Showing papers on "Point source published in 2023"

Detailed Analysis of the TeV γ-Ray Sources 3HWC J1928+178, 3HWC J1930+188, and the New Source HAWC J1932+192

Abstract: The latest High Altitude Water Cherenkov (HAWC) point-like source catalog up to 56 TeV reported the detection of two sources in the region of the Galactic plane at galactic longitude 52° < ℓ < 55°, 3HWC J1930+188 and 3HWC J1928+178. The first one is associated with a known TeV source, the supernova remnant SNR G054.1+00.3. It was discovered by one of the currently operating Imaging Atmospheric Cherenkov Telescope (IACT), the Very Energetic Radiation Imaging Telescope Array System (VERITAS), detected by the High Energy Stereoscopic System (H.E.S.S), and identified as a composite SNR. However, the source 3HWC J1928+178, discovered by HAWC and coincident with the pulsar PSR J1928+1746, was not detected by any IACT despite their long exposure on the region, until a recent new analysis of H.E.S.S. data was able to confirm it. Moreover, no X-ray counterpart has been detected from this pulsar. We present a multicomponent fit of this region using the latest HAWC data. This reveals an additional new source, HAWC J1932+192, which is potentially associated with the pulsar PSR J1932+1916, whose γ-ray emission could come from the acceleration of particles in its pulsar wind nebula. In the case of 3HWC J1928+178, several possible explanations are explored, in an attempt to unveil the origins of the very-high-energy γ-ray emission.

Numerical Simulation Study on Spatial Diffusion Behavior of Non-Point Source Fugitive Dust under Different Enclosure Heights

Abstract: Non-point source fugitive dust produced during municipal road construction is one of the main ambient air pollutants gravely threatening the life and health of construction workers and residents around construction areas. In this study, a gas-solid two-phase flow model is used to simulate the diffusion behavior of non-point source dust with different enclosure heights under wind loads. Moreover, the inhibitory effect of the enclosure on the diffusion of non-point source dust from construction to residential areas is analyzed. The results show that the physical blocking and reflux effects of the enclosure can effectively restrain dust diffusion. When the enclosure height is 3–3.5 m, the concentration of particulate matter in most sections of residential areas can be reduced to less than 40 μg/m3. Moreover, when the wind speed is 1–5 m/s and the enclosure height is 2–3.5 m, the diffusion height of non-point source dust particles above the enclosure is concentrated in the range 1.5–2 m. This study provides a scientific basis for setting the heights of enclosures and atomization sprinklers at construction sites. Further, effective measures are proposed to reduce the impact of non-point source dust on the air environment of residential areas and health of residents.

Diffraction of directional sources by rigid wedges or barriers.

Abstract: The secondary-edge-source method (SESM) has found widespread application in predicting edge diffraction. The method, however, is limited to point sources. Based on the concept of weighted summation of diffractive contribution from each propagation path, this letter extends the SESM to directional sources with equivalent acoustic centers. The validity of the proposed method is demonstrated by comparing theoretical results to those obtained using the omnidirectional monopole superposition method.

Estimation of Source Direction with Spherical Array Receiver in Molecular Communication

Abstract: This paper proposes a method to estimate the direction of a point source in a diffusive molecular communication (MC) system. The estimator neither requires the knowledge of the number of released molecules nor the distance from the source. The goal is achieved via a specifically designed spherical array of receivers composed of a finite number of fully-absorbing (FA) elements, i.e. receivers. It is proved that the source direction estimation without the knowledge of the number of released molecules is feasible only for the symmetrical configuration of receivers on the array. The proposed approach is based on a model for the temporal asymptotic number of molecules absorbed by multiple FA receivers that takes into account their reciprocal interaction. The accuracy of the estimation is measured in terms of root mean squared error for different scenarios.

