Showing papers on "Point source published in 1999"

Auditory localization of nearby sources. II. Localization of a broadband source.

Abstract: Although many researchers have examined auditory localization for relatively distant sound sources, little is known about the spatial perception of nearby sources. In the region within 1 m of a listener’s head, defined as the “proximal region,” the interaural level difference increases dramatically as the source approaches the head, while the interaural time delay is roughly independent of distance. An experiment has been performed to evaluate proximal-region localization performance. An auditory point source was moved to a random position within 1 m of the subject’s head, and the subject responded by pointing to the perceived location of the sound with an electromagnetic position sensor. The overall angular error (17°) was roughly comparable to previously measured results in distal-region experiments. Azimuth error increased slightly as the sound source approached the head, but elevation performance was essentially independent of source distance. Distance localization performance was generally better than has been reported in distal-region experiments and was strongly dependent on azimuth, with the stimulus–response correlation ranging from 0.85 to the side of the head to less than 0.4 in the median plane. The results suggest that the enlarged binaural difference cues found in the head-related transfer function (HRTF) for nearby sources are important to auditory distance perception in the proximal region.

One-channel time-reversal in chaotic cavities: Experimental results

Abstract: Experiments are presented that show the feasibility of reconstructing a point source using acoustic time-reversal with a single transmitter/receiver. The propagation medium is a closed 2-D silicon cavity with chaotic ray dynamics and negligible absorption. Injection of a short pulse at one point yields a long signal at a second one; by reversing a part of this signal, we obtain a focus at the initial injection point. The characterization of the focus was observed by scanning with an optical interferometer and by measuring the signal at the focal spot. With circular converging wavefronts, the reconstructed focus was excellent (corresponding to an aperture of 360°), but not perfect. The increase in quality of the focus with growing length of the reversed signal is described by a statistical ray model. Despite the irreversibility in classical chaos (due to strong sensitivity to initial conditions), the underlying chaotic ray dynamics is useful in this case.

Investigations of the proximity effect for pollutants in the indoor environment.

Abstract: More than a dozen indoor air quality studies have reported a large discrepancy between concentrations measured by stationary indoor monitors (SIMs) and personal exposure monitors (PEMs). One possible cause of this discrepancy is a source proximity effect, in which pollutant sources close to the respondent cause elevated and highly variable exposures. This paper describes three sets of experiments in a home using real-time measurements to characterize and quantify the proximity effect relative to a fixed distant location analogous to a SIM. In the first set of experiments, using sulfur hexafluoride (SF6) as a continuously emitting tracer pollutant from a point source, measurements of pollutant concentrations were made at different distances from the source under different air exchange rates and source strengths. A second set of experiments used a continuous point source of carbon monoxide (CO) tracer pollutant and an array of high time resolution monitors to collect simultaneous concentration readings at different locations in the room. A third set of experiments measured particle count density and particle-bound polycyclic aromatic hydrocarbon (PAH) concentrations emitted from a continuous particle point source (an incense stick) using two particle counters and two PAH monitors, and included human activity periods both before and during the source emission period. Results from the SF6 and CO experiments show that while the source is emitting, a source proximity effect can be seen in the increases in the mean and median and in the variability of concentrations closest to the source, even at a distance of 2.0 m from the source under certain settings of air exchange rate and source strength. CO concentrations at locations near the source were found to be higher and more variable than the predictions of the mass balance model. For particles emitted from the incense source, a source proximity effect was evident for the fine particle sizes (0.3 to 2.5 µm) and particle-bound PAH up to at least 1.0 m from the source. Analysis of spatial and temporal patterns in the data for the three tracer pollutants reveal marked transient elevations of concentrations as seen by the monitor, referred to as “microplumes,” particularly at locations close to the source. Mixing patterns in the room show complex patterns and directional effects, as evidenced by the variable intensity of the microplume activity at different locations. By characterizing the spatial and temporal variability of pollutant concentrations in the home, the proximity effect can be quantified, leading to improved indoor monitoring designs and models of human exposure to air pollutants.

