 mGy per pulse. X-ray radiography of small biological sample is demonstrated to illustrate a potential application of the present X-ray source.

Chandra point-source counts in distant galaxy clusters

Abstract: Context. With the superb angular resolution of the Chandra Observatory, it is now possible to detect X‐ray point sources, either e mbedded in galaxy clusters or along the cluster line of sight, which cou ld not be resolved by previous instruments. This now allows studies of source counts in distant cluster fields. Aims. We want to analyze the inner region of clusters of galaxies to check for the presence of any overdensity of X-ray point sources embedded in the gas diffuse emission. These point sources are possible AGN belonging to the clusters and could contaminate the cluster emission. Methods. We used a sample of 18 distant (0.25 < z < 1.01) galaxy clusters from the Chandra archive to construct the log N− log S , in both the soft and hard energy bands, for the X‐ray point sources detected in the central cluster region to be compared with the counts of point sources detected in similarly deep fields without clusters. Results. We find a ∼ 2 � excess of cluster region sources at the bright end of the log N− log S . The radial distribution of the brightest X‐ray point sources confirms this excess and indicates that it is co nfined to the inner 0.5 Mpc of the cluster region. Conclusions. The results suggest the possible existence of X‐ray sources belonging to the cluster (most probably AGN, given their 0.5‐10 keV luminosity ranging from 10 43 to 10 44 erg s −1 ): on average one every three clusters. Unlike previous studies, which have mainly investigated the point-source population in the vicinity of the galaxy clust ers, the present study analyzes the content of point sources within the 1 Mpc region covered by the cluster extent. Our work confirms the findings o f other investigators who analyzed the central 1 Mpc region of more massive clusters and/or groups in a similar redshift range. The X‐ray source excess found here is much smaller than the excess of radio galaxies found recently in high-z X‐ray selected clusters, possibly due to the better sensitivity of the radio observations.

A Hybrid Inversion Technique for the Evaluation of Source, Path, and Site Effects Employing S-Wave Spectra for Subduction and Upper-Crustal Earthquakes in El Salvador

Abstract: We propose a hybrid inversion analysis (hia) technique aimed at incorporating earthquakes from different seismogenic sources in one single inversion scheme. The application of this technique provides estimations of the source, path, and site effects at 23 strong-motion stations in El Salvador, Central America. The strong-motion dataset comprises 404 triaxial accelerograms corresponding to 63 subduction and upper-crustal earthquakes with M S magnitudes between 3.0 and 7.8. Application of the hia technique reveals: (1) good matching of the estimated source spectra with those derived from the ω 2 point source model; (2) rapid and low attenuation patterns characterizing the upper-crustal volcanic belt and subduction zone, respectively; and (3) soil transfer functions characterized by amplification ratios larger than those derived from spectral ratio techniques by a factor of two. Reference- site-dependent techniques yield similar soil transfer functions at all stations, whereas the horizontal-to-vertical spectral ratio technique yields similar results at sites characterized by clearly predominant peaks.

Waveform inversion of volcano-seismic signals for an extended source

Abstract: [1] We propose a method to investigate the dimensions and oscillation characteristics of the source of volcano-seismic signals based on waveform inversion for an extended source. An extended source is realized by a set of point sources distributed on a grid surrounding the centroid of the source in accordance with the source geometry and orientation. The source-time functions for all point sources are estimated simultaneously by waveform inversion carried out in the frequency domain. We apply a smoothing constraint to suppress short-scale noisy fluctuations of source-time functions between adjacent sources. The strength of the smoothing constraint we select is that which minimizes the Akaike Bayesian Information Criterion (ABIC). We perform a series of numerical tests to investigate the capability of our method to recover the dimensions of the source and reconstruct its oscillation characteristics. First, we use synthesized waveforms radiated by a kinematic source model that mimics the radiation from an oscillating crack. Our results demonstrate almost complete recovery of the input source dimensions and source-time function of each point source, but also point to a weaker resolution of the higher modes of crack oscillation. Second, we use synthetic waveforms generated by the acoustic resonance of a fluid-filled crack, and consider two sets of waveforms dominated by the modes with wavelengths 2L/3 and 2W/3, or L and 2L/5, where W and L are the crack width and length, respectively. Results from these tests indicate that the oscillating signature of the 2L/3 and 2W/3 modes are successfully reconstructed. The oscillating signature of the L mode is also well recovered, in contrast to results obtained for a point source for which the moment tensor description is inadequate. However, the oscillating signature of the 2L/5 mode is poorly recovered owing to weaker resolution of short-scale crack wall motions. The triggering excitations of the oscillating cracks are successfully reconstructed.