On the Gamma-Ray Emission of the Andromeda Galaxy M31

Abstract: Using the γ-ray data obtained with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (Fermi) for ∼14 yr, we examine the high-energy emission emanating from the center of the Andromeda galaxy M31. Different from previously reported results, which show a seemingly extended source, we instead find two individual point sources, one consistent with being at the center and one 0.°4 southeast of the center. The emission of the former is well described using a log-parabola model, similar to those of previous studies, and that of the latter can be fitted with a power law. We discuss the possible origins for the two sources. M31's central source, now consistent with being a point source, necessitates a revisit of its previously discussed originations with this new property taken into consideration, in particular those cosmic rays or dark matter scenarios involving extended source distributions. The SE source appears to have a projected distance of ∼6 kpc from M31's center, and the investigation is required as to whether it is a source locally associated with M31, or is instead a background extragalactic one.

Extension of sound field reconstruction based on element radiation superposition method in a sparsity framework

Abstract: Nearfield acoustic holography (NAH) is a powerful tool for realizing source identification and sound field reconstruction. The wave superposition (WS)-based NAH is appropriate for the spatially extended sources and does not require the complex numerical integrals. Equivalent source method (ESM), as a classical WS approach, is widely used due to its simplicity and efficiency. In the ESM, a virtual source surface is introduced, on which the virtual point sources are taken as the assumed sources, and an optimal retreat distance needs to be considered. A newly proposed WS-based approach, the element radiation superposition method (ERSM), uses piston surface source as the assumed source with no need to choose a virtual source surface. To satisfy the application conditions of piston pressure formula, the sizes of pistons are assumed to be as small as possible, which results in a large number of pistons and sampling points. In this paper, transfer matrix modes (TMMs), which are composed of the singular vectors of the vibro-acoustic transfer matrix, are used as the sparse basis of piston normal velocities. Then, the compressive ERSM based on TMMs is proposed. Compared with the conventional ERSM, the proposed method maintains a good pressure reconstruction when the number of sampling points and pistons are both reduced. Besides, the proposed method is compared with the compressive ESM in a mathematical sense. Both simulations and experiments for a rectangular plate demonstrate the advantage of the proposed method over the existing methods.

Location and Activity Characterization of Gamma-Ray Point Sources Concealed in Shipping Containers Using Iterative Reconstruction and Modeling Cargo-Specific Attenuation

Abstract: Simulated measurements of traditional shipping container screening infrastructure based on large-area polyvinyl-toluene (PVT) or sodium-iodide detectors (NaI) are used alongside an iterative reconstruction algorithm to characterize the activity and location of a radioactive point source concealed within a shipping container loaded with cargo. A maximum likelihood expectation maximization reconstruction method is employed to reconstruct the source distribution under the assumption that there exists a single point source in the scenario.To account for shielding by the cargo, it is assumed that the encompassing cargo, which was chosen to represent iron cargo, such as scrap metal or machine parts, is homogeneously distributed throughout the 32.2-m3 container at realistic loaded container densities of 0.0, 0.2, or 0.6 gcm−3. When the material properties of the cargo are assumed known and provided to the algorithm, the method is capable of localizing the source to within 40.5 cm and estimating the activity to the correct order of magnitude for cases with no cargo and 0.2 gcm−3 iron cargo completely filling the 32.2-m3 volume. With iron cargo at a density of 0.6 gcm−3, the localization and activity estimation is significantly worse, which is attributed to the method of accounting for attenuation in the cargo, a decreased signal-to-noise ratio, and the use of gross-count data that include the effect of buildup radiation. Using 662-keV photopeak data from a NaI-based radiation portal monitor (RPM) achieves better results than gross-count data from a PVT- or NaI-based RPM with the correct order of magnitude activity estimates for all cargo densities.For scenarios where the material of the cargo is unknown, but its density and distribution are known, a brute-force search is performed to find the optimum mass attenuation coefficient that describes the cargo. From the range of mass attenuation coefficients obtained, the method is not capable of differentiating between different types of common cargo, but demonstrates the principle of the method for characterizing shipping container cargo. Ultimately, the largest limiting factor in this method is the use of a simple average to estimate the path length traveled from a point in the container through the cargo in the direction of a detector. The large area detectors result in a high variance in this path length, and the degree of attenuation is exponentially dependent on this value.Despite the simple method of accounting for attenuation in the cargo, the maximum likelihood expectation maximization point source (MLEM PS) method is able to characterize a concealed point source well in the case with a PVT-based RPM and 0.2 gcm−3, which is cargo above the average density of a shipping container, drastically reducing the search area in secondary screening processes. The MLEM PS algorithm, therefore, represents a means of enhancing shipping container screening procedures without requiring significant changes in infrastructure and hardware.