On the Theory of Pulse Propagation and Two-Frequency Field Statistics in Irregular Interstellar Plasmas

Abstract: We have computed two-frequency second moments and pulse profiles for plane and spherical waves both propagating in an extended plasma and incident on a thin plasma phase screen located between the source and the observer. The various models we consider for the power spectrum of the interstellar electron density fluctuations versus wavenumber are the simple power-law model, the power-law model with an inner scale, the "β=4" model, and the square-law structure function model. The power-law model with spectral exponent β=11/3 corresponds to the Kolmogorov turbulence spectrum. The β=4 model describes the random distribution in location and orientation of discrete objects with relatively sharp boundaries, such as shocks, across the line of sight. An outer scale is included in the β=4 model to account for the average size of such objects. The results for the various spectral models generally lie between the extremes represented by the simple Kolmogorov and square-law structure function models. In the strong scattering limit, the diffractive decorrelation bandwidth is related to the scattering broadening time via a Fourier uncertainty relation. We have computed numerical values for the uncertainty constant for the various geometries and spectral models and find values ranging over a factor of 2. For the Kolmogorov spectrum, the value is less than that in the widely-used Taylor, Manchester, & Lyne (1993) catalog by factors of 1.98 and 2.34 for a point source in an extended medium and in screen geometries, respectively. Thus observers must use the appropriate constant when comparing observations with a specific spectral and geometrical model. We also note that high signal-to-noise ratio observations of the scattering tails of pulsar pulses may be used to constrain the various spectral models of the Galactic electron density fluctuations, independent of the distribution of the scattering material along the line of sight.

Polarimetry and Unification of Low-Redshift Radio Galaxies

Abstract: We have made high-quality measurements of the polarization spectra of 13 FR II radio galaxies and taken polarization images for 11 of these with the Keck telescopes. Seven of the eight narrow-line radio galaxies (NLRGs) are polarized, and six of the seven show prominent broad Balmer lines in polarized light. The broad lines are also weakly visible in total flux. Some of the NLRGs show bipolar regions with roughly circumferential polarization vectors, revealing a large reflection nebula illuminated by a central source. Our observations powerfully support the hidden quasar hypothesis for some NLRGs. According to this hypothesis, the continuum and broad lines are blocked by a dusty molecular torus, but can be seen by reflected, hence polarized, light. Classification as a NLRG, a broad-line radio galaxy (BLRG), or a quasar therefore depends on orientation. However, not all objects fit into this unification scheme. Our sample is biased toward objects known in advance to be polarized, but the combination of our results with the 1996 findings of Hill, Goodrich, and DePoy show that at least six out of a complete, volume and flux-limited sample of nine FR II NLRGs have broad lines, seen either in polarization or P{alpha}.The BLRGs in ourmore » sample range from 3C 382, which has a quasar-like spectrum, to the highly reddened IRAS source FSC 2217+259. This reddening sequence suggests a continuous transition from unobscured quasar to reddened BLRG to NLRG. Apparently the obscuring torus does not have a distinct edge. The BLRGs have polarization images that are consistent with a point source broadened by seeing and diluted by starlight. We do not detect extended nebular or scattered emission, perhaps because it is swamped by the nuclear source. Our starlight-corrected BLRG spectra can be explained with a two-component model: a quasar viewed through dust and quasar light scattered by dust. The direct flux is more reddened than the scattered flux, causing the polarization to rise steeply to the blue. Strong rotations of the electric vector position angle across H{alpha} in 3C 227 and 3C 445 may be explained by systematic orbital motions in an equatorial broad-line region. (c) (c) 1999. The American Astronomical Society.« less

