Current Issue : October - December Volume : 2016 Issue Number : 4 Articles : 6 Articles
In accreting neutron star (NS) low-mass X-ray binaries (LMXBs), the turbulent flow in accretion\ndisk may show magnetic structures. Its emission will vary in time due to inhomogeneous motions\nthrough and with the accretion flow. These emissions contribute to considerable X-ray variability\non a wide range of timescales in all wavelengths, and down to milliseconds. In this article, we give\na brief review for quasi-periodic oscillations (QPOs), one of a periodic X-ray variability, in NS/\nLMXBs. Firstly, we give a brief introduction to NS/LMXBs and the fruitful QPO components. As an\nexample, the energy dependence of normal branch oscillations in Scorpius X-1 is discussed. We\nmostly focus on the properties and mechanism of kilohertz QPOsââ?¬â?the fastest variability components\nthat have the same order as the dynamical timescales of the innermost regions of accretion\nflow. Finally, we discuss the success and questions for theoretical interpretations and present the\npossible entry for investigation of nature of QPOs....
The inverse-Compton X-ray emission model for supernovae has been well established to explain the X-ray properties of many supernovae for over 30 years. However, no observational case has yet been found to connect the X-rays with the optical lights as they should be. Here, we report the discovery of a hard X-ray source that is associated with a Type II-b supernova. Simultaneous emission enhancements have been found in both the X-ray and optical light curves twenty days after the supernova explosion. While the enhanced X-rays are likely dominated by inverse-Compton scatterings of the supernova's lights from the Type II-b secondary peak, we propose a scenario of a high-speed supernova ejecta colliding with a low-density pre-supernova stellar wind that produces an optically thin and high-temperature electron gas for the Comptonization. The inferred stellar wind mass-loss rate is consistent with that of the supernova progenitor candidate as a yellow supergiant detected by the Hubble Space Telescope, providing an independent proof for the progenitor. This is also new evidence of the inverse-Compton emission during the early phase of a supernova....
Ultrafast electron transfer in dissociating iodomethane and fluoromethane molecules\nwas studied at the Linac Coherent Light Source free-electron laser using an ultravioletpump,\nX-ray-probe scheme. The results for both molecules are discussed with respect to\nthe nature of their UV excitation and different chemical properties. Signatures of longdistance\nintramolecular charge transfer are observed for both species, and a quantitative\nanalysis of its distance dependence in iodomethane is carried out for charge states up to\nI21+ . The reconstructed critical distances for electron transfer are in good agreement\nwith a classical over-the-barrier model and with an earlier experiment employing a nearinfrared\npump pulse...
The design and evaluation of the expected performance of optical systems requires sophisticated and\nreliable information about the surface topography of planned optical elements before they are fabricated. The\nproblem is especially severe in the case of x-ray optics for modern diffraction-limited-electron-ring and free-electron-\nlaser x-ray facilities, as well as x-ray astrophysics missions, such as the X-ray Surveyor under development.\nModern x-ray source facilities are reliant upon the availability of optics of unprecedented quality, with surface\nslope accuracy <0.1 �¼rad. The unprecedented high angular resolution and throughput of future x-ray space\nobservatories require high-quality optics of 100 m2 in total area. The uniqueness of the optics and limited number\nof proficient vendors make the fabrication extremely time-consuming and expensive, mostly due to the limitations\nin accuracy and measurement rate of metrology used in fabrication. We continue investigating the possibility of\nimproving metrology efficiency via comprehensive statistical treatment of a compact volume of metrology of\nsurface topography, which is considered the result of a stochastic polishing process. We suggest, verify,\nand discuss an analytical algorithm for identification of an optimal symmetric time-invariant linear filter\nmodel with a minimum number of parameters and smallest residual error. If successful, the modeling could\nprovide feedback to deterministic polishing processes, avoiding time-consuming, whole-scale metrology measurements\nover the entire optical surface with the resolution required to cover the entire desired spatial frequency\nrange. The modeling also allows forecasting of metrology data for optics made by the same vendor and technology.\nThe forecast data are vital for reliable specification for optical fabrication, evaluated from numerical simulation\nto be exactly adequate for the required system performance, avoiding both over- and underspecification.\n�© The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in\npart requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.OE.55.7.074106]...
We report the generation of coherent water-window soft x-ray harmonics in a neon-filled semiinfinite\ngas cell driven by a femtosecond multi-mJ mid-infrared optical parametric chirped-pulse\namplification (OPCPA) system at a 1 kHz repetition rate. The cutoff energy was extended to\nâË?¼450 eV with a 2.1 Ã?¼m driver wavelength and a photon flux of 1.5 106 photons/s/1%\nbandwidth was obtained at 350 eV. A comparable photon flux of 1.0 106 photons/s/1%\nbandwidth was observed at the nitrogen K-edge of 410 eV. This is the first demonstration of\nwater-window harmonic generation up to the nitrogen K-edge from a kHz OPCPA system.\nFinally, this system is suitable for time-resolved soft x-ray near-edge absorption spectroscopy.\nFurther scaling of the driving pulseÃ?¼s energy and repetition rate is feasible due to the availability\nof high-power picosecond Yb-doped pump laser technologies, thereby enabling ultrafast,\ntabletop water-window x-ray imaging....
The room temperature X-ray responses as functions of time of two n type cubic GaN Schottky diodes (200â���¼m and 400â���¼m diameters) are reported. The current densities as functions of time for both diodes showed fast turn-on transients and increases in current density when illuminated with X-ray photons of energy up to 35â��keV. The diodes were also electrically characterized: capacitance, implied depletion width and dark current measurements as functions of applied bias at room temperature are presented. At -5â��V reverse bias, the capacitances of the diodes were measured to be (84.05â���±â��0.01) pF and (121.67â���±â��0.02) pF, respectively. At -5â��V reverse bias, the dark current densities of the diodes were measured to be (347.2â���±â��0.4) mA cm(-2) and (189.0â���±â��0.2) mA cm(-2), respectively. The Schottky barrier heights of the devices (0.52â���±â��0.07) eV and (0.63â���±â��0.09) eV, respectively, were extracted from the forward dark current characteristics....
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