Showing papers on "Point source published in 2001"
TL;DR: In this article, a model of a horizontal circular crack in a semi-infinite elastic solid was proposed and exact expressions for vertical and horizontal displacements of the free surface of a half-space were derived for a special case of a uniformly pressurized crack.
Abstract: We consider deformation due to sill-like magma intrusions using a model of a horizontal circular crack in a semi-infinite elastic solid. We present exact expressions for vertical and horizontal displacements of the free surface of a half-space, and calculate surface displacements for a special case of a uniformly pressurized crack. We derive expressions for other observable geophysical parameters, such as the volume of a surface uplift/subsidence, and the corresponding volume change due to fluid injection/withdrawal at depth. We demonstrate that for essentially oblate (i.e. sill-like) source geometries the volume change at the source always equals the volume of the displaced material at the surface of a half-space. Our solutions compare favourably to a number of previously published approximate models. Surface deformation due to a ‘point’ crack (that is, a crack with a large depth-to-radius ratio) differs appreciably from that due to an isotropic point source (‘Mogi model’). Geodetic inversions that employ only one component of deformation (either vertical or horizontal) are unlikely to resolve the overall geometry of subsurface deformation sources even in a simplest case of axisymmetric deformation. Measurements of a complete vector displacement field at the Earth's surface may help to constrain the depth and morphology of active magma reservoirs. However, our results indicate that differences in surface displacements due to various axisymmetric sources may be subtle. In particular, the sill-like and pluton-like magma chambers may give rise to differences in the ratio of maximum horizontal displacements to maximum vertical displacements (a parameter that is most indicative of the source geometry) that are less than 30 per cent. Given measurement errors in geodetic data, such differences may be hard to distinguish.
312 citations
TL;DR: The location of the asymptotic virtual origin of positively buoyant turbulent plumes with a deficit of initial momentum flux when compared with equivalent pure plumes is investigated in this paper.
Abstract: The location of the asymptotic virtual origin of positively buoyant turbulent plumes with a deficit of initial momentum flux when compared with equivalent pure plumes is investigated. These lazy plumes are generated by continuous steady releases of momentum, buoyancy and volume into a quiescent uniform environment from horizontal sources (at z = 0) of finite area, and are shown to be equivalent to the far-field flow above point source pure plumes, of buoyancy only, rising from the asymptotic virtual source located below the actual source at z = −zavs.An analytical expression for the location of the asymptotic virtual source relative to the actual source of the lazy plume is developed. The plume conservation equations are solved for the volume flow rate, and the position of the asymptotic virtual origin is deduced from the scaling for the volume flow rate at large distances from the source.The displacement zavs of the asymptotic virtual origin from the actual origin scales on the source diameter and is a function of the source parameter Γ ∝ Qˆ20Fˆ0/Mˆ5/20 which is a measure of the relative importance of the initial fluxes of buoyancy Fˆ0, momentum Mˆ0, and volume Qˆ0 in the plume. The virtual origin correction developed is valid for Γ > 1/2 and is therefore applicable to lazy plumes for which Γ > 1, pure plumes for which Γ = 1, and forced plumes in the range 1/2 < Γ < 1. The dimensionless correction z*avs decreases as Γ increases, and for Γ [Gt ] 1, z*avs → 0.853Γ−1/5. Comparisons made between the predicted location of the asymptotic virtual origin and the location inferred from measurements of lazy saline plumes in the laboratory show close agreement.
239 citations
TL;DR: 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.
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...
185 citations
TL;DR: In this paper, radio and X-ray pulsations at a period of 51.6 ms from the EGRET source 3EG J2227+6122 were detected, and a new supernova remnant was identified as a pulsar wind nebula.
