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  • 1
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    Jovian X-ray emissions (1996)
    In:  CASI
    Details
    Publikationsdatum: 2019-08-16
    Beschreibung: The Einstein and Rosat observations of X-ray emissions from Jupiter are summarized. Jupiter's soft X-ray emission is observed to originate from the planet's auroral zones, and specifically, from its equatorial region. The processes responsible for these emissions are not established. The brightness distribution of the Jovian X-rays is characterized by the dependence on central meridian longitude and by north-south and morning-afternoon asymmetries. The X-rays observed during the impact of the comet Shoemaker-Levy 9 are believed to be impact-induced brightenings of the X-ray aurora.
    Schlagwort(e): Space Radiation
    Materialart: ; 641-644
    Format: text
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  • 2
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    The Decay of Optical Emission from the gamma-Ray Burst GRB970228 (1997)
    In:  CASI
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    Publikationsdatum: 2019-07-13
    Beschreibung: The origin of gamma-ray bursts has been one of the great unsolved mysteries in high-energy astrophysics for almost 30 years. The recent discovery of fading sources at X-ray and optical wavelengths coincident with the location of the gamma-ray burst GRB970228 therefore provides an unprecedented opportunity to probe the nature of these high-energy events. The optical counterpart appears to be a transient point source embedded in a region of extended nebulosity, the latter having been tentatively identified as a high-redshift galaxy. This would seem to favour models that place gamma-ray bursts at cosmological distances, although a range of mechanisms for producing the bursts is still allowed. A crucial piece of information for distinguishing between such models is how the brightness of the optical counterpart evolves with time. Here we re-evaluate the existing photometry of the optical counterpart of GRB970228 to construct an optical light curve for the transient event. We find that between 21 hours and six days after the burst, the R-band brightness decreased by a factor of approximately 40, with any subsequent decrease in brightness occurring at a much slower rate. As the point source faded, it also became redder. The initial behaviour of the source appears to be consistent with the 'fireball' model, but the subsequent decrease in the rate of fading may prove harder to explain.
    Schlagwort(e): Space Radiation
    Materialart: NASA-TM-112889 , NAS 1.15:112889 , Letters to Nature; 387; 479-481
    Format: application/pdf
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  • 3
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    X-ray QSO evolution from a very deep ROSAT survey (1996)
    In:  CASI
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    Publikationsdatum: 2004-12-03
    Beschreibung: Some 32 quasi-stellar objects (QSOs) were identified to a flux limit of 2 x 10(exp -15) erg/sq cm in the deepest optically identified X-ray survey performed to date. This Rosat position sensitive proportional counter survey (PSPC) is spectroscopically 86 percent complete. The surface density of quasars at this flux limit is 210 +/- 37 per sq deg. The QSO X-ray luminosity function was measured at low luminosities and high redshifts, the highest of which was 3.4.
    Schlagwort(e): Space Radiation
    Materialart: ; 457-458
    Format: text
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  • 4
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    Comparison of interplanetary type 2 radio burst observations by ISEE-3, Ulysses, and WIND with applications to space weather prediction (1997)
    In:  CASI
    Details
    Publikationsdatum: 2004-12-03
    Beschreibung: Interplanetary (IP) type 2 radio bursts are produced by IP shocks driven by solar ejecta, presumably involving shock acceleration of electrons that leads to radio emission. These radio bursts, which can be detected remotely by a sensitive spacecraft radio receiver, provide a method of tracking the leading edge of solar ejecta moving outward from the sun. Consequently, observations of these bursts sometimes provide advance warning of one or more days prior to the onset of geomagnetic activity induced by the solar ejecta. A robust lower limit on the fraction of intense geomagnetic storms, that are preceded by IP type 2 bursts, is provided. It is shown that 41 percent of the geomagnetic storms occurring during the interval September 1978 to February 1983 were preceded by type 2 events in this catalog, and reasons why the fraction is not larger are addressed. Differences in the observing capabilities of the International Sun-Earth Explorer (ISEE) 3, Ulysses, and WIND, to explain why each of these similar spacecraft radio investigations provides a different perspective of IP type 2 emissions are reviewed.
    Schlagwort(e): Space Radiation
    Materialart: Proceedings of the 31st ESALB Symposium on Correlated Phenomena at the Sun, in the Heliosphere and in Geospace; 533-538; ESA-SP-415
    Format: text
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  • 5
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    Ulysses and WIND simultaneous observations of the radio emission associated with the 6-7 January 1997 coronal mass ejection (1997)
    In:  CASI
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    Publikationsdatum: 2004-12-03
    Beschreibung: The three dimensional source location of interplanetary type 2 radio bursts is intended to be determined from two spacecraft observations, performed by the radio receivers onboard the WIND and Ulysses spacecraft and associated with the interplanetary coronal mass ejection detected by the large advanced spectrometer coronagraph (LASCO) from the SOHO spacecraft. The intensity time profiles recorded by WIND and Ulysses were compared and their directivity is found to vary from one component of radio emission to another. The three dimensional location was obtained by radio triangulation and was deduced from the direction measured at WIND and the difference of the arrival times measured at both spacecraft. The sensitivity of both determination methods to wave scattering and refraction was discussed.