Optical Counterparts of Two Candidate Ultraluminous X-ray Sources in NGC 4536

Abstract: Archival {\it XMM-Newton}, {\it Chandra} and {\it Hubble Space Telescope (HST)} data have been used to study the X-ray and optical properties of two candidate ultraluminous X-ray sources (ULXs) in NGC\,4536. In order to search for potential optical counterparts, relative astrometry between {\it Chandra} and {\it HST} was improved, and as a result, optical counterparts were detected for both X-ray sources. To complement our findings (based on the archival data), ground-based optical spectra of the counterparts were obtained with the 6m BTA Telescope located at the Special Astrophysical Observatory (SAO). The calculated redshift (z = 0.4391$\pm$0.0010) for one of the sources (X-3) indicates that the source is, in fact, a background active galactic nucleus (AGN). Two possible optical counterparts (s1 and s2) were found for X-2. Whether s1 is point-like or an extended source is unclear: If it is point-like and the emission is dominated by the donor its spectral type indicates O-B star. The second source (s2) is point-like and is consistent with the colors and absolute magnitudes of a red supergiant.

Experimental Verification of a One-dimensional Diffraction-limit Coronagraph

Abstract: We performed an experimental verification of a coronagraph. As a result, we confirmed that, at the focal region where the planetary point spread function exists, the coronagraph system mitigates the raw contrast of a star-planet system by at least 1 × 10−5 even for the 1-λ/D star-planet separation. In addition, the verified coronagraph keeps the shapes of the off-axis point spread functions when the setup has the source angular separation of 1λ/D. The low-order wave front error and the non-zero extinction ratio of the linear polarizer may affect the currently confirmed contrast. The sharpness of the off-axis point spread function generated by the sub-λ/D separated sources is promising for the fiber-based observation of exoplanets. The coupling efficiency with a single mode fiber exceeds 50% when the angular separation is greater than 3–4×10−1 λ/D. For sub-λ/D separated sources, the peak positions (obtained with Gaussian fitting) of the output point spread functions are different from the angular positions of sources; the peak position moved from about 0.8λ/D to 1.0λ/D as the angular separation of the light source varies from 0.1λ/D to 1.0λ/D. The off-axis throughput including the fiber-coupling efficiency (with respect to no focal plane mask) is about 40% for 1-λ/D separated sources and 10% for 0.5-λ/D separated ones (excluding the factor of the ratio of pupil aperture width and Lyot stop width), where we assumed a linear-polarized-light injection. In addition, because this coronagraph can remove point sources on a line in the sky, it has another promising application for high-contrast imaging of exoplanets in binary systems.

Weak solutions to the near-field reflector problem with spatial restrictions approached with generalized reflectors constructed from ellipsoids

Abstract: We motivate then formulate a novel variant of the near-field reflector problem and call it the near-field reflector problem with spatial restrictions. Let $O$ be an anisotropic point source of light and assume that we are given a bounded open set $U$. Suppose that the light emitted from the source at $O$ in directions defined by the aperture $D\subseteq S^2$, of radiance $g(m)$ for $m\in D$, is reflected off $R\subset \overline{U}$, creating the irradiance $f(x)$ for $x\in T$. The inverse problem consists of constructing the reflector $R\subseteq \overline{U}$ from the given position of the source $O$, the input aperture $D$, radiance $g$, `target' set $T$, and irradiance $f$. We focus entirely on the case where the target set $T$ is finite.