Abstract: We report the detection of radio and X-ray pulsations at a period of 51.6 ms from the X-ray source RX/AX J2229.0+6114 in the error box of the EGRET source 3EG J2227+6122. An ephemeris derived from a single ASCA observation and multiple epochs at 1412 MHz from Jodrell Bank indicates steady spin-down with = 7.83 × 10-14 s s-1. From the measured P and , we derive spin-down power = 2.2 × 1037 ergs s-1, magnetic field Bp = 2.0 × 1012 G, and characteristic age P/2 = 10,460 yr. An image from the Chandra X-Ray Observatory reveals a point source surrounded by centrally peaked diffuse emission that is contained within an incomplete radio shell. We assign the name G106.6+2.9 to this new supernova remnant, which is evidently a pulsar wind nebula. For a distance of 3 kpc estimated from X-ray absorption, the ratio of X-ray luminosity to spin-down power is ≈8 × 10-5, smaller than that of most pulsars but similar to the Vela pulsar. If PSR J2229+6114 is the counterpart of 3EG J2227+6122, then its efficiency of γ-ray production, if isotropic, is 0.016 (d/3 kpc) 2. It obeys an established trend of γ-ray efficiency among known γ-ray pulsars, which, in combination with the demonstrated absence of any other plausible counterpart for 3EG J2227+6122, makes the identification compelling. If confirmed, this identification bolsters the pulsar model for unidentified Galactic EGRET sources.
150 citations
TL;DR: The salient features of the photoacoustic point source are that rapid heat deposition and nonlinear thermal expansion dominate the production of ultrasound.
Abstract: We investigate the photoacoustic effect generated by heat deposition at a point in space in an inviscid fluid. Delta-function and long Gaussian optical pulses are used as sources in the wave equation for the displacement potential to determine the fluid motion. The linear sound-generation mechanism gives bipolar photoacoustic waves, whereas the nonlinear mechanism produces asymmetric tripolar waves. The salient features of the photoacoustic point source are that rapid heat deposition and nonlinear thermal expansion dominate the production of ultrasound.
147 citations
TL;DR: Experimental results show that it is possible to obtain a robust estimation on a single realization of disorder, taking advantage of the wide frequency bandwidth, and the quality of focusing saturates, which the authors believe is linked to the Thouless factor g.
Abstract: This is the second article in a series of two dealing with the statistical moments of ultrasonic waves transmitted through a disordered medium with resonant multiple scattering. Second-order moments in time and space are considered here. An ultrasonic pulsed wave is transmitted from a point source to a 128-element receiving array through two-dimensional samples with various thicknesses. The samples consist of random collections of parallel steel rods immersed in water. The scattered waves are recorded, time reversed, and sent back into the medium. The time-reversed waves are converging back to their source and the quality of spatial and temporal focusing on the source is related to the second-order moments of the scattered wave (correlation) in time and in space. Experimental results show that it is possible to obtain a robust estimation on a single realization of disorder, taking advantage of the wide frequency bandwidth. The spatial resolution of the system is only limited by the correlation length of the scattered field, and no longer by the array aperture. As the sample thickness is increased, the quality of focusing saturates, which we believe is linked to the Thouless factor g. In the thickest sample, $g\ensuremath{\sim}30,$ which is still well above the localization threshold.
146 citations
TL;DR: In this article, the authors present spectra and high-resolution images of the hard X-ray feature along the southern edge of the supernova remnant IC443 and argue that this object is a synchrotron nebula powered by the compact source that is physically associated with IC443.
Abstract: We present spectra and high resolution images of the hard X-ray feature along the southern edge of the supernova remnant IC443. Data from the Chandra X-ray Observatory reveal a comet-shaped nebula of hard emission, which contains a softer point source at its apex. We also present 20cm, 6cm, and 3.5cm images from the Very Large Array that clearly show the cometary nebula. Based on the radio and X-ray morphology and spectrum, and the radio polarization properties, we argue that this object is a synchrotron nebula powered by the compact source that is physically associated with IC443. The spectrum of the soft point source is adequately but not uniquely fit by a black body model (kT=0.71 +/- 0.08 keV, L=(6.5 +/- 0.9) * 10^31 erg/s). The cometary morphology of the nebula is the result of the supersonic motion of the neutron star (V_NS=250 +/- 50 km/s), which causes the relativistic wind of the pulsar to terminate in a bow shock and trail behind as a synchrotron tail. This velocity is consistent with an age of 30,000 years for the SNR and its associated neutron star.
145 citations
TL;DR: In this paper, an X-ray point source near the center of the 300-year old Cas A supernova remnant was detected in the Chandra/HRC data and the spectral properties and the timing limits of the point source were analyzed.