Elimination of Free Surface-related Multiples Without Need of the Source Wavelet

Abstract: This paper presents a new, wave‐equation based method for eliminating the effect of the free surface from marine seismic data without destroying primary amplitudes and without any knowledge of the subsurface. Compared with previously published methods which require an estimate of the source wavelet, the present method has the following characteristics: it does not require any information about the marine source array and its signature, it does not rely on removal of the direct wave from the data, and it does not require any explicit deghosting. Moreover, the effect of the source signature is removed from the data in the multiple elimination process by deterministic signature deconvolution, replacing the original source signature radiated from the marine source array with any desired wavelet (within the data frequency‐band) radiated from a monopole point source. The fundamental constraint of the new method is that the vertical derivative of the pressure or the vertical component of the particle velocity is...

Isotropic Wavelets: a Powerful Tool to Extract Point Sources from CMB Maps

Abstract: It is the aim of this paper to introduce the use of isotropic wavelets to detect and determine the flux of point sources appearing in CMB maps. The most suited wavelet to detect point sources filtered with a Gaussian beam is the Mexican Hat. An analytical expression of the wavelet coefficient obtained in the presence of a point source is provided and used in the detection and flux estimation methods presented. For illustration the method is applied to two simulations (assuming Planck Mission characteristics) dominated by CMB (100 GHz) and dust (857 GHz) as these will be the two signals dominating at low and high frequency respectively in the Planck channels. We are able to detect bright sources above 1.58 Jy at 857 GHz (82% of all sources) and above 0.36 Jy at 100 GHz (100% of all) with errors in the flux estimation below 25%. The main advantage of this method is that nothing has to be assumed about the underlying field, i.e. about the nature and properties of the signal plus noise present in the maps. This is not the case in the detection method presented by Tegmark and Oliveira-Costa 1998. Both methods are compared producing similar results.

Analytical Approximations for Calculating the Escape and Absorption of Radiation in Clumpy Dusty Environments

Abstract: We present analytical approximations for calculating the scattering, absorption, and escape of nonionizing photons from a spherically symmetric two-phase clumpy medium, with either a central point source of isotropic radiation, a uniform distribution of isotropic emitters, or uniformly illuminated by external sources. The analytical approximations are based on the mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman, combined with escape and absorption probability formulae for homogeneous media. The accuracy of the approximations is examined by comparison with three-dimensional Monte Carlo simulations of radiative transfer, including multiple scattering. Our studies show that the combined mega-grains and escape/absorption probability formulae provide a good approximation of the escaping and absorbed radiation fractions for a wide range of parameters characterizing the clumpiness and optical properties of the medium. A realistic test of the analytic approximations is performed by modeling the absorption of a starlike source of radiation by interstellar dust in a clumpy medium and by calculating the resulting equilibrium dust temperatures and infrared emission spectrum of both the clumps and the interclump medium. In particular, we find that the temperature of dust in clumps is lower than in the interclump medium if the clumps are optically thick at wavelengths at which most of the absorption occurs. Comparison with Monte Carlo simulations of radiative transfer in the same environment shows that the analytic model yields a good approximation of dust temperatures and the emerging UV-FIR spectrum of radiation for all three types of source distributions mentioned above. Our analytical model provides a numerically expedient way to estimate radiative transfer in a variety of interstellar conditions and can be applied to a wide range of astrophysical environments, from clumpy star-forming regions to starburst galaxies.

An Edge-brightened Bicone in the Nuclear Regions of Cygnus A

Abstract: Infrared-imaging observations that span the wavelength range of 08-235 μm have been obtained for the archetypal powerful radio galaxy Cygnus A using the Near-Infrared Camera and Multiobject Spectrograph on board the Hubble Space Telescope At 225 μm, the images are dominated by the presence of a nuclear point source (FWHM < 021), whose flux is a factor of ~4 times less than the limits deduced from previous ground-based studies The observations also reveal an edge-brightened biconical structure centered on the point source, which is strikingly similar to those observed around young stellar objects The high polarization and orientation of the bicone relative to the radio axis lead us to conclude that it is an illuminated structure, while the edge brightening provides evidence that the bicone is defined as much by outflows in the nuclear regions as by the polar diagram of the illuminating quasar radiation field A further implication of our observations is that not all of the anisotropy in the nuclear radiation field is caused by extinction on a scale less than 100 pc in the torus; some of the anisotropy must be generated by absorption and scattering in the dust lane on a 1 kpc scale