Diffuse Sources, Clustering and the Excess Anisotropy of the Radio Synchrotron Background

Abstract: We present the largest low frequency (120~MHz) arcminute resolution image of the radio synchrotron background (RSB) to date, and its corresponding angular power spectrum of anisotropies (APS) with angular scales ranging from $3^\circ$ to $0.3^\prime$. We show that the RSB around the North Celestial Pole has a significant excess anisotropy power at all scales over a model of unclustered point sources based on source counts of known source classes. This anisotropy excess, which does not seem attributable to the diffuse Galactic emission, could be linked to the surface brightness excess of the RSB. To better understand the information contained within the measured APS, we model the RSB varying the brightness distribution, size, and angular clustering of potential sources. We show that the observed APS could be produced by a population of faint clustered point sources only if the clustering is extreme and the size of the Gaussian clusters is $\lesssim 1'$. We also show that the observed APS could be produced by a population of faint diffuse sources with sizes $\lesssim 1'$, and this is supported by features present in our image. Both of these cases would also cause an associated surface brightness excess. These classes of sources are in a parameter space not well probed by even the deepest radio surveys to date.

Constraints on the origins of the Galactic neutrino flux

Abstract: Galactic and extragalactic objects in the universe are sources of high-energy neutrinos that can be detected by the IceCube neutrino detector, with the former being easier to resolve due to comparatively smaller distances. Recently, a study done using cascade-like events seen by IceCube reported neutrino emission from the Galactic plane with $>$4$\sigma$ significance. In this work, we put a limit on the number of Galactic sources required to explain this emission. To achieve this, we make use of a simulation package created to simulate point sources in the Galaxy along with the neutrino and gamma-ray flux emissions originating from them. Along with making use of past IceCube sensitivity curves, we also account for Eddington bias effects due to Poisson fluctuations in the number of detected neutrino events. By making use of a toy-Monte Carlo simulation method, we find that there should be more than 10 sources, each with luminosities $10^{35}$ erg/s responsible for the Galactic neutrino emission. Our results constrain the number of individual point-like emission regions, which applies both to discrete astrophysical sources and to individual points of diffuse emission.

Modelling HOE performance with an extended source: experimental investigation using misaligned point sources

Abstract: Holographic Optical Elements (HOEs) have the potential to enable more compact, versatile and lightweight optical designs, but many challenges remain. Volume HOE’s have the advantage of high diffraction efficiency but they present both chromatic selectivity and chromatic dispersion which impact on their use with wide spectrum light sources. Single-colour LED sources have a narrow spectrum that reduces these issues and this makes them better suited for use with volume HOEs. However, the LED source size must be taken into consideration for compact volume HOE-LED systems. To investigate the design limits for compact HOE-LED systems, a theoretical and experimental study was carried out on the effects of an extended source on the HOE output for different holographic lenses, with focal lengths from 2.5-10 cm. The lenses were recorded in Bayfol HX200 material and their diffraction efficiency was characterised across the lens aperture by measuring the Bragg angular selectivity curve at each location. Offset point sources were used to experimentally study the effects of a non-point source on the HOEs and the system was also modelled using Matlab and Zemax.

[Validation of calculation method for dose distribution around radioactive iodine-125 particles based on AAPM TG43 report].

