Point source

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

Extended X-Ray Emission around 4C 41.17 at z = 3.8

Abstract: We present sensitive, high-resolution, X-ray imaging from Chandra of the high-redshift radio galaxy 4C 41.17 (z = 3.8). Our 150 ks Chandra exposure detects strong X-ray emission from a point source coincident with the nucleus of the radio galaxy. In addition, we identify extended X-ray emission with a luminosity of ~1045 ergs s-1 covering a 100 kpc (15'') diameter region around the radio galaxy. The extended X-ray emission follows the general distribution of radio emission in the radio lobes of this source and the distribution of a giant Lyα emission-line halo, while the spectrum of the X-ray emission is nonthermal and has a power-law index consistent with that of the radio synchrotron. We conclude that the X-ray emission is most likely inverse Compton scattering of far-infrared photons from a relativistic electron population probably associated with past and current activity from the central object. Assuming an equipartition magnetic field, the cosmic microwave background energy density at z = 3.8 can account for at most only 40% of the inverse Compton emission. Published submillimeter maps of 4C 41.17 have detected an apparently extended and extremely luminous far-infrared emission around the radio galaxy. We demonstrate that this photon component and its spatial distribution, in combination with the cosmic microwave background, can reproduce the observed X-ray luminosity. We propose that photoionization by these inverse Compton X-ray photons plays a significant role in this system and provides a new physical feedback mechanism to preferentially affect the gas within the most massive halos at high redshift. This is the highest redshift example of extended X-ray emission around a radio galaxy currently known and points toward an extraordinary halo around such systems, where cool dust, relativistic electrons, neutral and ionized gas, and intense infrared and X-ray radiation all appear to coexist.

Three-dimensional air pollutant modeling in the lower atmosphere

Abstract: A steady-state, three-dimensional turbulent diffusion equation describing the concentration distribution of an air pollutant from an elevated point source in the lower atmosphere is solved analytically. The same formulation can be used to obtain solutions from line, area or other kinds of sources. The solution is developed for the cases in which the velocity, vertical and lateral diffusivities are given by the power law. The model preserves the beauty of analytical solution without sacrificing much on the accuracy of approximating the velocity and eddy diffusivities. Methods of evaluating the parameters, which are required for the model applications, are discussed. Results indicate that the ratio of the plume width to the plume length increases with decreasing stability and with increasing source height. These consequences are in response to the variations of the size of eddies in the vertical direction.

X-ray jet emission from the black hole X-ray binary XTE J1550-564 with CHANDRA in 2000

Abstract: We have discovered an X-ray jet due to material ejected from the black hole X-ray transient XTE J1550-564. The discovery was first reported in 2002 by Corbel and coworkers, and here we present an analysis of the three Chandra observations made between 2000 June and September. For these observations, a source is present that moves in an eastward direction away from the point source associated with the compact object. The separation between the new source and the compact object changes from 213 in June to 234 in September, implying a proper motion of 21.2 ± 7.2 mas day-1, a projected separation of 0.31-0.85 pc, and an apparent jet velocity between 0.34 ± 0.12 and 0.93 ± 0.32 times the speed of light for a source distance range of d = 2.8-7.6 kpc. These observations represent the first time that an X-ray jet proper-motion measurement has been obtained for any accretion-powered Galactic or extragalactic source. While this work deals with the jet to the east of the compact object, the western jet has also been detected in the X-ray and radio bands. The most likely scenario is that the eastern jet is the approaching jet and that the jet material was ejected from the black hole in 1998. Along with a 1998 VLBI proper-motion measurement, the Chandra proper motion indicates that the eastern jet decelerated between 1998 and 2000. There is evidence that the eastern jet is extended by ±2''-3'' in the direction of the proper motion. The upper limit on the source extension in the perpendicular direction is ±15, which corresponds to a jet opening angle of less than 75. The X-ray jet energy spectrum is well but not uniquely described by a power law with an energy index of α = -0.8 ± 0.4 (Sν ∝ να) and interstellar absorption. The eastern jet was also detected in the radio band during an observation made within 7.4 days of the June Chandra observation. The overall radio flux level is consistent with an extrapolation of the X-ray power law with α = -0.6. The 0.3-8 keV X-ray jet luminosity is in the range (3-24) × 1032 ergs s-1 for the June observation using the distance range above but is a factor of ~2-3 lower for the later observations. We cannot definitively determine the X-ray emission mechanism, but a synchrotron origin is viable and may provide the simplest explanation for the observations.

Modeling local earthquakes as shear dislocations in a layered half space

Abstract: The SH impulse response of layered medium resulting from the application of a point source shear dislocation is studied by the generalized ray technique. Numerical seismograms of ground displacement in the range 10–100 km are constructed for a number of different crustal models and source descriptions. The results for shallow events show severe wave form modifications at epicentral distances as small as twice the source depth when models contain soft surface layers. A comparison of synthetic seismograms with observations from a central California earthquake of magnitude 4.6 indicates a moment of 2.1×1022 ergs with a faulting duration of 0.5 s. This moment is noticeably smaller than that estimated from the low frequency level of the whole seismogram spectrum assuming the usual homogeneous half-space model. This occurs because the energy arriving with the surface waves and deep crustal reflections boosts the long period portion of the spectrum, giving the impression of an anomalously large moment when calculated in the conventional manner. The close agreement between synthetic results and the actual data both in the time and in the Fourier transform domain suggests that our deterministic approach can be quite useful in understanding the complexity of seismograms recorded in the local field.