Showing papers on "Point source published in 2005"

First GLIMPSE Results on the Stellar Structure of the Galaxy

Abstract: The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ~30 million mid-infrared sources toward the inner Galaxy, 10° ≤ |l| ≤ 65° and |b| ≤ 1°, was used to determine the distribution of stars in Galactic longitude, l, latitude, b, and apparent magnitude, m. The counts versus longitude can be approximated by the modified Bessel function N = N0(l/l0)K1(l/l0), where l0 is insensitive to limiting magnitude, band choice, and side of Galactic center: l0 = 17°-30° with a best-fit value in the 4.5 μm band of l0 = 24° ± 4°. Modeling the source distribution as an exponential disk yields a radial scale length of H* = 3.9 ± 0.6 kpc. There is a pronounced north-south asymmetry in source counts for |l| 30°, with ~25% more stars in the north. For l = 10°-30°, there is a strong enhancement of stars of m = 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length Rbar = 4.4 ± 0.5 kpc, tilted by = 44° ± 10° to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l = 26°-28°, 315-34°, and 306°-309° are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (mK-m[8.0] > 3) peak. In these areas, the counts are reduced by extinction due to molecular gas, high diffuse backgrounds associated with star formation, or both.

Point‐Source Extraction with MOPEX

Abstract: MOPEX (Mosaicking and Point‐source Extraction) is a package developed at the Spitzer Science Center for astronomical image processing. We report on the point‐source extraction capabilities of MOPEX. Point‐source extraction is implemented as a two‐step process: point‐source detection and profile fitting. Nonlinear matched filtering of input images can be performed optionally to increase the signal‐to‐noise ratio and improve detection of faint point sources. Point response function (PRF) fitting of point sources produces the final point‐source list, which includes the fluxes and improved positions of the point sources, along with other parameters characterizing the fit. Passive and active deblending allows for successful fitting of confused point sources. Aperture photometry can also be computed for every extracted point source for an unlimited number of aperture sizes. The PRF is estimated directly from the input images. The implementation of efficient methods of background and noise estimation an...

The extended Chandra deep field-south survey: Chandra point-source catalogs

Abstract: We present Chandra point‐source catalogs for the Extended Chandra Deep Field–South (E‐CDF‐S) survey. The E‐CDF‐S consists of four contiguous 250 ks Chandra observations covering an approximately square region of total solid angle ≈0.3 deg2, which flank the existing ≈1 Ms Chandra Deep Field–South (CDF‐S). The survey reaches sensitivity limits of ≈ and ≈ ergs cm−2 s−1 for the 0.5–2.0 and 2–8 keV bands, respectively. We detect 762 distinct X‐ray point sources within the E‐CDF‐S exposure; 589 of these sources are new (i.e., not previously detected in the ≈1 Ms CDF‐S). This brings the total number of X‐ray point sources detected in the E‐CDF‐S region to 915 (via the E‐CDF‐S and ≈1 Ms CDF‐S observations). Source positions are determined using matched‐filter and centroiding techniques; the median positional uncertainty is ≈035. The basic X‐ray and optical properties of these sources indicate a variety of source types, although absorbed active galactic nuclei (AGNs) seem to dominate. In addition to our main Chandra catalog, we constructed a supplementary source catalog containing 33 lower significance X‐ray point sources that have bright optical counterparts (R < 23). These sources generally have X‐ray–to–optical flux ratios expected for normal and starburst galaxies, which lack a strong AGN component. We present basic number‐count results for our main Chandra catalog and find good agreement with the ≈1 Ms CDF‐S for sources with 0.5–2.0 and 2–8 keV fluxes greater than and ergs cm−2 s−1, respectively. Furthermore, three extended sources are detected in the 0.5–2.0 keV band, which are found to be likely associated with galaxy groups or poor clusters at ; these have typical rest‐frame 0.5–2.0 keV luminosities of ergs s−1.