Modeling of buried monopole and dipole sources of acoustic emission with a finite element technique

Abstract: The requirements for dynamic finite-element modeling of the source dynamics and wave propagation of buried acoustic-emission point sources were examined. Maximum permissible source and cell sizes for point sources were determined as a function of the minimum wavelength for frequencies of interest These wavelengths were calculated from the source rise-times. For both buried monopoles and dipoles, finite-element predictions for both plates and half-spaces were compared with published results obtained from other approaches. The modeled signals were evaluated from the epicenter to a distance of 15 plate thicknesses for plates with a thickness of s 25 mm. The finite-element method provided accurate results and a practical means to analyze \ finite specimens, unlike most alternate approaches.

Method of synthesizing an audio signal

Abstract: A method of synthesizing an audio signal having left and right channels corresponding to an extended virtual sound source at a given apparent location in space relative to a preferred position of a listener in use is described. The information in the channels includes cues for perception of the direction of said virtual sound source from the preferred position. The extended source comprises a plurality of point virtual sources, the sound from each point source being spatially related to the sound from the other point sources, such that sound appears to be emnitted from an extended region of space. If the signal from two sound sources is the same, they are modified to be sufficiently different from one another to be separately distinguishable by a listener when they are disposed symmetrically on either side of the listener. This modification can be accomplished by filtering the two point sources using different comb filters.

The effect of point sources on satellite observations of the cosmic microwave background

Abstract: We study the effect of extragalactic point sources on satellite observations of the cosmic mi-crowavebackground(CMB). In order to separate the contributionsdue to differentforegroundcomponents, a maximum-entropy method is applied to simulated observations by the PlanckSurveyor satellite. In addition to point sources, the simulations include emission from theCMB and the kinetic and thermal Sunyaev-Zel’dovich (SZ) effects from galaxy clusters, aswell as Galactic dust, free-free and synchrotron emission.We find that the main input com-ponents are faithfully recovered and, in particular, that the quality of the CMB reconstructionis only slightly reduced by the presence of point sources. In addition, we find that it is pos-sible to recover accurate point source catalogues at each of the Planck Surveyor observingfrequencies.Key words: methods: data analysis – techniques: image processing – cos mic microwavebackground. 1 INTRODUCTIONA new generation of cosmic microwave background (CMB) satel-lite missions are currently in the final stages of design. The NASAMAP satellite is expected to be launched by 2000, followed by theESA Planck Surveyor in 2007 (Bersanelli et al. 1996). Both mis-sions will provide detailed all-sky maps of the CMB anisotropies,leading to definitive measurements of the CMB power spectrum .This should allow tight constraints to be placed on fundamentalcosmological parameters and distinguish between competing the-ories of structure formation in the early Universe such as inflationand topological defects.The maps produced by these satellites will, however, containcontributions from various foreground components, most notablyGalactic dust, free-free and synchrotron emission as well as thekinetic and thermal SZ effects from galaxy clusters. In addition,significant contamination from extragalactic point source s is alsoexpected. It is therefore clear that in order to obtain maps of theCMB anisotropies alone, it is necessary to separate the emissiondue to these various components.In a previous paper, Hobson et al. (1998) (hereafter Paper I)use a non-linear maximum-entropy method (MEM) to separate theemission due to the different foreground components from simu-lated Planck Surveyor observations of a 10 ×10 deg

Refinements to the geometry factor used in the AAPM Task Group Report No. 43 necessary for brachytherapy dosimetry calculations