Abstract: The spectacular ii —rst light ˇˇ observation by the Chandra X-Ray Observatory revealed an X-ray point source near the center of the 300 yr old Cas A supernova remnant. We present an analysis of the public X-ray spectral and timing data. No coherent pulsations were detected in the Chandra/HRC data. The 3 p upper limit on the pulsed fraction is less than 35% for P ( 20 ms. The Chandra/ACIS spectrum of the point source may be —tted with an ideal blackbody (kT \ 0.5 keV) or with blackbody models modi—ed by the presence of a neutron star atmosphere (kT \ 0.25¨0.35 keV), but the temperature is higher and the inferred emitting area lower than expected for a 300 yr old neutron star according to standard cooling models. The spectrum may also be —tted with a power-law model (photon index ! \ 2.8¨3.6). Both the spectral properties and the timing limits of the point source are inconsistent with a young Crab-like pulsar but are quite similar to the properties of the anomalous X-ray pulsars. The spectral parameters are also very similar to those of the other radio-quiet X-ray point sources in the supernova remnants Pup A, RCW 103, and PKS 1209(52. Current limits on an optical counterpart for the Cas A point source rule out models that invoke fallback accretion onto a compact object if fallback disk properties are similar to those in quiescent low-mass X-ray binaries. However, the optical limits are mar- ginally consistent with plausible alternative assumptions for a fallback disk. In this case, accreting neutron star models can explain the X-ray data, but an accreting black hole model is not promising. Subject headings: accretion, accretion disksblack hole physicsstars: neutron ¨ supernovae: individual (Cassiopeia A) ¨ supernova remnantsX-rays: stars
127 citations
TL;DR: The EUROMET project 428 examines efficiency transfer results for Ge gamma-ray spectrometers when the efficiency is known for a reference point source geometry and concludes that these codes cannot be used directly for metrological purposes.
114 citations
TL;DR: In this paper, the authors derived the scaling laws for the image size and the pulse broadening of a point source and the observed pulse shape of a scattered impulse in the presence of disks and filaments.
Abstract: This paper considers scattering screens that have arbitrary spatial variations of scattering strength transverse to the line of sight, including screens that are spatially well confined, such as disks and filaments. We calculate the scattered image of a point source and the observed pulse shape of a scattered impulse. The consequences of screen confinement include (1) source image shapes that are determined by the physical extent of the screen rather than by the shapes of much smaller diffracting microirregularities (these include image elongations and orientations that are frequency dependent); (2) variation with frequency of angular broadening that is much weaker than the trademark ν-2 scaling law (for a cold, unmagnetized plasma), including frequency-independent cases; and (3) similar departure of the pulse-broadening time from the usually expected ν-4 scaling law. We briefly discuss applications that include scattering of pulses from the Crab pulsar by filaments in the Crab Nebula; image asymmetries from Galactic scattering of the sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active galactic nuclei by intervening galaxies. We also address the consequences for inferences about the shape of the wavenumber spectrum of electron density irregularities, which depend on scaling laws for the image size and the pulse broadening. Future low-frequency (<100 MHz) array observations will also be strongly affected by the Galactic structure of scattering material. Our formalism is derived in the context of radio scattering by plasma density fluctuations. It is also applicable to optical, UV, and X-ray scattering by grains in the interstellar medium.
107 citations
TL;DR: The unprecedented combination of RGS and EPIC images reveal that the limb- brightening of the plumes is mostly seen in higher ionization emission lines, while in the lower ionization lines, and below 0.5 keV, the plume is more homogeneously structured, points to new interpretations as to the make up of the starburst-driven outflow.
Abstract: We describe the first XMM-Newton observations of the starburst galaxy NGC 253. As known from previous X-ray observations, NGC 253 shows a mixture of extended (disk and halo) and point-source emission. The high XMM-Newton throughput allows for the first time a detailed investigation of the spatial, spectral and variability properties of these components simultaneously. We detect a bright X-ray transient approx. 70 sec SSW of the nucleus and show the spectrum and light curve of the brightest point source (approx. 30 sec S of the nucleus, most likely a black-hole X-ray binary, BHXRB). The unprecedented combination of RGS and EPIC also sheds new light on the emission of the complex nuclear region, the X-ray plume and the disk diffuse emission. In particular, EPIC images reveal that the limb-brightening of the plume is mostly seen in higher ionization emission lines, while in the lower ionization lines, and below 0.5 keV, the plume is more homo- geneously structured, pointing to new interpretations as to the make up of the starburst-driven outflow. Assuming that type IIa supernova remnants (SNRs) are mostly responsible for the E greater than 4 keV emission, the detection with EPIC of the 6.7 keV line allows us to estimate a supernova rate within the nuclear starburst of 0.2 /yr.
