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    Publication Date: 2008-09-05
    Description: The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3 mm that set a size of 37(+16)(-10) microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Doeleman, Sheperd S -- Weintroub, Jonathan -- Rogers, Alan E E -- Plambeck, Richard -- Freund, Robert -- Tilanus, Remo P J -- Friberg, Per -- Ziurys, Lucy M -- Moran, James M -- Corey, Brian -- Young, Ken H -- Smythe, Daniel L -- Titus, Michael -- Marrone, Daniel P -- Cappallo, Roger J -- Bock, Douglas C-J -- Bower, Geoffrey C -- Chamberlin, Richard -- Davis, Gary R -- Krichbaum, Thomas P -- Lamb, James -- Maness, Holly -- Niell, Arthur E -- Roy, Alan -- Strittmatter, Peter -- Werthimer, Daniel -- Whitney, Alan R -- Woody, David -- England -- Nature. 2008 Sep 4;455(7209):78-80. doi: 10.1038/nature07245.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Massachusetts Institute of Technology (MIT) Haystack Observatory, Off Route 40, Westford, Massachusetts 01886, USA. sdoeleman@haystack.mit.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/18769434" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
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    Publication Date: 2012-09-29
    Description: Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 +/- 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Doeleman, Sheperd S -- Fish, Vincent L -- Schenck, David E -- Beaudoin, Christopher -- Blundell, Ray -- Bower, Geoffrey C -- Broderick, Avery E -- Chamberlin, Richard -- Freund, Robert -- Friberg, Per -- Gurwell, Mark A -- Ho, Paul T P -- Honma, Mareki -- Inoue, Makoto -- Krichbaum, Thomas P -- Lamb, James -- Loeb, Abraham -- Lonsdale, Colin -- Marrone, Daniel P -- Moran, James M -- Oyama, Tomoaki -- Plambeck, Richard -- Primiani, Rurik A -- Rogers, Alan E E -- Smythe, Daniel L -- SooHoo, Jason -- Strittmatter, Peter -- Tilanus, Remo P J -- Titus, Michael -- Weintroub, Jonathan -- Wright, Melvyn -- Young, Ken H -- Ziurys, Lucy M -- New York, N.Y. -- Science. 2012 Oct 19;338(6105):355-8. doi: 10.1126/science.1224768. Epub 2012 Sep 27.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉MIT Haystack Observatory, Off Route 40, Westford, MA 01886, USA. sdoeleman@haystack.mit.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23019611" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 4
    Publication Date: 2016-01-20
    Description: Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intrahour variability associated with these fields.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Johnson, Michael D -- Fish, Vincent L -- Doeleman, Sheperd S -- Marrone, Daniel P -- Plambeck, Richard L -- Wardle, John F C -- Akiyama, Kazunori -- Asada, Keiichi -- Beaudoin, Christopher -- Blackburn, Lindy -- Blundell, Ray -- Bower, Geoffrey C -- Brinkerink, Christiaan -- Broderick, Avery E -- Cappallo, Roger -- Chael, Andrew A -- Crew, Geoffrey B -- Dexter, Jason -- Dexter, Matt -- Freund, Robert -- Friberg, Per -- Gold, Roman -- Gurwell, Mark A -- Ho, Paul T P -- Honma, Mareki -- Inoue, Makoto -- Kosowsky, Michael -- Krichbaum, Thomas P -- Lamb, James -- Loeb, Abraham -- Lu, Ru-Sen -- MacMahon, David -- McKinney, Jonathan C -- Moran, James M -- Narayan, Ramesh -- Primiani, Rurik A -- Psaltis, Dimitrios -- Rogers, Alan E E -- Rosenfeld, Katherine -- SooHoo, Jason -- Tilanus, Remo P J -- Titus, Michael -- Vertatschitsch, Laura -- Weintroub, Jonathan -- Wright, Melvyn -- Young, Ken H -- Zensus, J Anton -- Ziurys, Lucy M -- New York, N.Y. -- Science. 2015 Dec 4;350(6265):1242-5. doi: 10.1126/science.aac7087. Epub 2015 Dec 3.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. mjohnson@cfa.harvard.edu. ; Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA. ; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA. ; Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA. ; Department of Astronomy, Radio Astronomy Laboratory, 501 Campbell, University of California Berkeley, Berkeley, CA 94720-3411, USA. ; Department of Physics MS-057, Brandeis University, Waltham, MA 02454-0911. ; Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA. National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan. Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. ; Institute of Astronomy and Astrophysics, Academia Sinica, Post Office Box 23-141, Taipei 10617, Taiwan. ; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. ; Academia Sinica Institute for Astronomy and Astrophysics (ASIAA), 645 N. A'ohoku Pl. Hilo, HI 96720, USA. ; Department of Astrophysics/Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen, Post Office Box 9010, 6500 GL Nijmegen, Netherlands. ; Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON N2L 2Y5, Canada. Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. ; Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany. ; James Clerk Maxwell Telescope, East Asia Observatory, 660 N. A'ohoku Place, University Park, Hilo, HI 96720, USA. ; Department of Physics, Joint Space-Science Institute, University of Maryland at College Park, Physical Sciences Complex, College Park, MD 20742, USA. ; National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan. Graduate University for Advanced Studies, Mitaka, 2-21-1 Osawa, Mitaka, Tokyo 181-8588. ; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA. Department of Physics MS-057, Brandeis University, Waltham, MA 02454-0911. ; Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany. ; Owens Valley Radio Observatory, California Institute of Technology, 100 Leighton Lane, Big Pine, CA 93513-0968, USA. ; Haystack Observatory, Route 40, Massachusetts Institute of Technology, Westford, MA 01886, USA. Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany. ; Department of Astrophysics/Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen, Post Office Box 9010, 6500 GL Nijmegen, Netherlands. Leiden Observatory, Leiden University, Post Office Box 9513, 2300 RA Leiden, Netherlands.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/26785487" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 2035-2037 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report attaining Ga-terminated (4×2) surface reconstruction on virgin GaAs substrates using a completely dry process at temperatures below the oxide sublimation temperature and without group V overpressure. The native oxides are removed with an electron cyclotron resonance hydrogen plasma treatment, followed by annealing at 500 °C in ultrahigh vacuum, which yields a reconstructed surface suitable for epitaxial overgrowth. Characterization by secondary ion mass spectroscopy and transmission electron microscopy reveals the complete removal of O, reduced C, and high structural order at the epilayer/substrate interface when this preparation method is used before molecular beam epitaxy. Annealing the substrate at a lower temperature yields a nonreconstructed surface possessing significant impurity concentrations, and leads to dislocation defects at the epilayer/substrate interface.
