Point source

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

Ray theory for a locally layered random medium

Abstract: We consider acoustic pulse propagation in inhomogeneous media over relatively long propagation distances. Our main objective is to characterize the spreading of the travelling pulse due to microscale variations in the medium parameters. The pulse is generated by a point source and the medium is modelled by a smooth three-dimensional background that is modulated by stratified random fluctuations. We refer to such media as locally layered . We show that, when the pulse is observed relative to its random arrival time, it stabilizes to a shape determined by the slowly varying background convolved with a Gaussian. The width of the Gaussian and the random travel time are determined by the medium parameters along the ray connecting the source and the point of observation. The ray is determined by high-frequency asymptotics (geometrical optics). If we observe the pulse in a deterministic frame moving with the effective slowness , it does not stabilize and its mean is broader because of the random compone...

Quantum limits to optical point-source localization

Abstract: Motivated by the importance of optical microscopes to science and engineering, scientists have pondered for centuries how to improve their resolution and the existence of fundamental resolution limits. In recent years, a new class of microscopes that overcome a long-held belief about the resolution have revolutionized biological imaging. Termed “super-resolution” microscopy, these techniques work by accurately locating optical point sources from far field. To investigate the fundamental localization limits, here I derive quantum lower bounds on the error of locating point sources in free space, taking full account of the quantum, nonparaxial, and vectoral nature of photons. These bounds are valid for any measurement technique, as long as it obeys quantum mechanics, and serve as general no-go theorems for the resolution of microscopes. To arrive at analytic results, I focus mainly on the cases of one and two classical monochromatic sources with an initial vacuum optical state. For one source, a lower bound on the root-mean-square position estimation error is of the order of λ0/N, where λ0 is the free-space wavelength and N is the average number of radiated photons. For two sources, owing to the statistical effect of nuisance parameters, the error bound diverges when their radiated fields overlap significantly. The use of squeezed light to further enhance the accuracy of locating one classical point source and the localization limits for partially coherent sources and single-photon sources are also discussed. The theory presented establishes a rigorous quantum statistical inference framework for the study of super-resolution microscopy and points to the possibility of using quantum techniques for true resolution enhancement.

Mechanism of deep earthquakes from spectrums of isolated body-wave signals: 1. The Banda Sea earthquake of March 21, 1964

Abstract: Spectrums of teleseismic body waves have been obtained for the period range 20–100 seconds from the records of a group of USCGS standardized seismograph stations around the source of a deep-focus earthquake in the Banda Sea. Corrections were applied for all the factors which took part in shaping the waveform of the signal during the course of propagation, and the spectrums were extrapolated to the vicinity of the source according to the equalization procedure described in a previous paper [Ben-Menahem et al., 1965]. Radiation patterns drawn from the corrected P wave spectrums over the above period range show that the spatial factor of the source function is independent of frequency up to 0.1 cps. Through a comparison of the empirical radiation patterns with those calculated from an assumed double-couple point source, it was possible to determine the source parameters. These are in good agreement with the solution furnished by first motions. Similar results from S waves confirm the double-couple assumption. All the corrected spectrums confirm the theory that the gross structure of the source time function is of the form (1 − e-t/τ)H(t), with 0 ≤ τ ≤ 10 seconds. From the equalized amplitudes, a quantity L0ds (source displacement times the area of a virtual fault plane), is found to be 0.125 km3. Further, by the concept of equivalent source theory, a virtual moment of a volume source can be deduced which leads in a simple way to the total energy of the source.

X-ray sources in the starburst spiral galaxy M83: nuclear region and discrete source population

Abstract: Chandra has resolved the starburst nuclear region of the face-on grand-design spiral M83. Eighty-one point sources are detected above 3.5 sigma in the ACIS S3 image, and 15 of them are within the inner 16-arcsec region of the galaxy. A point source with L_x ~ 3 x 10^(38) erg/s in the 0.3--8.0 keV band is found to coincide with the infra-red nuclear photometric peak, one of the two dynamical nuclei of the galaxy. No point-like sources are resolved (at a 2.5-sigma level) at the centre of symmetry of the outer optical isophote ellipses, suspected to be another dynamical nucleus. About 50% of the total emission in the nuclear region is unresolved; of this, about 70% can be attributed to hot thermal plasma, and the rest is probably due to unresolved point sources (eg, faint X-ray binaries). The azimuthally-averaged radial distribution of the unresolved emission has a King-like profile, with no central cusp. Strong emission lines are seen in the spectrum of the optically thin plasma component. The high abundances of C, Ne, Mg, Si and S with respect to Fe suggest that the interstellar medium in the nucleus is enriched and heated by type-II supernova explosions and winds from massive stars. The cumulative luminosity distribution of the discrete X-ray sources is neither a single nor a broken power law. Separating the sources in the nuclear region (within a distance of 60 arcsec from the X-ray centre) from the rest reveals that the two groups have different luminosity distributions. The log[N(>S)]--log S curve of the sources in the inner region (nucleus and stellar bar) is a single power law, which we interpret as due to continuous, ongoing star formation. Outside the central region, there is a smaller fraction of sources brighter than the Eddington limit for an accreting neutron star.

The chandra view of ngc 4178: the lowest mass black hole in a bulgeless disk galaxy?

Abstract: Using high-resolution Chandra data, we report the presence of a weak X-ray point source coincident with the nucleus of NGC 4178, a late-type bulgeless disk galaxy known to have high-ionization mid-infrared (mid-IR) lines typically associated with active galactic nuclei (AGNs). Although the faintness of this source precludes a direct spectral analysis, we are able to infer its basic spectral properties using hardness ratios. X-ray modeling, combined with the nuclear mid-IR characteristics, suggests that NGC 4178 may host a highly absorbed AGN accreting at a high rate with a bolometric luminosity on order of 1043 erg s–1. The black hole mass estimate, based on our Chandra data and archival Very Large Array data using the most recent fundamental plane relations, is ~104-105 M ☉, possibly the lowest mass nuclear black hole currently known. There are also three off-nuclear sources, two with a similar brightness to the nuclear source at 36'' and 32'' from the center. As with the nuclear source, hardness ratios are used to estimate spectra for these two sources, and both are consistent with a simple power-law (PL) model with absorption. These two sources have X-ray luminosities of the order of ~1038 erg s–1, which place them at the threshold between X-ray binaries and ultraluminous X-ray sources (ULXs). The third off-nuclear source, located 49'' from the center, is the brightest source detected, with an X-ray luminosity of ~1040 erg s–1. Its spectrum is well fit with an absorbed PL model, suggesting that it is a ULX. We also fit its spectrum with the Bulk Motion Comptonization model and suggest that this source is consistent with an intermediate-mass black hole of mass (6 ± 2) × 103 M ☉.