XBootes: An X-Ray Survey of the NDWFS Bootes Field. I. Overview and Initial Results

Abstract: We obtained a 5 ks deep Chandra X-ray Observatory ACIS-I map of the 9.3 deg2 Bootes field of the NOAO Deep Wide-Field Survey. Here we describe the data acquisition and analysis strategies leading to a catalog of 4642 (3293) point sources with 2 or more (4 or more) counts, corresponding to a limiting flux of roughly 4(8) ? 10-15 ergs cm-2 s-1 in the 0.5-7 keV band. These Chandra XBootes data are unique in that they constitute the widest contiguous X-ray field yet observed to such a faint flux limit. Because of the extraordinarily low background of the ACIS, we expect only 14% (0.7%) of the sources to be spurious. We also detected 43 extended sources in this survey. The distribution of the point sources among the 126 pointings (ACIS-I has a 16' ? 16' field of view) is consistent with Poisson fluctuations about the mean of 36.8 sources per pointing. While a smoothed image of the point source distribution is clumpy, there is no statistically significant evidence of large-scale filamentary structure. We do find however, that for ? > 1', the angular correlation function of these sources is consistent with previous measurements, following a power law in angle with slope ~-0.7. In a 1.4 deg2 sample of the survey, approximately 87% of the sources with 4 or more counts have an optical counterpart to R ~ 26 mag. As part of a larger program of optical spectroscopy of the NDWFS Bootes area, spectra have been obtained for ~900 of the X-ray sources, most of which are quasars or active galactic nuclei.

XBootes: An X-Ray Survey of the NDWFS Bootes Field - Paper I Overview and Initial Results

Abstract: We obtained a 5 ksec deep Chandra X-ray Observatory ACIS-I map of the 9.3 square degree Bootes field of the NOAO Deep Wide-Field Survey. Here we describe the data acquisition and analysis strategies leading to a catalog of 4642 (3293) point sources with 2 or more (4 or more) counts, corresponding to a limiting flux of roughly 4(8)x10^{-15} erg cm^{-2}s^{-1} in the 0.5-7 keV band. These Chandra XBootes data are unique in that they consitute the widest contiguous X-ray field yet observed to such a faint flux limit. Because of the extraordinarily low background of the ACIS, we expect only 14% (0.7%) of the sources to be spurious. We also detected 43 extended sources in this survey. The distribution of the point sources among the 126 pointings (ACIS-I has a 16 x 16 arcminute field of view) is consistent with Poisson fluctuations about the mean of 36.8 sources per pointing. While a smoothed image of the point source distribution is clumpy, there is no statistically significant evidence of large scale filamentary structure. We do find however, that for theta>1 arcminute, the angular correlation function of these sources is consistent with previous measurements, following a power law in angle with slope -0.7. In a 1.4 deg^{2} sample of the survey, approximately 87% of the sources with 4 or more counts have an optical counterpart to R ~26 mag. As part of a larger program of optical spectroscopy of the NDWFS Bootes area, spectra have been obtained for \~900 of the X-ray sources, most of which are QSOs or AGN.

Size is everything : universal features of quasar microlensing with extended sources

Abstract: We examine the effect that the shape of the source brightness profile has on the magnitude fluctuations of images in quasar lens systems due to microlensing. We do this by convolving a variety of accretion disk models (including Gaussian disks, uniform disks, "cones," and a Shakura-Sunyaev thermal model) with two magnification maps in the source plane, one with convergence κ = 0.4 and shear γ = 0.4 (positive parity) and the other with κ = γ = 0.6 (negative parity). By looking at magnification histograms of the convolutions and using χ2 tests to determine the number of observations that would be necessary to distinguish histograms associated with different disk models, we find that, for circular disk models, the microlensing fluctuations are relatively insensitive to all properties of the models except the half-light radius of the disk. Shakura-Sunyaev models are sufficiently well constrained by observed quasar properties that we can estimate the half-light radius at optical wavelengths for a typical quasar. If Shakura-Sunyaev models are appropriate, the half-light radii are very much smaller than the Einstein rings of intervening stars, and the quasar can be reasonably taken to be a point source except in the immediate vicinity of caustic-crossing events.