Restless rays, steady wave fronts

Abstract: Observations of underwater acoustic fields with vertical line arrays and numerical simulations of long-range sound propagation in an ocean perturbed by internal gravity waves indicate that acoustic wave fronts are much more stable than the rays comprising these wave fronts. This paper provides a theoretical explanation of the phenomenon of wave front stability in a medium with weak sound-speed perturbations. It is shown analytically that at propagation ranges that are large compared to the correlation length of the sound-speed perturbations but smaller than ranges at which ray chaos develops, end points of rays launched from a point source and having a given travel time are scattered primarily along the wave front corresponding to the same travel time in the unperturbed environment. The ratio of root mean square displacements of the ray end points along and across the unperturbed wave front increases with range as the ratio of ray length to correlation length of environmental perturbations. An intuitive physical explanation of the theoretical results is proposed. The relative stability of wave fronts compared to rays is shown to follow from Fermat’s principle and dimensional considerations.

Bioluminescence imaging of point sources implanted in small animals post mortem: evaluation of a method for estimating source strength and depth

Abstract: The performance of a simple approach for the in vivo reconstruction of bioluminescent point sources in small animals was evaluated. The method uses the diffusion approximation as a forward model of light propagation from a point source in a homogeneous tissue to find the source depth and power. The optical properties of the tissue are estimated from reflectance images obtained at the same location on the animal. It was possible to localize point sources implanted in mice, 2-8 mm deep, to within 1 mm. The same performance was achieved for sources implanted in rat abdomens when the effects of tissue surface curvature were eliminated. The source power was reconstructed within a factor of 2 of the true power for the given range of depths, even though the apparent brightness of the source varied by several orders of magnitude. The study also showed that reconstructions using optical properties measured in situ were superior to those based on data in the literature.

Scalar transport from a point source in flows over wavy walls

Abstract: Simultaneous measurements of the velocity and concentration field in fully developed turbulent flows over a wavy wall are described. The concentration field originates from a low-momentum plume of a passive tracer. PLIF and digital particle image velocimetry are used to make spatially resolved measurements of the structure of the scalar distribution and the velocity. The measurements are performed at three different Reynolds numbers of Re b = 5,600, Re b = 11,200 and Re b = 22,400, respectively, based on the bulk velocity u b and the total channel height 2h. The velocity field and the scalar field are investigated in a water channel with an aspect ratio of 12:1, where the bottom wall of the test section consists of a train of sinusoidal waves. The wavy wall is characterized by the amplitude to wavelength ratio α = 0.05 and the ratio β between the wave amplitude and the half channel height where β = 0.1. The scalar is released from a point source at the wave crest. For the concentration measurements, Rhodamine B is used as tracer dye. At low to moderate Reynolds number, the flow field is characterized through a recirculation zone which develops after the wave crest. The recirculation zone induces high intensities of the fluctuations of the streamwise velocity and wall-normal velocity. Furthermore, large-scale structures are apparent in the flow field. In previous investigations it has been shown that these large-scale structures meander laterally in flows over wavy bottom walls. The investigations show a strong effect of the wavy bottom wall on the scalar mixing. In the vicinity of the source, the scalar is transported by packets of fluid with a high scalar concentration. As they move downstream, these packets disintegrate into filament-like structures which are subject to strong gradients between the filaments and the surrounding fluid. The lateral scale of the turbulent plume is smaller than the lateral scale of the large-scale structures in the flow field and the plume dispersion is dominated by the structures in the flow field. Due to the lateral meandering of the large-scale structures of the flow field, also the scalar plume meanders laterally. Compared to turbulent plumes in plane channel flows, the wavy bottom wall enhances the mixing effect of the turbulent flow and the spreading rate of the scalar plume is increased.

Enhanced sound transmission from water to air at low frequencies

Abstract: Excitation of acoustic radiation into the air from a low-frequency point source under water is investigated using plane wave expansion of the source spectrum and Rayleigh reflection/transmission coefficients. Expressions are derived for the acoustic power radiated into air and water as a function of source depth and given to lowest order in the air/water density ratio. Near zero source depth, the radiation into the water is quenched by the source's acoustic image, while the power radiated into air reaches about 1% of the power that would be radiated into unbounded water.