TL;DR: In this article, the authors presented VLBI observations of Sagittarius A* at 86 GHz using a six station array, including the VLBA antennas at Pie Town, Fort Davis, and Los Alamos, the 12 m antenna at Kitt Peak, and the millimeter arrays at Hat Creek and Owens Valley.
Abstract: At radio wavelengths, images of the compact radio source Sagittarius A* (Sgr A*) in the Galactic center are scatter broadened with a j2 dependence due to an intervening ionized medium. We present VLBI observations of Sgr A* at 86 GHz using a six station array, including the VLBA antennas at Pie Town, Fort Davis, and Los Alamos, the 12 m antenna at Kitt Peak, and the millimeter arrays at Hat Creek and Owens Valley. To avoid systematic errors due to imperfect antenna calibration, the data were modeled using interferometric closure information. The data are best modeled by a circular Gaussian brightness distribution of FWHM 0.18 ^ 0.02 mas. The data are also shown to be consistent with an elliptical model corresponding to the scattering of a point source. The source structure in the northsouth direction, which is less well determined than in the east-west direction because of the limited north-south u-v coverage of the array, is constrained to be less than 0.27 mas by these measurements. These results are consistent with extrapolations of intrinsic structure estimates obtained with VLBI at a 7 mm wavelength, assuming the intrinsic size of Sgr A* has a greater dependence than j0.9 with wavelength.
TL;DR: In a 52 ks long Chandra X-Ray Observatory ACIS-S observation of the nearby starburst galaxy NGC 3628, obtained to study the starburst-driven outflow from this galaxy, the authors have detected a very luminous (LX ≈ 1.1 × 1040 ergs s-1 in the 0.3-8.0 keV energy band) point source located at least 20'' (~970 pc) from the nucleus of the galaxy.
Abstract: In a 52 ks long Chandra X-Ray Observatory ACIS-S observation of the nearby starburst galaxy NGC 3628, obtained to study the starburst-driven outflow from this galaxy, we have detected a very luminous (LX ≈ 1.1 × 1040 ergs s-1 in the 0.3-8.0 keV energy band) point source located at least 20'' (~970 pc) from the nucleus of the galaxy. No radio, optical, or near-IR counterpart to this source has been found. This is most probably the reappearance of the strongly variable X-ray luminous source discovered by Dahlem, Heckman, & Fabbiano, which faded by a factor of 27 between 1991 December and 1994 March (at which point it had faded below the detection limit in a ROSAT HRI observation). This source is clearly a member of an enigmatic class of X-ray sources that are considerably more luminous than conventional X-ray binaries but less luminous than active galactic nuclei and which are not found at the dynamical center of the host galaxy. The Chandra spectrum is best fitted by an absorbed power-law model with a photon index of Γ = 1.8 ± 0.2, similar to that seen in Galactic black hole binary candidates in their hard state. Bremsstrahlung models or multicolor disk models (the favored spectral model for objects in this class on the basis of ASCA observations) can provide statistically acceptable fits only if the data at energies E > 5 keV are ignored. This is one of the first X-ray spectra of such an object that is unambiguously that of the source alone, free from the spectral contamination by X-ray emission from the rest of the galaxy that affects previous spectral studies of these objects using ASCA.
TL;DR: In this paper, a method of atmospheric Cherenkov imaging which reconstructs the unique arrival direction of TeV gamma rays using a single telescope is presented. But the method is derived empirically and utilizes several features of gamma-ray-induced air showers which determine, to a precision of 0.12°, the arrival direction.
TL;DR: In this article, near-infrared (J, H, and K) photometry and moderate-resolution (?/?? = 3000) K-band spectroscopy of the embedded stellar cluster in the giant H II region W31 is presented.
Abstract: We present near-infrared (J, H, and K) photometry and moderate-resolution (?/?? = 3000) K-band spectroscopy of the embedded stellar cluster in the giant H II region W31. Four of the brightest five cluster members are early O-type stars based on their spectra. We derive a spectrophotometric distance for W31 of 3.4 ? 0.3 kpc using these new spectral types and infrared photometry. The brightest cluster source at K is a red object that lies in the region of the J-H versus H-K color-color plot inhabited by stars with excess emission in the K band. This point source has an H- plus K-band spectrum that shows no photospheric features, which we interpret as being the result of veiling by local dust emission. Strong Brackett series emission and permitted Fe II emission are detected in this source; the latter feature is suggestive of a dense inflow or outflow. The near-infrared position of this red source is consistent with the position of a 5 GHz thermal radio source seen in previous high angular resolution VLA images. We also identify several other K-band sources containing excess emission with compact radio sources. These objects may represent stars in the W31 cluster still embedded in their birth cocoons.