Chandra Deep X-Ray Observation of a Typical Galactic Plane Region and Near-Infrared Identification

Abstract: Using the Chandra Advanced CCD Imaging Spectrometer Imaging array (ACIS-I), we have carried out a deep hard X-ray observation of the Galactic plane region at (l,b) approx. (28.5 deg,0.0 deg), where no discrete X-ray source has been reported previously. We have detected 274 new point X-ray sources (4 sigma confidence) as well as strong Galactic diffuse emission within two partidly overlapping ACIS-I fields (approx. 250 sq arcmin in total). The point source sensitivity was approx. 3 x 10(exp -15)ergs/s/sq cm in the hard X-ray band (2-10 keV and approx. 2 x 10(exp -16) ergs/s/sq cm in the soft band (0.5-2 keV). Sum of all the detected point source fluxes account for only approx. 10 % of the total X-ray fluxes in the field of view. In order to explain the total X-ray fluxes by a superposition of fainter point sources, an extremely rapid increase of the source population is required below our sensitivity limit, which is hardly reconciled with any source distribution in the Galactic plane. Therefore, we conclude that X-ray emission from the Galactic plane has truly diffuse origin. Only 26 point sources were detected both in the soft and hard bands, indicating that there are two distinct classes of the X-ray sources distinguished by the spectral hardness ratio. Surface number density of the hard sources is only slightly higher than observed at the high Galactic latitude regions, strongly suggesting that majority of the hard X-ray sources are active galaxies seen through the Galactic plane. Following the Chandra observation, we have performed a near-infrared (NIR) survey with SOFI at ESO/NTT to identify these new X-ray sources. Since the Galactic plane is opaque in NIR, we did not see the background extragalactic sources in NIR. In fact, only 22 % of the hard sources had NIR counterparts which are most likely to be Galactic origin. Composite X-ray energy spectrum of those hard X-ray sources having NIR counterparts exhibits a narrow approx. 6.7 keV iron emission line, which is a signature of Galactic quiescent cataclysmic variables (CVs).

Source localization by spatially distributed electronic noses for advection and diffusion

Abstract: Based on continuous concentration measurements from spatially distributed electronic noses, the location of a point source is to be determined. It is assumed that the emitted substance is transported by advection caused by a known homogeneous wind field and by isotropic diffusion. A new two-step approach for solving the source localization problem is presented. In the first step, for each sensor i, the set of points P/sub i/ is determined, on which the source can lie, taking only the specific concentration measurement C/sub i/ at sensor i into account. In the second step, an estimate for the source position is evaluated by intersecting the sets P/sub i/. The new approach overcomes the problem of poor convergence of iterative algorithms, which try to minimize the least squares output error. Finally, experimental results showing the capability of the new approach are presented.

Identification of a point source in a linear advection?dispersion?reaction equation: application to a pollution source problem

Abstract: We consider the problem of identification of a pollution source in a river. The mathematical model is a one-dimensional linear advection?dispersion?reaction equation with the right-hand side spatially supported at a point (the source) and a time-dependent intensity, both unknown. Assuming that the source becomes inactive after the time T*, we prove that it can be identified by recording the evolution of the concentration at two points, one of which is strategic.

Quantitative point source photoacoustic inversion formulas for scattering and absorbing media.

Abstract: We present here an expression for the photoacoustic contribution of an optical point source in a diffusive and absorbing medium. By using this measurement as a reference, we present a direct inversion formula that recovers the absorption map quantitatively, at the same time accounting for instrumental factors such as the source strength, the shape of the optical pulse, and the impulse response and finite size of the transducers. We further validate this expression through accurate numerical simulations showing that the absorption map is recovered quantitatively in the presence of a rotating geometry. We finally discuss how the presented solutions for point sources within the photoacoustic problem enable the use of concurrent fluorescence and ultrasound measurements as appropriate for a hybrid tomographic system. The proposed system could retrieve absorption information using photoacoustic measurements, and use these data to more accurately describe the fluorescence problem and improve reconstruction fidelity.

