Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.

Qualitative Aspects of Quasar Microlensing with Two Mass Components: Magnification Patterns and Probability Distributions

Abstract: It has been conjectured that the distribution of magnifications of a point source microlensed by a randomly distributed population of intervening point masses is independent of its mass spectrum. We present gedanken experiments that cast doubt on this conjecture and numerical simulations that show it to be false.

The orion nebula in the mid-ir

Abstract: We present two wide-field (≈ 5 ′ × 3.5 ′ ), diffraction limited (λ/D ≃ 0.5 ′′ at 10 � m), broad-band 10 � m and 20 � m images of the Orion Nebula, plus six 7–13 � m narrow-band (λ/�λ ≃ 1) images of the BN/KL complex taken at the 3.8m UKIRT 1 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 technique 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 HII region and reveal a new remarkable group of arc-like 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 B0.5V star θ 1 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 � m absorption feature. Assuming a simple two component model, we map with ≃ 0. 5 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 0. 5 north of the deeply embedded HII region “I”. We analyze the spectral profile of the 10 � m 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 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 � m is present up to ≈ 1 mag level on a time-scale ∼ 2 yr. With the possible exception of a pair of OB stars, all point sources detected at shorter wavelengths display 10 � m emission well above the photospheric level, that we attribute to disks circumstellar emission. The model of Robberto, Beckwith, & Panagia (2002, ApJ. 578, 897) provides the simplest explanation for the observed mid-IR excess. Subject headings: nebulae: Orion Nebula, Infrared, ISM, Data Analysis — Infrared

Development and Evaluation of the Offshore and Coastal Dispersion Model

Abstract: The Offshore and Coastal Dispersion (OCD) model has been developed for the Minerals Management Service (MMS) to determine the impact of offshore and onshore emissions from point sources on the air quality of coastal regions. Constructed on the framework of the EPA guideline model MPTER, the OCD model incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. Hourly meteorological data are needed from both offshore and onshore locations, including wind direction and speed, mixing height, overwater air temperature and relative humidity, and the sea surface temperature. Observed turbulence intensities are preferred by the model but are not mandatory. Building downwash and plume rise are incorporated into the OCD model similar to their use in the Buoyant Line and Point Source (BLP) model. Partial plume penetration into elevated inversions is treated using Briggs’ model. Dispersion coefficients are proportional to turbulence intensities. A virtual so...

A thorough study of the intriguing X-ray emission from the Cartwheel ring

Abstract: We present the results from the high resolution Chandra observation of the Cartwheel galaxy. Many individual sources are resolved in the image, mostly associated with the outer ring. All detected sources have a very high X-ray luminosity (${\geq} 10^{39}$ erg s -1 ) that classifies them as Ultra Luminous X-ray sources (ULX). The brightest of them is possibly the most luminous individual non-nuclear source observed so far, with $L_{\rm X} \sim 10^{41}$ erg s -1 (at $D=122$ Mpc). The spatial extent of this source is consistent with a point source at the Chandra resolution. The luminosity function of individual X-ray sources extends about an order of magnitude higher than previously reported in other galaxies. We discuss this in the context of the “universal” luminosity function for High Mass X-ray Binaries and we derive a Star Formation Rate higher than in other starburst galaxies studied so far. A diffuse component, associated with hot gas, is present. However, deeper observations that we will obtain with XMM-Newton are needed to constrain its properties.

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.