TL;DR: In this paper, an estimation of the number and amplitude (in flux) of the extragalactic point sources that will be observed by the Planck Mission is presented, based on the Mexican Hat wavelet formalism.
Abstract: An estimation of the number and amplitude (in flux) of the extragalactic point sources that will be observed by the Planck Mission is presented in this paper. The study is based on the Mexican Hat wavelet formalism introduced by Cayon et al. Simulations at Planck observing frequencies are analysed, taking into account all the possible cosmological, Galactic and extragalactic emissions together with noise. With the technique used in this work, the Planck Mission will produce a catalogue of extragalactic point sources above the following flux values: 1.03 Jy (857 GHz), 0.53 Jy (545 GHz), 0.28 Jy (353 GHz), 0.24 Jy (217 GHz), 0.32 Jy (143 GHz), 0.41 Jy (100 GHz, high-frequency instrument), 0.34 Jy (100 GHz, low-frequency instrument), 0.57 Jy (70 GHz), 0.54 Jy (44 GHz) and 0.54 Jy (30 GHz), which are only slightly model dependent (see text). Amplitudes of these sources are estimated with errors below ∼15 per cent. Moreover, we also provide a complete catalogue (for the point-source simulations analysed) with errors in the estimation of the amplitude below ∼10 per cent. In addition we discuss the possibility of identifying different point-source populations in the Planck catalogue by estimating their spectral indices.
TL;DR: In this article, a new formulation for the eddy diffusivities to be used in atmospheric dispersion models has been derived, accounting for the current knowledge of the convective boundary layer structure and characteristics.
Abstract: Accounting for the current knowledge of the convective boundary layer structure and characteristics, a new formulation for the eddy diffusivities to be used in atmospheric dispersion models has been derived. That is, expressions for the eddy diffusivities, depending on source distance, for inhomogeneous turbulence are proposed. The classical statistical diffusion theory, the observed spectral properties, and observed characteristics of energy-containing eddies are used to estimate these parameters. In addition, a vertical eddy diffusivity was introduced into an air pollution model, validated with the data of Copenhagen experiments, and compared with an improved Gaussian model. The results of this new method are shown to agree with the measurements of Copenhagen and also with those of the Gaussian model. Furthermore, the current study suggests that the inclusion of the memory effect, important in regions near an elevated continuous point source, improves the description of the turbulent transport ...
TL;DR: In this paper, Chandra X-Ray Observatory was used to resolve spatially and spectrally the X-ray emission from the Circinus galaxy, which made up ≈34% of the total 0.5-10 keV emission.
Abstract: We have used the Chandra X-Ray Observatory to resolve spatially and spectrally the X-ray emission from the Circinus galaxy. We report here on the nature of the X-ray emission from the off-nuclear point sources associated with the disk of Circinus, which make up ≈34% of the total 0.5–10 keV emission. We find that many of the serendipitous X-ray sources are concentrated along the optical disk of the galaxy, although few have optical counterparts within 1'' of their X-ray positions down to limiting magnitudes of mV = 23–25. At the distance of Circinus (≈3.8 Mpc), their intrinsic 0.5–10 keV luminosities range from ≈2 × 1037 ergs s-1 to ≈4 × 1039 ergs s-1. One-fourth of the sources are variable over the duration of the 67 ks observation, and spectral fitting of these off-nuclear sources shows a diverse range of spectral properties. The overall characteristics of the point sources suggest that most are X-ray binaries and/or ultraluminous supernova remnants within Circinus. We are able to analyze the two strongest off-nuclear sources in greater detail and find both to have remarkable properties. The average X-ray luminosities of the two sources are 3.7 × 1039 ergs s-1 and 3.4 × 1039 ergs s-1. The former displays large and periodic flux variations every 7.5 hr and is well fitted by a multicolor blackbody accretion disk model with Tin = 1.35 keV, properties consistent with an eclipsing 50 M⊙ black hole binary. The latter appears to be a young supernova remnant, as it coincides with a nonthermal radio counterpart and an Hα-detected H II region. This source exhibits both long-term (≈4 yr) X-ray variability and a 6.67–6.97 keV iron emission-line blend with a 1.6 keV equivalent width. These two objects further support the notion that super-Eddington X-ray sources in nearby galaxies can be explained by a mixture of intermediate-mass black holes in X-ray binaries and young supernova remnants.