Fundamental solution for a layered porous half space subject to a vertical point force or a point fluid source

Abstract: The focus of this contribution is to develop a transmission and reflection matrices (TRM) method for a layered porous half-space subject to a point force or a fluid point source. Applying the Hankel and the Fourier transformation, the general solutions for the displacements, stresses and pore pressure are derived from the potentials for the solid skeleton and the pore fluid as well as the governing equations of Biot’s theory. The transmission and reflection matrices (TRM) for each interface are obtained by using the general solutions as well as the continuity conditions at the interface. The TRM method for the layered porous medium is developed on the basis of the transmission and reflection matrices (TRM) and the boundary conditions as well as the source terms for the point force or the fluid point source. The fundamental solutions of the point force and the point fluid source in both the frequency domain and the time domain are obtained by using the proposed TRM method. Some numerical examples are given in the paper.

The optical and ultraviolet spectral energy distributions of short-period black hole X-ray transients in outburst

Abstract: We compile optical and UV spectra of a sample of "typical" short-period black hole X-ray transients in outburst. We also survey determinations of interstellar extinction and distance to deredden spectra and compare absolute fluxes. Hence, we perform a comparative study of the broadband spectral energy distributions (SEDs). We find that, given such a homogeneous sample of typical sources, the optical SEDs form a relatively uniform set, all exhibiting quasi-power-law spectra with Fν ∝ να, where 0.5 α 1.5 (steeper than the canonical ν1/3 disk spectrum). All become flatter in the UV, although there is more diversity here. The SEDs studied can be broadly divided into two optical-UV spectral states. The UV-hard spectra, e.g., A0620-00 and GS 1124-684, continue to rise in the far-UV. The UV-soft spectra, e.g., GRO J0422+32, drop off. XTE J1859+226 evolved from UV-soft to UV-hard as it decayed, indicating that this effect is a real difference, not a dereddening artifact. All the spectra can be fitted by a generalized blackbody disk model with two forms of heating, resulting in the two states. The UV-soft state is consistent with a disk illuminated by a central point source, with irradiative heating dominating over viscous. The UV-hard state is well described by a viscously heated disk, although this requires very high mass flow rates in the case of GS 1124-684. Alternatively, a UV-hard spectrum can be produced if the disk is illuminated by a vertically extended X-ray source such as a central scattering corona or jet. Since scattering is assumed by some numerical simulations, it is worth emphasizing that when illumination comes from (nonlocal) scattering high above the disk, we generically expect a steeper radial dependence of X-ray heating (F ∝ R-3) than is usually assumed; it is this steep dependence that leads to the UV-hard spectrum.

The X-ray emission from Young Stellar Objects in the rho Ophiuchi cloud core as seen by XMM-Newton

Abstract: We observed the main core F of the rho Ophiuchi cloud, an active star-forming region located at ~140 pc, using XMM-Newton with an exposure of 33 ks. We detect 87 X-ray sources within the 30' diameter field-of-view of the it EPIC imaging detector array. We cross-correlate the positions of XMM-Newton X-ray sources with previous X-ray and infrared (IR) catalogs: 25 previously unknown X-ray sources are found from our observation; 43 X-ray sources are detected by both XMM-Newton and Chandra; 68 XMM-Newton X-ray sources have 2MASS near-IR counterparts. We show that XMM-Newton and Chandra have comparable sensitivity for point source detection when the exposure time is set to ~30 ks for both. We detect X-ray emission from 7 Class I sources, 26 Class II sources, and 17 Class III sources. The X-ray detection rate of Class I sources is very high (64 %), which is consistent with previous Chandra observations in this area. We propose that 15 X-ray sources are new class III candidates, which doubles the number of known Class III sources, and helps to complete the census of YSOs in this area. We also detect X-ray emission from two young bona fide brown dwarfs, GY310 and GY141, out of three known in the field of view. GY141 appears brighter by nearly two orders of magnitude than in the Chandra observation. We extract X-ray light curves and spectra from these YSOs, and find some of them showed weak X-ray flares. We observed an X-ray flare from the bona fide brown dwarf GY310. We find as in the previous Chandra observation of this region that Class I sources tend to have higher temperatures and heavier X-ray absorptions than Class II and III sources.