Abstract: Determination of the geometry factor is necessary for brachytherapydosimetry calculations as recommended by the AAPM Task Group No. 43 (TG-43). The equivalence and errors associated with use of a point source approximation for an extended line segment source are examined. For all angles, the error using the point source approximation is less than 2% for distances in which the ratio of radius to active source length, (r/L), exceed about 3.6. A novel approach to determining the geometry factor using Monte Carlo methods is discussed in which the particle flux emanates from the active source and streams with no interactions occurring within the source or phantom. This method was performed for determining the geometry factor along the transverse axis for six brachytherapy sources. Differences in the geometry factor exceeding 2% between the point source approximation and that obtained using Monte Carlo methods occurred at distances ranging from 0.5 to 5 mm from the source center along the transverse plane. The merits of the Monte Carlo approach for solving the geometry factor are discussed in light of using a point or line source approximation for calculating additional brachytherapydosimetry parameters.

A persistent ~1 HZ quasi-periodic oscillation in the dipping low-mass x-ray binary 4U 1323-62

Abstract: We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistent emission, the dips, and the type I X-ray bursts of the low-mass X-ray binary 4U 1323-62. The rms amplitude of the QPO is approximately 9%, only weakly depending on photon energy. The amplitude is consistent with being constant throughout the persistent emission, the dips, and the bursts in all but one observation, where it is much weaker during one dip. These properties suggest that we have observed a new type of QPO, which is caused by quasi-periodic obscuration of the central X-ray source by a structure in the accretion disk. This can only occur when the binary inclination is high, consistent with the fact that 4U 1323-62 is a dipping source. The quasi-periodic obscuration could take place by partial covering of an extended central X-ray source by a near-opaque medium or by covering of a point source by a medium having suitable characteristics to produce the relatively energy-independent oscillations.

Investigating the geometry of quasars with microlensing

Abstract: When a source crosses the caustics of a gravitational microlens at cosmological distances, a high-magnification event occurs. This is seen as a change in the total flux of the image of the source. We present an analytic result for the magnification of a point source near a parabolic fold caustic. This is a higher-order approximation to the existing solution for a straight fold caustic. We show from a study of fold-crossing events that we can distinguish between disc and annular source geometries. One important application of this work will be the determination of the geometric structure of the luminous regions of quasars. By observing the flux changes of a quasar image during a microlensing event, it is possible to determine features of quasar structure that would otherwise be below the resolution limit available with existing optical methods.

Robust retrieval of a seismic point-source time function

Abstract: SUMMARY A comparison of two waveform-inversion methods designed to retrieve the mechanism of a seismic source and of its time function is presented using vertical-component synthetic signals, computed for velocity with a maximum frequency of 10 Hz. The geometry of the array of recording stations simulates the northeastern Italy Seismometric Network (OGS Trieste), consisting of 16 stations of which 12 are short period, vertical component only, three are short period, three components, and one is broad band. The synthetic seismograms are inverted using an inconsistent forward modelling technique; that is, by means of Green’s functions (GFs) constructed for a structural model diVerent from those used to generate the synthetic data. The approach based on ‘overparametrization’ of the rupture process, by means of independent moment tensor rate functions (MTRFs), and their subsequent reduction to the source time function (STF) (Method I) is shown to be superior to a traditional approach where the rupture process is constrained a priori (Method II). With Method I, the eVects of inconsistent structural modelling are partially absorbed into the uncorrelated parts of the MTRFs and their reverse slips, which allows us to eliminate them by subsequent retention of the STF as their positively constrained correlated part. Method I is shown to be able to yield a reasonable estimate of the STF even in the case when the traditional approach fails completely. Inadequacy of the GF, which may occur due to mislocation of the hypocentre, is taken into account by comparing the two approaches: the source depth is optimized simultaneously with the determination of the mechanism and the source time function. In addition to its capacity to handle inaccurate structural models, the overparametrization yielding a linear inverse scheme is completely independent from the starting model of the mechanism: Method II, using a gradient scheme, can proceed properly only if the starting source parameters are suYciently close to the true ones. The extension of the comparison of the performances between the two methods to an M d 3.0 earthquake near Friuli, in February 1988, recorded by seven stations of the OGS Trieste Network gives results in good agreement with the synthetic tests. The orientation of the nodal planes retrieved using Method I is in good agreement with the orientation of the source mechanism retrieved from the polarity of first arrivals, while Method II gives consistent results only when starting from the source parameters retrieved using Method I.