TL;DR: In this paper, the authors describe laboratory experiments to determine the dependence of the "front" formation and stratication on the source momentum and buoyancy on the single source, and on the location and relative strengths of two sources from which momentum and buoys were supplied separately, and show that for a single source with a nonzero input of momentum, the rate of descent of the front is more rapid than for the case of zero source momentum, and increases with increasing momentum input.
TL;DR: In this paper, a maximum entropy method (MEM) and Mexican Hat wavelet (MHW) joint analysis is presented to recover the different components of the microwave sky from simulated observations by the ESA Planck satellite in a small patch of the sky.
Abstract: We present a maximum–entropy method (MEM) and ‘Mexican Hat’ wavelet (MHW) joint analysis to recover the different components of the microwave sky from simulated observations by the ESA Planck satellite in a small patch of the sky (12.8×12.8 deg 2 ). This combined method allows one to improve the CMB, Sunyaev– Zel’dovich and Galactic foregrounds separation achieved by the MEM technique alone. In particular, the reconstructed CMB map is free from any bright point source contamination. The joint analysis also produces point source catalogues at each Planck frequency which are more complete and accurate than those obtained by each method on its own. The results are especially improved at high frequencies where infrared galaxies dominate the point source contribution. Although this joint technique has been performed on simulated Planck data, it could be easily applied to other multifrequency CMB experiments, such as the forthcoming NASA MAP satellite or the recently performed Boomerang and MAXIMA experiments.
TL;DR: In this article, Chandra ACIS-S observations have detected a point source at the central star of NGC 6543 and confirmed the point source nature of the hard X-ray emission from NGC 7293.
Abstract: Recent Chandra ACIS-S observations have detected a point source at the central star of NGC 6543 and confirmed the point source nature of the hard X-ray emission from NGC 7293. The X-ray spectra of both sources peak between 0.5 keV and 1.0 keV and show line features indicating a thin plasma at temperatures of a few times 10^6 K. Their X-ray luminosities are 10^30 erg/s and 3x10^29 erg/s, respectively. We have considered four different mechanisms to explain the nature of these sources. The X-ray emission from the central star of NGC 6543 may originate from the coronal activity of an undetected companion star or from shocks in its fast stellar wind, while the hard X-ray emission from NGC 7293 might be ascribed to an undetected dMe companion. Follow-up observations are needed to determine the existence and natures of these stellar companions.
TL;DR: The first field experiment designed to evaluate a new approach for quantifying gaseous fugitive emissions of area air pollution sources found that this approach is applicable to many types of industrial areas or volume sources, given the use of an adequate PI-ORS system.
Abstract: This paper describes results from the first field experiment designed to evaluate a new approach for quantifying gaseous fugitive emissions of area air pollution sources. The approach combines path-integrated concentration data acquired with any path-integrated optical remote sensing (PI−ORS) technique and computed tomography (CT) technique. In this study, an open-path Fourier transform infrared (OP−FTIR) instrument sampled path-integrated concentrations along five radial beam paths in a vertical plane downwind from the source. A meteorological station collected measurements of wind direction and wind speed. Nitrous oxide (N2O) was released from a controlled area source simulator. The innovative CT technique, which applies the smooth basis function minimization method to the beam data in conjunction with measured wind data, was used to estimate the total flux from the simulated area source. The new approach estimates consistently underestimated the true emission rates in unstable atmospheric conditions an...
TL;DR: In this article, the authors used the Chandra X-ray Observatory to resolve spatially and spectrally the Xray emission from the Circinus Galaxy and found that many of the serendipitous sources are concentrated along the optical disk of the galaxy, but few have optical counterparts within 1" of their positions down to V=23-25.