The Orion Nebula in the Mid-Infrared

Abstract: We present two wide-field (≈5' × 35), diffraction-limited (λ/D 05 at 10 μm), broadband 10 and 20 μm images of the Orion Nebula, plus six 7–13 μm narrowband (λ/Δλ 1) images of the BN/KL complex taken at the 3.8 m UKIRT telescope with the MPIA MAX camera. The wide-field images, centered on the Trapezium and BN/KL regions, are mosaics of 35'' × 35'' frames obtained with standard chopping and nodding techniques and reconstructed using a new restoration method developed for this project. They show the filamentary structure of the dust emission from the walls of the H II region and reveal a new remarkable group of arclike structures ≈1' to the south of the Trapezium. The morphology of the Ney-Allen Nebula, produced by wind-wind interaction in the vicinity of the Trapezium stars, suggests a complex kinematical structure at the center of the cluster. We find indications that one of the most massive members of the cluster, the B0.5 V star θ1 Ori D, is surrounded by a photoevaporated circumstellar disk. Among the four historic Trapezium OB stars, this is the only one without a binary companion, suggesting that stellar multiplicity and the presence of massive circumstellar disks may be mutually exclusive. In what concerns the BN/KL complex, we find evidence for extended optically thin silicate emission on top of the deep 10 μm absorption feature. Assuming a simple two-component model, we map with 05 spatial resolution the foreground optical depth, color temperature, and mid-IR luminosity of the embedded sources. We resolve a conspicuous point source at the location of the IRc2-A knot, approximately 05 north of the deeply embedded H II region I. We analyze the spectral profile of the 10 μm silicate absorption feature and find indication for grain crystallization in the harsh nebular environment. In the OMC-1 South region, we detect several point sources and discuss their association with the mass-loss phenomenology observed at optical and millimeter wavelengths. Finally, we list the position and photometry of 177 point sources, the large majority of which are detected for the first time in the mid-IR. Twenty-two of them lack a counterpart at shorter wavelengths and are therefore candidates for deeply embedded protostars. The comparison of photometric data obtained at two different epochs reveals that source variability at 10 μm is present up to a level of ≈1 mag on a timescale of ~2 yr. With the possible exception of a pair of OB stars, all point sources detected at shorter wavelengths display 10 μm emission well above the photospheric level, which we attribute to disk circumstellar emission. The recent model of Robberto et al. provides the simplest explanation for the observed mid-IR excess.

Numerical study of broadband sound pulse propagation in three-dimensional oceanic waveguides

Abstract: In this paper, the propagation of a broadband sound pulse in three-dimensional (3D) shallow water waveguides is investigated numerically. Two cases are examined: (i) the 3D ASA benchmark wedge, and (ii) the 3D Gaussian canyon. The numerical method used to solve the four-dimensional acoustic problem is based on a Fourier synthesis technique. The frequency-domain calculations are carried out using the fully 3D parabolic equation based model 3DWAPE, recently modified to include a wide-angle paraxial approximation for the azimuthal component. A broadband sound pulse with a central frequency of 25 Hz and a bandwith of 40 Hz is considered. For both test cases, 3D results corresponding to a 25 Hz cw point source are first presented and compared with predictions from a 3D adiabatic modal model. Then, the acoustic problem is solved considering the broadband source pulse. The modal structure of the received signals is analyzed and exhibits multiple mode arrivals of the propagating signal.