The power spectrum of the point source catalogue redshift survey

Abstract: We measure the redshift-space power spectrum P(k) for the recently completed IRAS Point Source Catalogue (PSC) redshift survey, which contains 14500 galaxies over 84% of the sky with 60 micron flux � 0.6 Jansky. Comparison with simulations shows that our estimated errors on P(k) are realistic, and that systematic errors due to the finite survey volume are small for wavenumbers k > � 0.03hMpc 1 . At large scales our power spectrum is intermediate between those of the earlier QDOT and 1.2 Jansky surveys, but with considerably smaller error bars; it falls slightly more steeply to smaller scales. We have fitted families of CDM-like models using the Peacock-Dodds formula for non-linear evolution; the results are somewhat sensitive to the assumed small-scale

The Midcourse Space Experiment Point Source Catalog Version 1.2 Explanatory Guide

Abstract: : This Explanatory Guide to the MSX Point Source Catalog briefly describes the instrumentation, and the experiments, which emphasized the areas that IRAS did not survey, and the Galactic Plane A detailed discussion of the calibration and data processing follows This guide then describes the catalog contents and presents analyses of the reliability of the quoted fluxes and positions, and the completeness and reliability of the catalog as a whole

Diffraction-limited Imaging and Photometry of NGC 1068

Abstract: The nearby Seyfert 2 galaxy NGC 1068 was observed with speckle imaging techniques in the near-infrared H band (1.6 μm) with the 200-inch Hale Telescope on Palomar Mountain and K band (2.2 μm) with the 10 m Keck I Telescope at the W. M. Keck Observatory. Images with diffraction-limited or near–diffraction-limited resolutions of 0”.05–0”.1 were obtained and used to search for structure in the nuclear region. Images of the nucleus of NGC 1068 reveal an extended region of emission, accounting for nearly 50% of the nuclear flux in the K band. This region extends 10 pc on either side of an unresolved point-source nucleus that is at most 0”.02 or 1.4 pc in size. Both the point source and the newly imaged extended emission are very red, with identical H-K colors corresponding to a color temperature of 800 K. While the point source is of a size to be consistent with grains in thermal equilibrium with the nuclear source, the extended emission is not. It must consist either of nuclear emission that has been reflected off an extended dusty disk or of small grains raised to transiently high temperatures by reflected UV photons.

Phase analysis and focusing of synchrotron radiation

Abstract: High accuracy calculations of synchrotron radiation (SR) emitted by a relativistic electron show that the phase of the frequency domain electric field of SR differs from the phase of radiation of a virtual point source. These differences may result in the reduction of focusing efficiency of diffraction-limited SR, if the focusing is performed by conventional optical components optimised for point sources. We show that by applying a phase correction locally, one may transform the phase of SR electric field at a desired polarisation to that of a point source. Such corrections are computed for undulator radiation (planar and helical) and bending magnet radiation (central part and edges). The focusing of the corrected SR wavefront can result in the increase of peak intensity in the focused spot up to several times compared to the focusing without correction. For non-diffraction-limited radiation, the effect of the phase corrections is reduced. Due to this reason, the use of the proposed phase corrections in existing electron storage rings is essentially of interest in the photon energy range from infrared to VUV. All numerical calculations discussed in the paper were performed by means of the computer code SRW.