Abstract: We have used the Chandra X-ray Observatory to resolve spatially and spectrally the X-ray emission from the Circinus Galaxy. We report here on the nature of the X-ray emission from the off-nuclear point sources associated with the disk of Circinus. We find that many of the serendipitous X-ray sources are concentrated along the optical disk of the galaxy, but few have optical counterparts within 1" of their X-ray positions down to V=23-25. At 3.8 Mpc, their intrinsic 0.5-10 keV luminosities range from approx. 2E37 erg/s to 4E39 erg/s. One quarter of the sources are variable over the duration of the 67 ks observation, and spectral fitting of these off-nuclear sources shows a diverse range of spectral properties. The properties of the two strongest off-nuclear sources are remarkable, with average X-ray luminosities of 3.7E39 erg/s and 3.4E39 erg/s. The former displays large and periodic flux variations every 7.5 hr and is well fit by a multicolor blackbody accretion-disk model with T_in=1.35 keV, properties consistent with an eclipsing >50 M_sun black-hole binary. The latter appears to be a young supernova remnant, as it coincides with a non-thermal radio counterpart and an H\alpha-detected HII region. This source exhibits both long-term (approx. 4 yr) X-ray variability and a 6.67-6.97 keV iron emission-line blend with a 1.6 keV equivalent width. These two objects further support the notion that super-Eddington X-ray sources in nearby galaxies can be explained by a mixture of intermediate-mass black holes in X-ray binaries and young supernova remnants. (abridged)
TL;DR: In this paper, a phase-space analysis of a rotational-shear volume interferometer is presented, where it is shown that a defocus region of the source location exists near the inter-ferometer, which indicates that the usual Fourier transform has its focal point at an infinite distance.
Abstract: A phase-space analysis of a rotational-shear volume interferometer is presented. It is shown that, in a fairly general condition of this interferometer and by using a Fourier transform method to retrieve spectrodirectional images, a defocus region of the source location exists near the interferometer. This indicates that the usual Fourier transform method has its focal point at an infinite distance. By focusing on finite-depth sources located in the defocus region, a new interferometric method to obtain both three-dimensional spatial information and spectral information of a stationary, quasi-homogeneous, polychromatic source distribution is developed. A key element of this method is the use of a filter function that acts on the volume interferogram of the rotational-shear volume interferometer. This filter function realizes, for a particular position of a monochromatic point source located within the defocus region, a diffraction-limited resolution of the interferometer for the spectrodirectional image.
TL;DR: In this article, a direct mathematical method is applied to calculate total and geometrical efficiencies of a parallelepiped (a × b × c ) detector for an arbitrarily positioned isotropic radiating point source.
TL;DR: In this article, the authors presented a detailed analysis of a pointed ASCA observation and archival ROSAT data of PSR J1119-6127 and its surroundings and revealed extended emission coincident with the newly-discovered radio supernova remnant G292.
Abstract: PSR J1119-6127 is a recently discovered 1700-year-old radio pulsar that has a very high inferred surface dipolar magnetic field. We present a detailed analysis of a pointed ASCA observation and archival ROSAT data of J1119-6127 and its surroundings. Both data sets reveal extended emission coincident with the newly-discovered radio supernova remnant G292.2-0.5, reported in a companion paper by Crawford et al. (astro-ph/0012287).. A hard point source, offset ~1.5' from the position of the radio pulsar, is seen with the ASCA GIS. No pulsations are detected at the radio period with a pulsed fraction upper limit of 61% (95% confidence). The limited statistics prevent a detailed spectral analysis, although a power-law model with photon index (Gamma=~1-2) describes the data well. Both the spectral model and derived X-ray luminosity are consistent with those measured for other young radio pulsars, although the spatial offset renders an identification of the source as the X-ray counterpart of the pulsar uncertain.
TL;DR: In this article, the authors present a fully relativistic approach to model both the continuum emission and the reflected fluorescent iron line from a primary X-ray source near a Kerr black hole.