A Northern Sky Survey for Steady TeV Gamma-Ray Point Sources Using the Tibet Air Shower Array

Abstract: Results on steady TeV $\gamma$-ray point source search using data taken from the Tibet HD (Feb. 1997 -- Sep. 1999) and Tibet III (Nov. 1999 -- Oct. 2001) arrays are presented. From $0^{\circ}$ to $60^{\circ}$ in declination, significant excesses from the well-known steady source Crab Nebula and the high state of the flare type source Markarian 421 are observed. Because the levels of significance from other positions are not sufficiently high, 90% confidence level upper limits on the flux are set assuming different power law spectra. To allow crosschecking, two independently developed analyses are used in this work.

A Time Domain Point Source Method for Inverse Scattering by Rough Surfaces

Abstract: In this paper we propose a new method to determine the location and shape of an unbounded rough surface from measurements of scattered electromagnetic waves. The proposed method is based on the point source method of Potthast (IMA J. Appl. Math. 61, 119–140, 1998) for inverse scattering by bounded obstacles. We propose a version for inverse rough surface scattering which can reconstruct the total field when the incident field is not necessarily time harmonic. We present numerical results for the case of a perfectly conducting surface in TE polarization, in which case a homogeneous Dirichlet condition applies on the boundary. The results show great accuracy of reconstruction of the total field and of the prediction of the surface location.

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

Abstract: We report on the discovery of a fifth lensed image in the large separation lensed quasar system SDSS J1004+4112. A faint point source located 0. 2 from the center of the brightest galaxy in the lensing cluster was detected in images taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth lensed image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves, and thereby the mass distribution of the lensing cluster.

Backward probability model using multiple observations of contamination to identify groundwater contamination sources at the Massachusetts Military Reservation

Abstract: [1] Backward location and travel time probability density functions characterize the possible former locations (or the source location) of contamination that is observed in an aquifer. For an observed contaminant particle the backward location probability density function (PDF) describes its position at a fixed time prior to sampling, and the backward travel time probability density function describes the amount of time required for the particle to travel to the sampling location from a fixed upgradient position. The backward probability model has been developed for a single observation of contamination (e.g., Neupauer and Wilson, 1999). In practical situations, contamination is sampled at multiple locations and times, and these additional data provide information that can be used to better characterize the former position of contamination. Through Bayes' theorem we combine the individual PDFs for each observation to obtain a PDF for multiple observations that describes the possible source locations or release times of all observed contaminant particles, assuming they originated from the same instantaneous point source. We show that the multiple-observation probability density function is the normalized product of the single-observation PDFs. The additional information available from multiple observations reduces the variances of the source location and travel time probability density functions and improves the characterization of the contamination source. We apply the backward probability model to a trichloroethylene (TCE) plume at the Massachusetts Military Reservation (MMR). We use four TCE samples distributed throughout the plume to obtain single-observation and multiple-observation location and travel time PDFs in three dimensions. These PDFs provide information about the possible sources of contamination. Under assumptions that the existing MMR model is properly calibrated and the conceptual model is correct the results confirm the two suspected sources of contamination and reveal that one or more additional sources is likely.