Abstract: Objective: According to the formula provided by the TG43 report [AAPM TG43 (2004)] proposed by the American Association of Physicists in Medicine (AAPM) in 2004, we calculated the dose distribution around the radioactive iodine-125 particles, and verified the calculation accuracy of the radioactive iodine-125 particles treatment planning system. Methods: AAPM TG43 (2004) report provides two calculation methods when calculating the dose around a single radioactive source. The calculation method that does not consider the geometric structure of the radioactive source is called point source calculation method, and the calculation method that considers the geometric structure of the radioactive source is called line source calculation method. Assuming a single Amersham 6711 radioactive iodine-125 particle with an activity of 100 U, the following point doses were calculated according to the two calculation methods provided by AAPM TG43 (2004) report, at 0°, 90° directions, distances 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5 and 6 cm; In the direction of 45°, the doses at 0.71, 1.41, 2.12, 2.83, 3.54, 4.24, 4.95, 5.66, 6.36, 7.07, 7.78 and 8.49 cm. On the clinically used brachytherapy planning system variseeds 8.0, the above two calculation methods are used to calculate the corresponding activity and the dose around the corresponding type of radioactive iodine-125 particles, and the function of capturing points to templates built in the planning system is used to accurately find the above corresponding point position, using a single measurement of the above corresponding point dose; and comparation of the results were performed to see if there is a statistical difference. Results: The AAPM TG43 report uses point source calculation method to calculate the dose of single Amersham 6711 radioactive iodine-125 particles with activity of 100 U at 0° and 90° directions. The points with the same distance and the same dose are 8 082.18, 1 870.08, 756.58, 381.47, 217.11, 131.91, 86.55, 58.32, 39.97, 27.42, 19.74, 14.13 Gy, respectively, at 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5 and 6 cm away from them. In the 45° direction, the doses at the distances of 0.71, 1.41, 2.12, 2.83, 3.54, 4.24, 4.95, 5.66, 6.36, 7.07, 7.78 and 8.49 cm are 3 957.37, 865.83, 329.99, 155.69, 84.10, 48.50, 28.49, 17.80, 11.37, 7.38, 4.98 and 3.39 Gy, respectively; For line source calculation method, radioactive particles are at the same distance as above. The doses at each point in the direction of 0° are 3 128.71, 755.44, 330.30, 180.53, 107.74, 68.56, 46.40, 32.22, 22.70, 16.00, 11.51, 8.24 Gy, respectively. The doses at each point in the direction of 90° are 8 306.46, 1 981.01, 802.74, 405.38, 230.60, 140.03, 91.83, 61.84, 42.36, 29.05, 20.91, 14.97 Gy; In the 45° direction, the dose at the corresponding distance as above is 4 020.78, 877.43, 333.49, 156.93, 84.69, 48.81, 28.65, 17.89, 11.42, 7.41, 4.99 and 3.40 Gy, respectively. The maximum dose difference (0.3%) between the two methods is 7.78 cm in the 45° direction, the maximum difference (-0.3%) between the two methods is 8.49 cm in the 45° direction, and the value of other sampling points is less than 0.3%. The closer the Amersham 6711 iodine-125 particles are to the source in the directions of 0°, 45°, and 90°, the faster the dose will drop, and the dose will drop gradually as the distance increases. Conclusion: The brachytherapy planning system variseeds 8.0 and the AAPM TG43 report calculate a maximum dose difference of 0.3%, which can accurately calculate the dose distribution around radioactive iodine-125 seeds, and provide a reliable tool for the clinical implementation of radioactive iodine-125 particles implantation for tumor treatment.

Impact source identification on pipes using acoustic emission energy

Abstract: Impact is a common source of damage in pipes and pipeline systems and detecting the location and nature of damage is vital for reliability and safety of these systems. The main difficulty in the use of AE technology for such applications is being able to investigate the extent to which the temporal structure of such a non-impulsive event can be reconstructed using sensors located on the external surface of a pipe at some distance from the source. There is currently no reliable way of relating the temporal structure of the generating event to the temporal structure of an AE source. This work uses a set of matching AE experiments and finite element simulations to study the relationship between the generating AE event (dropped ball on a steel surface) and the resulting stress-time history recorded at a given point on the surface of the pipe. Two test objects were used; a solid cylindrical steel block of diameter 307mm and length 166mm and a 2m pipe length of diameter 100mm and wall thickness 10mm. The AE resulting from the surface impacts was recorded over a period of 2 seconds for both experiments and simulations. The work builds on an earlier study with the same test objects using impulsive sources. The results confirmed that a mechanical disturbance which is extended in time can be identified from its energy-time imprint carried on the stress wave.

Microseismic monitoring of wave propagation through heterogeneous media: a tool for brownfield site investigation?