    Schlagwort(e): Space Radiation
    Materialart: Proceedings of the 31st ESALB Symposium on Correlated Phenomena at the Sun, in the Heliosphere and in Geospace; 429-430; ESA-SP-415
    Format: text
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  • 6
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    The Radio-to-X-Ray Spectrum of GRB 970508 on 1997 May 21.0 UT (1998)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: We have reconstructed the spectrum of the afterglow of GRB 970508 on 1997 May 21.0 UT (12.1 days after the gamma-ray burst) on the basis of observations spanning the X-ray-to-radio range. The low-frequency power-law index of the spectrum, alpha = 0.44 +/- 0.07 (F(sub nu) proportional to nu(exp alpha)), is in agreement with the expected value alpha = 1/3 for optically thin synchrotron radiation. The 1.4 GHz emission is self-absorbed. We infer constraints on the break frequencies nu(sub c) and nu(sub m) on 1997 May 21.0 UT from a spectral transition from F(sub nu) approx. nu(exp -0.6) to F(sub nu) approx. nu(exp -1.1) in the optical passband around 1.4 days. A model of an adiabatically expanding, blast wave emitting synchrotron radiation, in which a significant fraction of the electrons cool rapidly, provides a successful and consistent description of the afterglow observations over nine decades in frequency, ranging in time from trigger until several months later.
    Schlagwort(e): Space Radiation
    Materialart: Astrophysical Journal; 500; L97-L100
    Format: text
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  • 7
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    Correlations Between the Cosmic X-Ray and Microwave Backgrounds: Constraints on a Cosmological Constant (1998)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: In universes with significant curvature or cosmological constant, cosmic microwave background (CMB) anisotropies are created very recently via the Rees-Sciama or integrated Sachs-Wolfe effects. This causes the CMB anisotropies to become partially correlated with the local matter density (z less than 4). We examine the prospects of using the hard (2- 10 keV) X-ray background as a probe of the local density and the measured correlation between the HEAO1 A2 X-ray survey and the 4-year COBE-DMR map to obtain a constraint on the cosmological constant. The 95% confidence level upper limit on the cosmological constant is OMega(sub Lambda) less than or equal to 0.5, assuming that the observed fluctuations in the X-ray map result entirely from large scale structure. (This would also imply that the X-rays trace matter with a bias factor of b(sub x) approx. = 5.6 Omega(sub m, sup 0.53)). This bound is weakened considerably if a large portion of the X-ray fluctuations arise from Poisson noise from unresolved sources. For example, if one assumes that the X-ray bias is b(sub x) = 2, then the 95% confidence level upper limit is weaker, Omega(sub Lambda) less than or equal to 0.7. More stringent limits should be attainable with data from the next generation of CMB and X-ray background maps.
    Schlagwort(e): Space Radiation
    Materialart: New Astronomy (ISSN 1384-1076); 3; 275-291
    Format: text
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  • 8
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    Radio and Optical Follow-Up Observations and Improved IPN Position of GRB 970111 (1997)
    In:  Other Sources
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    Publikationsdatum: 2019-07-17
    Beschreibung: We report on Westerbork 840 MHz, 1.4 and 5 GHz radio observations of the improved IPN-WFC error box of the gamma ray burst GRB 970111, between 26.4 hours and 120 days after the event onset. In the approximately 16 sq arcmin area defined by the IPN (BATSE and Ulysses) annulus and the published refined BeppoSAX Wide Field Camera (WFC) error box we detected no steady sources brighter than 0.56 mJy (4sigma), and no varying radio emission, down to 1.0 mJy (4sigma). We also report on B, V, R and I band observations of the error box with the 4.2 m William Herschel Telescope at La Palma. Subject headings: gamma rays: bursts - gamma rays: individual (GRB 9701 1 1)
    Schlagwort(e): Space Radiation
    Materialart: Astrophysical Journal
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 9
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    Observations of a Major Outburst of BL Lacertae in the Active Galactic Nuclei Monitoring Program in the Center for Automated Space Science (1998)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: BL Lacertae (BL Lac) is the prototype for a class of Active Galactic Nuclei (AGN) known as BL Lac objects. These objects are characterized by a featureless optical continuum that is both highly polarized and extremely variable. Historically, this object is known to have an optical variability range of 5 magnitudes (Webb, et al., 1987, Carini, et al. 1992) and exhibits episodic periods in which the source is observed to be bright (V = 12.0) and other periods when the source is observed to be quite faint (V = 16.5). In May, 1997, we found, in the course of our routine monitoring of these objects, that BL Lac was in an elevated state, with a V magnitude peaking near V = 12.5. The purpose of this paper is to report on our observations of the behavior of the optical continuum flux of BL Lac during this major outburst.
    Schlagwort(e): Space Radiation
    Materialart: NASA University Research Centers Technical Advances in Aeronautics, Space Sciences and Technology, Earth Systems Sciences, Global Hydrology, and Education; 2 and 3; 57-60; NONP-NASA-CD-1999011585
    Format: text
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  • 10
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    Isolating the foreground of the X-ray background (1996)
    In:  CASI
    Details
    Publikationsdatum: 2019-08-27
    Beschreibung: The foreground component of the X-ray background can be studied by cross-correlating its intensity with galaxy catalogs. The two-point cross-correlation function between 67 Einstein X-ray fields and the APM galaxy catalogs was computed. A signal with an amplitude of W(sub xg) = 0.04 +/- 0.01, significant at the 3.5 sigma level, was computed. The reality of this signal was tested. The way in which this signal can be used to constrain models of the X-ray background is discussed.
    Schlagwort(e): Space Radiation
    Materialart: ; 337-338
    Format: text
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