Limits from the Hubble Space Telescope on a Point Source in SN 1987A

Abstract: We observed supernova 1987A (SN 1987A) with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) in 1999 September and again with the Advanced Camera for Surveys (ACS) on the HST in 2003 November. Our spectral observations cover ultraviolet (UV) and optical wavelengths from 1140 to 10266 A, and our imaging observations cover UV and optical wavelengths from 2900 to 9650 A. No point source is observed in the remnant. We obtain a limiting flux of Fopt<=1.6×10-14 ergs s-1 cm-2 in the wavelength range 2900-9650 A for any continuum emitter at the center of the supernova remnant (SNR). This corresponds to an intrinsic luminosity of Lopt<=5×1033 ergs s-1. It is likely that the SNR contains opaque dust that absorbs UV and optical emission, resulting in an attenuation of ~35% due to dust absorption in the SNR. Correcting for this level of dust absorption would increase our upper limit on the luminosity of a continuum source by a factor of 1.54. Taking into account dust absorption in the remnant, we find a limit of Lopt<=8×1033 ergs s-1. We compare this upper bound with empirical evidence from point sources in other supernova remnants and with theoretical models for possible compact sources. We show that any survivor of a possible binary system must be no more luminous than an F6 main-sequence star. Bright young pulsars such as Kes 75 or the Crab pulsar are excluded by optical and X-ray limits on SN 1987A. Other nonplerionic X-ray point sources have luminosities similar to the limits on a point source in SN 1987A; RCW 103 and Cas A are slightly brighter than the limits on SN 1987A, while Pup A is slightly fainter. Of the young pulsars known to be associated with SNRs, those with ages <=5000 yr are all too bright in X-rays to be compatible with the limits on SN 1987A. Examining theoretical models for accretion onto a compact object, we find that spherical accretion onto a neutron star is firmly ruled out and that spherical accretion onto a black hole is possible only if there is a larger amount of dust absorption in the remnant than predicted. In the case of thin-disk accretion, our flux limit requires a small disk, no larger than 1010 cm, with an accretion rate no more than 0.3 times the Eddington accretion rate. Possible ways to hide a surviving compact object include the removal of all surrounding material at early times by a photon-driven wind, a small accretion disk, or very high levels of dust absorption in the remnant. It will not be easy to improve substantially on our optical-UV limit for a point source in SN 1987A, although we can hope that a better understanding of the thermal infrared emission will provide a more complete picture of the possible energy sources at the center of SN 1987A.

Point source detection and characterization for vehicle radiation portal monitors

Abstract: Many international border crossings presently screen cargo for illicit nuclear material using radiation portal monitors (RPMs) that measure the gamma ray and/or neutron flux emitted by vehicles. The fact that many target sources have a point-like geometry can be exploited to detect subthreshold sources and filter out benign sources that frequently possess a distributed geometry. This report describes a two-step process, which has the potential to complement other alarm algorithms, for detecting and characterizing point sources. The first step applies a matched filter whereas step two uses a weighted nonlinear least squares method. In a base-case simulation, matched filtering detected a 250-cps source injected onto a white-noise background at a 95% detection probability and a 0.003 false alarm probability. For the same simulation, the maximum likelihood estimation technique performed well at source strengths of 250 and 400 cps. These simulations provided a best-case feasibility study for this technique, which will be extended to experimental data that possess false point-source signatures resulting from background shielding caused by vehicle design and cargo distribution

Two Pulsar Wind Nebulae: Chandra/XMM-Newton Imaging of GeV J1417?6100

Abstract: We report on Chandra ACIS and XMM-Newton MOS/pn imaging observations of two pulsar wind nebulae (K3/PSR J1420-6048 and G313.3+0.1, the "Rabbit") associated with the Galactic unidentified γ-ray source GeV J1417-6100. With excellent ACIS imaging the very energetic pulsar PSR J1420-6048 is separated from its surrounding nebula. This nebula has surprisingly little compact structure, although a faint arc is seen near the pulsar. Similarly, two point sources are resolved in the Rabbit Nebula. The large XMM-Newton collecting area provides useful spectral constraints on the Rabbit and the associated point sources. Based on spectra and X-ray morphology, we identify one point source as a plausible pulsar counterpart. Large backgrounds and low source counts limited pulse search sensitivities, but we report pulse upper limits and a candidate 108 ms period for the Rabbit Pulsar based on the XMM-Newton data and an ACIS CC observation. Comparison of the X-ray images with high-resolution ATCA radio maps shows that the nonthermal X-ray emission corresponds well with the radio structure.

