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  • 1
    Electronic Resource
    Electronic Resource
    [s.l.] : Macmillan Magazines Ltd.
    Nature 407 (2000), S. 445-445 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Sir — We would like to correct a misleading impression left by your News article on the new Green Bank Telescope (GBT) ( Nature 406, 816: 2000), which achieved first light on 22 August 2000. Funding for the GBT was appropriated by ...
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2013-08-31
    Description: The observed anticorrelation between diffuse soft X-ray flux and H I column density has been explained as absorption of soft X-rays produced in a hot galactic halo, assuming that the neutral interstellar material is sufficiently clumped to reduce the soft X-ray absorption cross section by a factor of two to three. A 21 cm emission line study of H I column density variations at intermediate and high galactic latitudes to 10' spatial resolution has been done. The results confirm conclusions from preliminary work at coarser resolution, and in combination with other data appear to rule out the hypothesis that clumping of neutral interstellar matter on any angular scale significantly reduces X-ray absorption cross sections in the 0.13 - 0.28 keV energy range. It is concluded therefore that the observed anticorrelation is not primarily a consequence of absorption of soft X-rays produced in a hot galactic halo.
    Keywords: ASTRONOMY
    Type: NAS 1.26:176674 , NASA-CR-176674
    Format: application/pdf
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  • 3
    Publication Date: 2013-08-31
    Description: Forty-five high-latitude, OB stars have been observed in the Ly alpha and 21 cm lines of HI in an effort to map out the vertical distribution and extent of the local HI halo. The 25 stars for which a reliable HI colum density can be obtained from Ly alpha lie between 60 and 3100 pc from the plane. The principal result is that the total column density of HI at z 1 kpc is, on the average, 5 + or - 3 x 10 the 19th power/sq cm, or 15% of the total sub HI. At relatively low z the data toward some stars suggest a low effective scale height and fairly high average foreground density, while toward others the effective scale height is large and the average density is low. This can be understood as the result of irregularities in the interstellar medium. A model with half of the HI mass in clouds having radii of a few pc and a Gaussian vertical distribution with sigma sub 2 = 135 pc, and half of the mass in an exponential component with a scale height of 500 pc, gives a satisfactory fit to the data. The technique of comparing Ly alpha and 21 cm column densities is also used to discuss the problem of estimating the distance to several possibly subluminous stars.
    Keywords: ASTRONOMY
    Type: NAS 1.26:176115 , NASA-CR-176115
    Format: application/pdf
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  • 4
    Publication Date: 2013-08-31
    Description: A detailed 21 cm study of areas of that have the smallest known amount of HI in the northern sky was performed. These observations were corrected for stray radiation. The region of main interest, around alpha = 10(h)45(m), delta = 57 deg 20', has a minimium N(HI) of 4.5 x 10 to the 19th power/sq cm. Spectra taken at 21' resolution over a field 4 x 3 deg in this direction show up to four HI line components. Two, near 0 and -50 km/s, are ubiquitous. There is also a narrow component at -10 km/s attributable to a diffuse cloud covering half of the field, and scattered patches of HI at v -100 km/s. the low and intermediate velocity components have a broad line width and are so smoothly distributed across the region that it is unlikely that they contain significant unresolved angular structure. Eight other low column density directions were also observed. Their spectra typically have several components, but the total column density is always 7 x 10 to the 19th power/sq cm and changes smoothly along a 2 deg strip. Half of the directions show narrow lines arising from weak diffuse HI clouds that contain 0.5 to 3.0 x 10 to the 19th power/sq cm.
    Keywords: ASTROPHYSICS
    Type: NASA-CR-175709 , NAS 1.26:175709
    Format: application/pdf
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  • 5
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    ISSN: 0066-4146
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    ISSN: 0066-4146
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 7
    Publication Date: 2011-08-19
    Description: The paper presents 21 cm observations made with the NRAO 43 m (140 foot) telescope of 20 randomly selected intermediate and high galactic latitude regions. The data are examined for evidence of the neutral gas clumping required by models in which a substantial fraction of the diffuse soft X-ray background (hv = 0.1 - 0.284 keV) originates outside the galactic disk and is absorbed by interstellar gas. No such evidence is found, and it is concluded that the degree of clumping required by such models must, if it exists, have characteristic angular scales less than 14 arcmin. Furthermore, an analysis of other data indicates that the required clumping does not exist on smaller size scales. It is therefore unlikely that a significant fraction of the X-ray flux originates in a galactic corona, unless some other explanation of the anomalously small apparent absorption can be found.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 290; 229-237
    Format: text
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  • 8
    Publication Date: 2011-08-18
    Description: New and existing observations of 21-cm emission lines toward 10 distant, high-latitude OB stars are combined with existing observations of interstellar Lyman-alpha absorption lines, in order to determine the ratio, N21/N-alpha, of the two different column densities of H I. This ratio, which is related to the fraction of the cool, neutral gas in the halo that lies beyond each star, decreases smoothly to about unity with increasing distance from the galactic plane. The column density of neutral gas beyond about 1 kpc can be as much as one-third of the total above the plane, but only relatively small amounts of such gas lie more than 2 kpc from the plane. The distances to, and the possible birthplaces of, these Population I stars in the halo are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 263
    Format: text
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  • 9
    Publication Date: 2014-09-27
    Description: Interstellar Lyman alpha observations toward high latitude stars, which give the column density of H I between Earth and the star, and 21 cm radio observations, which give the total H I column density to and beyond the star, can be combined to map out the amount of neutral gas as a function of distance from the Sun. Observations of the 21 cm and Lyman alpha lines made toward approximately 50 high latitude OB stars are presented. These data are used to discuss the distribution of H I in the solar neighborhood.
    Keywords: ASTROPHYSICS
    Type: NASA. Goddard Space Flight Center Local Interstellar Medium, No. 81; p 290
    Format: text
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  • 10
    Publication Date: 2019-07-13
    Description: We report the optical identification of an R = 18.3 mag, z = 2.432 quasar at the position of a 6 cm radio source and a faint ROSAT PSPC X-ray source. The quasar lies within the error circles of unidentified extreme-UV (EUV) detections by the EUVE and ROSAT WFC all-sky surveys at approximately 400 A and approximately 150 A, respectively. A 21 cm H I emission measurement in the direction of the quasar with a 21'-diameter beam yields a total H I column density of N(sub H) = 3.3 x 10(exp 20) /sq cm, two orders of magnitude higher than the maximum allowed for transparency through the Galaxy in the EUV The source of the EUV flux is therefore probably nearby (approximately 〈 100 pc), and unrelated to the quasar.
    Keywords: Space Radiation
    Type: Astronomy and Astrophysics; 308; 511-513
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