Point source

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

Polarimetry and Unification of Low-Redshift Radio Galaxies

Abstract: We have made high-quality measurements of the polarization spectra of 13 FR II radio galaxies and taken polarization images for 11 of these with the Keck telescopes. Seven of the eight narrow-line radio galaxies (NLRGs) are polarized, and six of the seven show prominent broad Balmer lines in polarized light. The broad lines are also weakly visible in total flux. Some of the NLRGs show bipolar regions with roughly circumferential polarization vectors, revealing a large reflection nebula illuminated by a central source. Our observations powerfully support the hidden quasar hypothesis for some NLRGs. According to this hypothesis, the continuum and broad lines are blocked by a dusty molecular torus, but can be seen by reflected, hence polarized, light. Classification as a NLRG, a broad-line radio galaxy (BLRG), or a quasar therefore depends on orientation. However, not all objects fit into this unification scheme. Our sample is biased toward objects known in advance to be polarized, but the combination of our results with the 1996 findings of Hill, Goodrich, and DePoy show that at least six out of a complete, volume and flux-limited sample of nine FR II NLRGs have broad lines, seen either in polarization or P{alpha}.The BLRGs in ourmore » sample range from 3C 382, which has a quasar-like spectrum, to the highly reddened IRAS source FSC 2217+259. This reddening sequence suggests a continuous transition from unobscured quasar to reddened BLRG to NLRG. Apparently the obscuring torus does not have a distinct edge. The BLRGs have polarization images that are consistent with a point source broadened by seeing and diluted by starlight. We do not detect extended nebular or scattered emission, perhaps because it is swamped by the nuclear source. Our starlight-corrected BLRG spectra can be explained with a two-component model: a quasar viewed through dust and quasar light scattered by dust. The direct flux is more reddened than the scattered flux, causing the polarization to rise steeply to the blue. Strong rotations of the electric vector position angle across H{alpha} in 3C 227 and 3C 445 may be explained by systematic orbital motions in an equatorial broad-line region. (c) (c) 1999. The American Astronomical Society.« less

Statistical characteristics of concentration fluctuations in dispersing plumes in the atmospheric surface layer

Abstract: Measurements have been made of concentration fluctuations in a dispersing plume from an elevated point source in the atmospheric surface layer using a recently developed fast-response photoionization detector. This detector, which has a frequency response (−6 dB point) of about 100 Hz, is shown to be capable of resolving the fluctuation variance contributed by the energetic subrange and most of the inertial-convective subrange, with a reduction in the fluctuation variance due to instrument smoothing of the finest scales present in the plume of at most 4%. Concentration time series have been analyzed to obtain the statistical characteristics of both the amplitude and temporal structure of the dispersing plume. We present alongwind and crosswind concentration fluctuation profiles of statistics of amplitude structure such as total and conditional fluctuation intensity, skewness and kurtosis, and of temporal structure such as intermittency factor, burst frequency, and mean burst persistence time. Comparisons of empirical concentration probability distributions with a number of model distributions show that our near-neutral data are best represented by the lognormal distribution at shorter ranges, where both plume meandering and fine-scale in-plume mixing are equally important (turbulent-convective regime), and by the gamma distribution at longer ranges, where internal structure or spottiness is becoming dominant (turbulent-diffusive regime). The gamma distribution provides the best model of the concentration pdf over all downwind fetches for data measured under stable stratification. A physical model is developed to explain the mechanism-induced probabilistic schemes in the alongwind development of a dispersing plume, that lead to the observed probability distributions of concentration. Probability distributions of concentration burst length and burst return period have been extracted and are shown to be modelled well with a powerlaw distribution. Power spectra of concentration fluctuations are presented. These spectra exhibit a significant inertial-convective subrange, with the frequency at the spectral peak decreasing with increasing downwind fetch. The Kolmogorov constant for the inertial-convective subrange has been determined from the measured spectra to be 0.17±0.03.

On Localisation of a Radiological Point Source

Abstract: The problem is to localise a source of gamma radiation using dose rate measurements taken with a gamma probe at various points in space. A statistical model of dose-rate counts is developed using experimental data recorded in a laboratory. The problem is then studied using the theoretical Cramer-Rao bound (CRB) analysis, which quantifies the accuracy with which it is possible to localise the source and estimate its intensity. Three estimation algorithms are implemented and their performance investigated by simulations. The algorithms are then applied to field trial radiological survey data. The maximum likelihood algorithm performs the best of the considered algorithms.

High-Energy Gamma-Ray Observations of Two Young, Energetic Radio Pulsars

Abstract: We present results of Compton Gamma-Ray Observatory/EGRET observations of the unidentified high-energy γ-ray sources 2EG J1049-5847 (GEV J1047-5840, 3EG J1048-5840) and 2EG J1103-6106 (3EG J1102-6103). These sources are spatially coincident with the young, energetic radio pulsars PSRs B1046-58 and J1105-6107, respectively. We find evidence for an association between PSR B1046-58 and 2EG J1049-5847. The γ-ray pulse profile, obtained by folding time-tagged photons having energies above 400 MeV using contemporaneous radio ephemerides, has probability of arising by chance of 1.2 × 10-4 according to the binning-independent H-test. A spatial analysis of the on-pulse photons reveals a point source of equivalent significance 10.2 σ. Off-pulse, the significance drops to 5.8 σ. Archival ASCA data show that the only hard X-ray point source in the 95% confidence error box of the γ-ray source is spatially coincident with the pulsar within the 1' uncertainty (Pivovaroff, Kaspi, & Gotthelf).The double peaked γ-ray pulse morphology and leading radio pulse are similar to those seen for other γ-ray pulsars and are well explained in models in which the γ-ray emission is produced in charge-depleted gaps in the outer magnetosphere. The inferred pulsed γ-ray flux above 400 MeV, (2.5 ± 0.6) × 10-10 ergs cm-2 s-1, represents 0.011 ± 0.003 of the pulsar's spin-down luminosity, for a distance of 3 kpc and 1 sr beaming. For PSR J1105-6107, light curves obtained by folding EGRET photons using contemporaneous radio ephemerides show no significant features. We conclude that this pulsar converts less than 0.014 of its spin-down luminosity into E > 100 MeV γ-rays beaming in our direction (99% confidence), assuming a distance of 7 kpc, 1 sr beaming and a duty cycle of 0.5.

Flux calibration of broad-band far-infrared and submillimetre photometric instruments: theory and application to Herschel-SPIRE

Abstract: Photometric instruments operating at far-infrared to millimetre wavelengths often have broad spectral passbands (λ/Δλ ∼ 3 or less), especially those operating in space. A broad passband can result in significant variation of the beam profile and aperture efficiency across the passband, effects which thus far have not generally been taken into account in the flux calibration of such instruments. With absolute calibration uncertainties associated with the brightness of primary calibration standards now in the region of 5 per cent or less, variation of the beam properties across the passband can be a significant contributor to the overall calibration accuracy for extended emission. We present a calibration framework which takes such variations into account for both antenna-coupled and absorber-coupled focal plane architectures. The scheme covers point source and extended source cases, and also the intermediate case of a semi-extended source profile. We apply the new method to the Spectral and Photometric Imaging Receiver (SPIRE) photometer on board the Herschel Space Observatory.