Abstract: Gheith Alfakri1&#160; &#160;Emmanouil Parastatidis1&#160; Stella Pytharouli1 Abstract: Previous studies present evidence that microseismic monitoring could be a favourable potential technology for brownfield land site investigations, e.g. in the identification of buried objects in the shallow subsurface (< 3 m). More specifically, the presence of buried objects change the characteristics (amplitude and frequency) of a mechanical wave that propagates through a medium where this object lies. These changes have to-date only been observed at recordings from stations that are located directly above the buried object. To investigate whether a buried object can be &#8216;seen&#8217; by more sensors located in the vicinity above the object, we carry out a series of numerical simulations. We examine the propagation of a sine wave emitted by a point source on the surface of a medium and study the frequency, amplitude and emitted energy from that sine wave and how these are affected by local changes in the mechanical properties of the model. For the duration of each simulation, we record the velocity history at a number of points on the free surface of the model. Numerical simulations are carried out in FLAC3D. First, we look on how the distance between the source and the monitoring points changes what we record. We examine two cases : In Case A, the monitoring stations and the buried object are at a distance less than 30 meters from the seismic wave source. In Case B, the monitoring stations and buried object are at a distance more than 30 meters from the seismic wave source. We apply spectral analysis to the resultant seismic velocity time histories as recorded at a number of monitoring stations at the free surface of the model. Our results for Case A show that an object can be detected at a monitoring station located directly above the object to a depth of 1-2 meters. Results for Case B show that an object can be detected at the monitoring station that is deployed directly above the object to a depth of up to 4-5 meters, and it can also be detected at neighbouring stations, at distances approximately equal to the depth of the object. In addition, we study factors having an impact on the amount of energy of the seismic wave emitted, i.e. depth of the object from the surface and its mechanical properties. Our analysis indicates that by increasing the depth of the object, the amount of reflected seismic energy decreases. The changes in the mechanical properties of the materials lead to a change in seismic wave propagation velocity and frequency. Results from our numerical simulations present evidence that microseismics can be used as a complementary, low-cost site investigation tool for applications where very shallow depths are of particular interest such as those at brownfield sites. This can have significant implications on the way site investigations on brownfield sites are carried out, with microseismics providing an alternative to sites where traditional non-intrusive methods such as GPR and/or resistivity tomography are limited due to ground properties.&#160;

Point-source Contribution to the Diffuse X-Ray Background below 1 keV and Its Effect on Our Understanding of the Circumgalactic Medium

Abstract: We studied the spectral signature of different components of the diffuse X-ray background (DXB), including Local Hot Bubble, solar wind charge exchange, galactic halo, and typically unresolved point sources (galaxies and active galactic nuclei), in the direction of the Chandra Deep Field South using the 4 Ms XMM-Newton survey and Chandra 4 Ms Source Catalog. In this paper, we present our results showing how the different components contribute to the DXB below 1 keV. In particular, we have found that ∼6% of the emission at 34 keV (all-sky average value ≈3 × 10−3 cm−6 pc), which is typically associated with galactic halo (GH) and circumgalactic medium (CGM), is, in fact, due to emission from typically unresolved galaxies. We will discuss the effect that this has on our understanding of GH and CGM and to our understanding of the missing CGM baryons.

Sensitivity to point-like sources of the ALTO atmospheric particle detector array, designed for $\rm 200\,GeV$--$\rm 50\,TeV$ $\gamma$-ray astronomy

Abstract: In the context of atmospheric shower arrays designed for $\gamma$-ray astronomy and in the context of the ALTO project, we present: a study of the impact of heavier nuclei in the cosmic-ray background on the estimated $\gamma$-ray detection performance on the basis of dedicated Monte Carlo simulations, a method to calculate the sensitivity to a point-like source, and finally the required observation times to reach a firm detection on a list of known point-like sources.