    Type of Medium: Electronic Resource
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    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Regrown/processed AlGaAs interfaces using secondary ion mass spectrometry, cross section transmission electron microscopy (TEM), and reflection high energy electron diffraction have been characterized. Two sets of samples, GaAs/Al0.4Ga0.6As (with GaAs on top) and Al0.4Ga0.6As/GaAs (with Al0.4Ga0.6As on top), are used as starting materials. For the GaAs/Al0.4Ga0.6As samples that are first exposed to atmosphere, the experiment is performed in an integrated processing system where etching and regrowth chambers are linked together by ultrahigh vacuum transfer modules. The etching process includes electron cyclotron resonance (ECR) hydrogen plasma cleaning of GaAs native oxides, ECR SiCl4 plasma anisotropic deep etching into Al0.4Ga0.6As, and an optional, brief Cl2 chemical etching. Regrowth is carried out using solid-source molecular beam epitaxy (MBE). Despite the in situ processing, significant amounts of C, Si, and O impurities at the 10, 5, and 50×1012 cm−2 levels exist at the interfaces. However, the impurity level is one order of magnitude smaller than that in air-exposed, ECR plasma etched and MBE regrown Al0.4Ga0.6As/GaAs of the set 2 samples. As revealed using TEM, isolated small particles (presumably correlated to aluminium oxides) exist at the regrown/processed interface of the set 1 samples, but no other defects such as dislocation are seen. Impurities and defects are mainly caused by the high reactivity of AlGaAs during ECR plasma etching.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 62 (1993), S. 735-737 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report a method for the removal of AlxGa1−xAs native oxides for 0≤x≤1, prior to molecular beam epitaxial overgrowth. The oxides formed on epilayers of AlGaAs after atmospheric exposure are removed in an electron cyclotron resonance hydrogen plasma with a substrate temperature less than 400 °C. Reflection high energy electron diffraction indicates the plasma-prepared AlGaAs surface are oxide-free and crystalline; after a vacuum anneal to 250–500 °C, GaAs or AlGaAs are epitaxially overgrown on these surfaces. Secondary ion mass spectroscopy detects C, O, and Si impurities at the interfaces, where their concentrations increase with increasing Al content of the exposed surface. The quality of the interface and the overgrown film, as observed by transmission electron microscopy, are found to be better for lower interface impurity densities.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 60 (1992), S. 1738-1740 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Epitaxial regrowth by solid-source molecular beam epitaxy (MBE) on dry etched heterostructures possessing exposed AlAs surfaces is accomplished for the first time using a vacuum integrated processing. Samples composed of multilayers of AlAs and AlGaAs are patterned with a SiO2 mask and are anisotropically etched using a low damage electron cyclotron resonance (ECR) SiCl4 plasma process. Etched samples are transferred in ultrahigh vacuum between the ECR and MBE chambers to avoid atmospheric exposure before regrowth. Microstructural analysis of the overgrown layers by scanning and transmission electron microscopy indicates that epitaxial regrowth on both the etched field and vertical sidewalls is achieved. The regrown material is found to contain microtwin plates, but few dislocations, indicating the good quality of the overgrowth.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 62 (1993), S. 2658-2660 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report a novel dry process to remove the surface contaminants C, Si, and O from GaAs substrates. This method utilizes an electron cyclotron resonance hydrogen plasma to remove the native oxides, followed by a very brief Cl2 chemical etching of GaAs to further reduce C and Si residues, and a final vacuum anneal. Characterization by secondary ion-mass spectrometry (SIMS) typically reveals the removal of C, Si, and O at the overgrown/processed interface to the levels below the SIMS detection limit. The as-processed GaAs surface, a Ga-stabilized reconstructed (4×6), is atomically smooth, and is as clean as a surface of freshly grown GaAs epilayers.
    Type of Medium: Electronic Resource
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