Search for Point Sources of Ultra-High-Energy Cosmic Rays above 4.0 × 1019 eV Using a Maximum Likelihood Ratio Test

Abstract: We present the results of a search for cosmic-ray point sources at energies in excess of 4.0 × 1019 eV in the combined data sets recorded by the Akeno Giant Air Shower Array and High Resolution Fly's Eye stereo experiments. The analysis is based on a maximum likelihood ratio test using the probability density function for each event rather than requiring an a priori choice of a fixed angular bin size. No statistically significant clustering of events consistent with a point source is found.

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

Abstract: We report the discovery of a fifth image in the large separation lensed quasar system SDSS J1004+4112. A faint point source located 0.2'' from the center of the brightest galaxy in the lensing cluster is detected in images taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves and thereby the mass distribution of the lensing cluster.

Emergence of a tracer source from air concentration measurements, a new strategy for linear assimilation

Abstract: The measurement of atmospheric concentrations by a monitoring network is a promising tool for the identification of the widespread sources of trace species. The paper addresses the case of the species scattered linearly by a known meteorology. The question is classical: what can be said about the source from a set of measurements? Is it possible to guess from the values observed by the measurements that the source is spread close to the detectors, or that the tracer comes from a remote region? And, if the source was a point source, would it be possible to understand it by just considering these values? A part of the answers is a matter of practical sense: the resolution with which an emission can be retrieved will always be limited and probably lower for a remote region, even if the detectors and dispersion model are error free. The paper proposes a linear strategy of inference: to any set of values taken by the observed concentrations is associated linearly an estimate of the source. Doubled values lead to a doubled estimate. The method, based on adjoint techniques, is intended to optimise the resolution by quantifying, with the concept of illumination, which regions are well, poorly or not seen at all. The illumination tied to ordinary adjoint functions becomes excessive close to the detectors thus leading to inversion artefacts. This may be corrected by attributing each point of the space time domain a geometric and statistical weight. The adjoint functions are transformed. The choice of this renormalising function is constrained by an unambiguous entropic criterion preventing any overestimation of the available information that would lead to artefacts. It amounts to evenly distribute the information between the points organised with their weights as a "known domain". The theory is illustrated by calculations performed with the experimental source ETEX1.

Analytical solutions for turbulent non-Boussinesq plumes

Abstract: Analytical solutions are developed for non-Boussinesq turbulent plumes rising from horizontal area sources in unconfined quiescent environments of uniform density. The approach adopted follows and extends an earlier approach for Boussinesq plumes and replaces the non-Boussinesq area source of interest and located at z=0 with an idealized point source located at a virtual origin z=z v such that the flow above the idealized source approximates that from the actual source. Asymptotic analytical expressions are developed for the location of the virtual source that are valid for large vertical distances above the non-Boussinesq source. The non-Boussinesq source is characterized by a non-dimensional parameter Γ nb which is a measure of the relative strengths of the mass, momentum and density deficit fluxes at, or at a specified height above, the source. The vertical distance between the actual and virtual sources scales on the length scale l that characterizes the height over which the flow is non-Boussinesq and expressions for z v /l are developed for lazy (Γ nb > 1) and forced plume (Γ ub < 1) sources. For pure-plume source conditions Γ nb = 1, and the virtual source provides an exact representation of the actual plume above z =0. The limiting cases of a nearly pure lazy plume and of a highly lazy plume are also explored analytically

Tycho-2 stars with infrared excess in the MSX Point Source Catalogue

Abstract: Stars of all evolutionary phases have been found to have excess infrared emission due to the presence of circumstellar material. To identify such stars, we have positionally correlated the infrared Mid-Course Space Experiment (MSX) Point Source Catalogue and the Tycho-2 optical catalogue. Near-mid-infrared colour criteria have been developed to select infrared excess stars. The search yielded 1938 excess stars; over half (979) have never previously been detected by IRAS. The excess stars were found to be young objects such as Herbig Ae/Be and Be stars, and evolved objects such as OH/IR (infrared) and carbon stars. A number of B-type excess stars were also discovered whose infrared colours could not be readily explained by known catalogued objects.