Characterizing the expected behavior of non-Poissonian template fitting

Abstract: We have performed a systematic study of the statistical behavior of non-Poissonian template fitting (NPTF), a method designed to analyze and characterize unresolved point sources in general counts datasets. In this paper, we focus on the properties and characteristics of the Fermi-LAT gamma-ray data set. In particular, we have simulated and analyzed gamma-ray sky maps under varying conditions of exposure, angular resolution, pixel size, energy window, event selection, and source brightness. We describe how these conditions affect the sensitivity of NPTF to the presence of point sources, for inner-galaxy studies of point sources within the Galactic Center excess, and for the simplified case of isotropic emission. We do not find opportunities for major gains in sensitivity from varying these choices, within the range available with current Fermi-LAT data. We provide an analytic estimate of the NPTF sensitivity to point sources for the case of isotropic emission and perfect angular resolution, and find good agreement with our numerical results for that case.

Optical counterparts of two candidate ultraluminous X-ray sources in NGC 4536

Abstract: Archival XMM-Newton, Chandra and Hubble Space Telescope (HST) data have been used to study the X-ray and optical properties of two candidate ultraluminous X-ray sources (ULXs) in NGC 4536. In order to search for potential optical counterparts, relative astrometry between Chandra and HST was improved, and as a result, optical counterparts were detected for both X-ray sources. To complement our findings (based on the archival data), ground-based optical spectra of the counterparts were obtained with the 6m BTA Telescope located at the Special Astrophysical Observatory (SAO). The calculated redshift (z = 0.4391±0.0010) for one of the sources (X-3) indicates that the source is, in fact, a background active galactic nucleus (AGN). Two possible optical counterparts (s1 and s2) were found for X-2. Whether s1 is point-like or an extended source is unclear: If it is point-like and the emission is dominated by the donor its spectral type indicates O-B star. The second source (s2) is point-like and is consistent with the colors and absolute magnitudes of a red supergiant.

Image Formation from a Concave Mirror

Abstract: Computing locations and extent of images, except in the most trivial configurations or special cases, is a complex task. Even rays emanating from a point source and passing through an optical system generally fail to converge at a single image point, highlighting the care needed to establish image locations. We use three approaches to study image formation in a simple configuration, that of a point source following reflection from a spherical concave mirror. We calculate the caustic surfaces and compute cross sections of flux densities on image surfaces, and compare the results with experimentally generated light intensity fields. One of the two caustic surfaces is linear while the other forms a surface. The latter, undergoes a metamorphosis from a distorted cone to an open surface as the source is moved away from the axis. Cross sections of the caustic surfaces with an image plane are found to coincide with peaks in the flux density. Experimental studies validate these conclusions.

Identification of acoustic point sources in a two-layered medium from multi-frequency sparse far field patterns

Abstract: We consider the reconstruction of point sources in a two layered medium from the multi-frequency sparse far field patterns taken on the upper half sphere. The point sources are located in both the upper half space and the lower half space, and consequently bring difficulty for the inverse problem because of the combination of two different types of far field patterns. After establishing the uniqueness of the point sources by the multi-frequency far field patterns at properly chosen sparse observation directions, we introduce a multi-step numerical scheme for identifying all the points sources. Numerical examples show that the proposed sampling methods work very well for locating the positions and the formulas for determining the corresponding scattering strengths are valid and stable with respect to the noises.

Об одном "парадоксе" в теории рассеяния

Abstract: The solution of the "paradox" in scattering theory is considered, according to which the extinction cross section is expressed in terms of the forward scattering amplitude (the so-called "optical theorem"), whereas for a point source, and as a consequence, for any emitter located at a finite distance from the scatterer, a similar ratio is often written through a scattered field near the emitter, i.e. determined by "backscattering". A clear picture of the formation of radiation losses during the transition of energy from the source to the scatterer is presented. It is shown that although the field backscattered to the source determines the change in its radiation characteristics (the Purcell effect), the optical theorem includes an extinction factor which is generally related to the work of the incident wave on the currents induced in the scatterer. This factor passes into the forward scattering amplitude in the limiting case of a plane incident wave.