Abstract: In this paper we present a fully relativistic approach to modelling both the continuum emission and the reflected fluorescent iron line from a primary X-ray source near a Kerr black hole. The X-ray source is located above an accretion disc orbiting around the black hole. The source is assumed to be a static point source located on an arbitrary position above the disc, on or off the axis of rotation. We carry out Monte Carlo simulations in order to estimate the iron line spectrum as well as its equivalent width. Because of the gravitational lensing effect, an enhancement of the iron line is expected when the primary source is located close to the central black hole. We find that for a source located on the axis of rotation the enhancement is relatively modest. An observer at inclination 30° would measure an equivalent width of ∼300 eV in the extreme case of a maximally rotating black hole and a source located at height 1.5 gravitational radii from the centre. This corresponds to an equivalent width enhancement factor of about 2 compared with the classical value where no lensing effect comes into play. However, when allowing the source to be located off the axis of rotation, much stronger enhancement can be obtained. In the extreme case of a maximally rotating black hole and a source located just above the approaching side of the disc, an observer at inclination 30° could measure an equivalent width as high as ∼1.5 keV (i.e., ∼10 times the classical value). We also find that observers located at high inclination angles observe a stronger line than observers at low inclination angles.
24 Jul 2001
TL;DR: In this article, the authors proposed a distributed point sources method (DPSM) for modeling magnetic and ultrasonic sensors, where the sensor surface is discretized into a finite number of elemental surfaces and a point source is placed at the centroid position of every surface.
Abstract: In this paper, we describe a model based on a spatial distribution of point sources, called 'DPSM' (Distributed Point Sources Method), applied to magnetic and ultrasonic sensors modelling. Magnetic and acoustic fields are theoretically generated for two types of sensors. The sensor surface is discretized into a finite number of elemental surfaces. A point source is placed at the centroid position of every elemental surface. Point source strength is proportional to the elemental surface area for acoustic sensors and it is obtained by inverting a matrix to satisfy the equipotential boundary conditions for magnetic sensors. Total field is computed at a given point by adding fields generated by all sources. The main difference between the magnetic and acoustic field modelling is that for a magnetic sensor the magnetic potential remains constant on the sensor surface and the magnetic flux varies from point to point, while for the acoustic sensor the particle velocity remains constant on the sensor surface and the acoustic pressure varies. This difference causes an additional matrix inversion in the magnetic field modeling, which is not necessary for the acoustic field modeling. Like other numerical modeling schemes, accuracy of the computation depends on the sensor surface discretization or mesh generation. Effect of the spacing between two neighboring point sources on the accuracy of the field computation is studied and the optimum spacing for accurate numerical computation is given. For accurately modelling acoustic fields the spacing between two neighboring sources should be less than the acoustic wavelength. Flat sensors with circular and rectangular cross-sections as well as point focused concave sensors have been modelled by this technique.
TL;DR: In this paper, Chandra observations of the powerful radio galaxy 3C294 showed clear evidence for a surrounding intracluster medium, which has an hour-glass shape in the N-S direction, extending to radii of at least 100 kpc, well beyond the radio source.
Abstract: We present a Chandra observation of the powerful radio galaxy 3C294 showing clear evidence for a surrounding intracluster medium. At a redshift of 1.786 this is the most distant cluster of galaxies yet detected in X-rays. The radio core is detected as a point source, which has a spectrum consistent with a heavily-absorbed power law implying an intrinsic 2-10 keV luminosity of ~10^45 erg/s. A small excess of emission is associated with the southern radio hotspots. The soft, diffuse emission from the intracluster medium is centred on the radio source. It has an hour-glass shape in the N-S direction, extending to radii of at least 100 kpc, well beyond the radio source. The X-ray spectrum of this extended component is fit by a thermal model with temperature ~5 keV, or by gas cooling from above 7 keV at rates of ~400-700 Msolar/yr. The rest-frame 0.3-10 keV luminosity of the cluster is ~4.5x10^44 erg/s. The existence of such a cluster is consistent with a low density universe.
TL;DR: In this article, the authors investigated the Green function for the case when the source point is located at or close to the free surface and showed that it oscillates with indefinitely increasing amplitude and indefinitely decreasing wavelength.
Abstract: The Green function used for analysing ship motions in waves is the velocity potential due to a point source pulsating and advancing at a uniform forward speed. The behaviour of this function is investigated, in particular for the case when the source is located at or close to the free surface. In the far field, the Green function is represented by a single integral along one closed dispersion curve and two open dispersion curves. The single integral along the open dispersion curves is analysed based on the asymptotic expansion of a complex error function. The singular and highly oscillatory behaviour of the Green function is captured, which shows that the Green function oscillates with indefinitely increasing amplitude and indefinitely decreasing wavelength, when a field point approaches the track of the source point at the free surface. This sheds some light on the nature of the difficulties in the numerical methods used for predicting the motion of a ship advancing in waves.