Vacuum spark and spherical pinch x-ray/EUV point sources

Abstract: The technology of x-ray/EUV point plasma sources is competing with the multiple beam synchrotrons as radiation sources for submicron lithography. The company ALFT has been doing research and development on two plasma point sources for several years now. They are the vacuum spark (VSX) and spherical pinch (SPX) technologies. Both have a long history of previous research to support the contention that are well qualified to be converted into technological tools for the manufacturing of the next generation of IC chips. The VSX is essentially a miniature discharge capable of emitting soft x-ray radiation. Because the plasma emits a small dose of x- ray in each spark, it is necessary to repeat the phenomenon at high frequency in order to meet the requirement for microlithography. The SPX is mainly a strong source of EUV/X-ray radiation that operates at a frequency of one hertz or more.

An ASCA Study of the Supernova Remnant G39.2–0.3

Abstract: We present the analysis of archival data from the Advanced Satellite for Cosmology and Astrophysics of the supernova remnant (SNR) G39.2-0.3. G39.2-0.3 has been sometimes characterized as a shell-like remnant in the literature, but our high-energy imaging and spectral analysis show the unambiguous composite nature of the remnant. We find that part of the contribution to the X-ray emission of G39.2-0.3 is distinctly nonthermal, best described by a power law with a photon index (2.53). The region of emission is consistent with a point source (extension consistent with the point-spread function of the detector at the off-axis angle of the observation) and is roughly defined by a circle of radius ~4'. A second contribution comes from a thermal component, which contributes flux primarily at low energies. Despite the absence of any pulsed emission detected from the compact source, we argue that the emission is most probably due to a rotating compact object that is powering the detected synchrotron nebula within the SNR.

High-Energy Emission from Relativistic Particles in Sagittarius A*

Abstract: EGRET onboard the Compton Gamma Ray Observatory has recently detected a high-energy source at the Galactic center. However, it is not yet clear whether the γ-rays are produced by the hypothesized massive black hole, Sgr A*, or whether the emitter is diffuse. The lack of variability in the γ-ray flux, up to an amplitude of about 20%, seems to suggest the latter. But hydrodynamical simulations of the accretion process point to a fluctuation amplitude in the inflowing mass rate below this level on a timescale comparable to the current observation time line. Thus, if the γ-ray flux is directly tied to the dissipation of gravitational energy, e.g., if it is produced by relativistic particles energized by a shock within the infalling plasma, we cannot rule Sgr A* out as the source of the γ-rays on this basis alone. In earlier work, we demonstrated that the γ-ray spectrum from the Galactic center may be consistent with the radiative decay of pions produced via proton-proton interactions in this shock acceleration scenario. However, this study was incomplete for several reasons. In this paper, we extend this work significantly by making several improvements to our treatment of the particle physics. We also make use of the new EGRET data published since our last paper. Our analysis now indicates that the hypothesized massive black hole is probably not the source of the γ-rays. Understanding the nature of the Galactic center γ-ray source may ultimately depend on whether or not future γ-ray detectors will have the capability of resolving it. In this paper we consider a point source, but if the γ-rays are in fact associated with diffuse emission, GLAST may be able to image the source with a spatial resolution of ~30'' to 5'.

Comparison of iterative and Monte Carlo methods for calculation of the aureole about a point source in the Earth’s atmosphere

Abstract: Point sources in the atmosphere are surrounded by aureoles because of atmospheric scattering. The properties of an aureole were calculated by use of a Monte Carlo approach and an iterative method for an isotropic source and an axially symmetric emission source inside an infinite homogeneous atmosphere. The influence of single-scattering albedo, optical depth between source and observer, and source intensity anisotropy were studied from both approaches. For each situation, the limits and advantages of the Monte Carlo technique and the iterative method are described.

