Point source

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

Subwavelength resolution in a two-dimensional photonic-crystal-based superlens.

Abstract: We experimentally and theoretically demonstrate single-beam negative refraction and superlensing in the valence band of a two-dimensional photonic crystal operating in the microwave regime. By measuring the refracted electromagnetic waves from a slab shaped photonic crystal, we find a refractive index of � 1:94 that is very close to the theoretical value of � 2:06. A scanning transmission measurement technique is used to measure the spatial power distribution of the focused electromagnetic waves that radiate from a point source. The full width at half maximum of the focused beam is measured to be 0:21� , which is in good agreement with the finite difference time domain method simulations. We also report a subwavelength resolution for the image of two incoherent point sources, which are separated by a distance of �= 3.

Modeling finite-fault radiation from the ωn spectrum

Abstract: The high-frequency seismic field near the epicenter of a large earth- quake is modeled by subdividing the fault plane into subelements and summing their contributions at the observation point. Each element is treated as a point source with an o92 spectral shape, where co is the angular frequency. Ground-motion contributions from the subsources are calculated using a stochastic model. Attenuation is based on simple geometric spreading in a whole space, coupled with regional anelastic atten- uation (Q operator). The form of the co n spectrum with natural n follows from point shear-dislocation theory with an appropriately chosen slip time function. The seismic moment and comer frequency are the two parameters defining the point-source spectrum and must be linked to the subfanlt size to make the method complete. Two coefficients, Aa and K, provide this link. Assigning a moment to a subfault of specified size introduces the stress parameter, Ao-. The relationship between comer frequency (dislocation growth rate) and fault size is established through the coefficient K, which is inherently nonunique. These two parameters control the number of subsources and the ampli- tudes of high-frequency radiation, respectively. Derivation of the model from shear- dislocation theory reveals the unavoidable uncertainty in assigning ogn spectrum to faults with finite size. This uncertainty can only be reduced through empirical vali- dation. The method is verified by simulating data recorded on rock sites near epicenters of the M8.0 1985 Michoacan (Mexico), the M8.0 1985 Valparaiso (Chile), and the M5.8 1988 Saguenay (Qurbec) earthquakes. Each of these events is among the largest for which strong-motion records are available, in their respective tectonic environ- ments. The simulations for the first two earthquakes are compared to the more de- tailed modeling of Somerville et al. (1991), which employs an empirical source function and represents the effects of crustal structure using the theoretical impulse response. Both methods predict the observations accurately on average. The precision of the methods is also approximately equal; the predicted acceleration amplitudes in our model are generally within 15% of observations. An unexpected result of this study is that a single value of a parameter K provides a good fit to the data at high frequencies for all three earthquakes, despite their different tectonic environments. This suggests a simplicity in the modeling of source processes that was unanticipated.

Source mechanisms of explosions at Stromboli Volcano, Italy, determined from moment-tensor inversions of very-long-period data

Abstract: [1] Seismic data recorded in the 2–30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source-centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and � 160 m northwest of the active vents. The source mechanisms include both moment-tensor and single-force components. The principal axes of the moment tensor have amplitude ratios 1:1:2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio n = 1/3, a value appropriate for hot rock. Both imaged cracks dip � 60� to the northwest and strike northeast–southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are � 200 m 3 for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 10 8 N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston-like rise of a slug of gas in the conduit. INDEX TERMS: 7215 Seismology: Earthquake parameters; 7280 Seismology: Volcano seismology (8419); 8414 Volcanology: Eruption mechanisms; KEYWORDS: very-long-period seismicity, moment tensor inversions, eruption mehanics

Irradiation of an accretion disc by a jet: general properties and implications for spin measurements of black holes

Abstract: X-ray irradiation of the accretion disc leads to strong reflection features, which are then broadened and distorted by relativistic effects. We present a detailed, general relativistic approach to model this irradiation for different geometries of the primary X-ray source. These geometries include the standard point source on the rotational axis as well as more jet-like sources, which are radially elongated and accelerating. Incorporating this code in the RELLINE model for relativistic line emission, the line shape for any configuration can be predicted. We study how different irradiation geometries affect the determination of the spin of the black hole. Broad emission lines are produced only for compact irradiating sources situated close to the black hole. This is the only case where the black hole spin can be unambiguously determined. In all other cases the line shape is narrower, which could either be explained by a low spin or an elongated source. We conclude that for those cases and independent of the quality of the data, no unique solution for the spin exists and therefore only a lower limit of the spin value can be given

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...