Localizing Gaseous Fugitive Emission Sources by Combining Real-Time Optical Remote Sensing and Wind Data

Abstract: This paper presents a new approach to localize point emissions from ground-level fugitive gaseous air pollution sources. We estimate the crosswind plume's ground-level peak location downwind from the source by combining smooth basis functions minimization (SBFM) with pathintegrated optical remote sensing concentration data acquired along the crosswind direction in alternating beam path lengths. Peak location estimates, in conjunction with real-time measured wind direction data, are used to reconstruct the fugitive source location. We conducted a synthetic data study to evaluate the proposed peak location SBFM reconstruction. Furthermore, the methodology was validated with open-path Fourier transform infrared concentration data collected with wind direction data downwind from a controlled point source. This approach was found to provide reasonable estimates of point source location. The field study reconstructed source location was within several meters of the real source location.

Application of the contouring method to extended microlensed sources

Abstract: The method devised by Lewis et al. (1993) for calculating the light curve of a microlensed point source is expanded to two dimensions to enable the calculation of light curves of extended sources. This method is significantly faster than the ray shooting method that has been used in the past. The increased efficiencyis used to obtain much higher resolution light curves over increased time scales. We investigate the signatures arising from different source geometries in a realistic microlensing model. We show that a large fraction of high magnification events (HMEs) in image A of Q2237+0305 involve only one caustic, and could therefore yield information on the structure of the quasar continuum through the recognition of a characteristic event shape. In addition, the cataloguing of HMEs into morphological type will, in theory, enable the direction of the transverse motion, as well as the source size to be obtained from long term monitoring.

Determination of the fault plane using a single near-field seismic station with a finite-dimension source model

Abstract: SUMMARY We explore the possibility of determining the actual fault plane of an earthquake from the inversion of near-source displacement seismograms of one station when a finitedimension source is used instead of a point source model and when the complete displacement is taken into account, including near-field waves. Tests on synthetic seismograms and real data recorded at local distances show that this is possible even with a single, three-component station. A single accelerogram available for the Erzincan, Turkey, 1992 March 13, M s =6.8 earthquake is inverted and the solution found is compatible with other seismological studies and with the mechanism expected for the North Anatolian Fault.

Properties of S waves near a kiss singularity: a comparison of exact and ray solutions

Abstract: SUMMARY We have studied the properties of S waves generated by a point source in a homogeneous, transversely isotropic, elastic medium, propagating in directions close to that of a kiss singularity, which coincides with the symmetry axis of the medium. We have proved analytically as well as numerically that the ray solution can describe the S waves correctly far from the source in all directions, including that of the kiss singularity. We have found that, in contrast to the far-field P wave, which can be reproduced satisfactorily by the zeroth-order ray approximation in all directions from the source, the far-field S waves can be reproduced satisfactorily by the zeroth-order ray approximation only in directions far from the kiss singularity. In directions near the kiss singularity, the first-order ray approximation must also be considered, because the zerothorder ray approximation yields distorted results. The first-order ray approximation can be of high frequency and can be detected in the far field.

Explicit Green's function of a boundary value problem for a sphere and trapped flux analysis in Gravity Probe B experiment

Abstract: Magnetic flux trapped on the surface of superconducting rotors of the Gravity Probe B (GP-B) experiment produces some signal in the superconducting quantum interference device readout. For the needs of GP-B error analysis and simulation of data reduction, this signal is calculated and analyzed in this article. We first solve a magnetostatic problem for a point source on the surface of a sphere, finding the closed form elementary expression for the corresponding Green’s function. Second, we calculate the flux through the pick-up loop as a function of the source position. Next, the time dependence of a source position, caused by rotor motion according to a symmetric top model, and thus the time signature of its flux are determined, and the spectrum of the trapped flux signal is analyzed. Finally, a multipurpose program of trapped flux signal generation based on the above results is described, various examples of the signal obtained by means of this program are given, and their features are discussed. Signals of up to 100 fluxons, i.e., 100 pairs of positive and negative point sources, are examined.