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  • Space Radiation  (13)
  • Astronomy  (3)
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  • 1
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    Evidence for Temporally-Extended, High-Energy Emission from Gamma Ray Burst 990104 (2002)
    In:  CASI
    Details
    Publikationsdatum: 2013-08-29
    Beschreibung: It is well known that high-energy emission (MeV - GeV) has been observed in several gamma ray bursts and temporally-extended emission from lower-energy gamma rays through radio wavelengths is well established. Observations of extended, high-energy emission are, however, scarce. Here we present evidence for a gamma ray burst emission that is both high-energy and extended, coincident with lower energy emissions. For the very bright and long burst, GRB 990104, we show light curves and spectra that confirm emission above 50 MeV, approximately 152 seconds after the BATSE (Burst and Transient Source Experiment) trigger and initial burst emission. Between the initial output and the main peak, seen at both low and high energy, there was a period of approx. 100 s during which the burst was relatively quiet. This burst was found as part of an ongoing search for high-energy emission in gamma ray bursts.
    Schlagwort(e): Space Radiation
    Format: application/pdf
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  • 2
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    Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare (1995)
    In:  CASI
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at GeV gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission.
    Schlagwort(e): Space Radiation
    Materialart: NASA-CR-200811 , NAS 1.26:200811
    Format: application/pdf
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  • 3
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    Stacking Searches for Greater Than 100 MeV Gamma Ray Emission from Radio Galaxies and Seyfert Galaxies (2003)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-18
    Beschreibung: The EGRET telescope on CGRO detected more than sixty sources of high-energy gamma radiation associated with active galactic nuclei (AGN). All but one of those belong to the blazar subclass; the only exception is the nearby radio galaxy Centaurus A. Since there is no obvious reason other than proximity to expect Cen A to be the only non-blazar AGN emitting in high-energy gamma rays, we have utilized the "stacking" technique to search for $〉100$-MeV emission from two non-blazar AGN subclasses, radio galaxies and Seyfert galaxies. Maps of gamma-ray counts, exposure, and diffuse background have been created, then co-added in varying numbers based on sorts by redshift, 5-GHZ flux density, and optical brightness, and finally tested for gamma-ray emission. No detection significance greater than $2\sigma$ has been found for any subclass, sorting parameter, or number of objects co-added. Monte Carlo simulations have also been performed, to validate the technique and estimate the significance of the results.
    Schlagwort(e): Astronomy
    Format: text
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  • 4
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    A Systematic Search for Short-term Variability of EGRET Sources (1999)
    In:  CASI
    Details
    Publikationsdatum: 2019-08-14
    Beschreibung: The 3rd EGRET Catalog contains 170 unidentified high-energy (E〉100 MeV) gamma-ray sources, and there is great interest in the nature of these sources. One means of determining sources class is the study of flux variability on time scales of days; pulsars are believed to be stable on these scales while blazars are known to be highly variable. In addition, previous work has led to the discovery of 2CG 135+01 and GRO J1838-04, candidates for a new high-energy gamma-ray source class. These sources display transient behavior but cannot be associated with any known blazars. These considerations have led us to conduct a systematic search for short-term variability in EGRET data, covering all viewing periods through cycle 4. Three unidentified sources show some evidence of variability on short time scales; the source displaying the most convincing variability, 3EG J2006-2321, is not easily identified as a blazar.
    Schlagwort(e): Space Radiation
    Materialart: Sep 15, 1999 - Sep 17, 1999; Portsmouth, NH; United States|EGRET Team Papers to be Published in the Proceedings of the Fifth Compton Symposium
    Format: text
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  • 5
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    Some Aspects of the Radio Emission of EGRET-Detected Blazars (1999)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: It has long been recognized that the high-latitude Energetic Gamma Ray Experiment Telescope (EGRET) sources can be identified with blazars of significant radio emission. Many aspects of the relation between high-energy gamma-ray emission and radio emission of EGRET-detected blazars remain uncertain. In this paper, we use the results of the recently published Third EGRET Source Catalog to examine in more detail to what extent the EGRET flux and the radio flux are correlated. In particular we examine the correlation (or the lack of it) in flux level, spectral shape, temporal variation, and detection limit. Many significant previous studies in these areas are also evaluated.
    Schlagwort(e): Space Radiation
    Materialart: EGRET Team Papers to be Published in the Proceedings of the Fifth Compton Symposium|Sep 15, 1999 - Sep 17, 1999; Portsmouth, NH; United States
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  • 6
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    EGRET/COMPTEL Observations of an Unusual, Steep-Spectrum Gamma-Ray Source (1999)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: During analysis of sources below the threshold of the third EGRET catalog, we have discovered a source, named GRO J1400-3956 based on the best position, with a remarkably steep spectrum. Archival analysis of COMPTEL data shows that the spectrum must have a strong turn-over in the energy range between COMPTEL and EGRET. The EGRET data show some evidence of time variability, suggesting an AGN, but the spectral change of slope is larger than that seen for most gamma-ray blazars. The sharp cutoff resembles the high-energy spectral breaks seen in some gamma-ray pulsars. There have as yet been no OSSE observations of this source.
    Schlagwort(e): Space Radiation
    Materialart: EGRET Team Papers to be Published in the Proceedings of the Fifth Compton Symposium|Sep 15, 1999 - Sep 17, 1999; Portsmouth, NH; United States
    Format: text
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  • 7
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    GeV Outbursts in Mrk 501 (1999)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: Mrk 501 is the third TeV blazar with a known GeV component. Previous multiwavelength campaigns on Mrk 501 showed well correlated outbursts at x-ray and TeV energies with no significant activity at GeV energies. We present here new evidence suggesting GeV outbursts in Mrk 501 when the spectrum appears to be extremely hard. However, this outburst appears uncorrelated with emission at x-ray energies. The resulting spectral energy distribution suggests a sharp cut off in the high-energy emission beyond a few hundred GeV.
    Schlagwort(e): Space Radiation
    Materialart: Sep 15, 1999 - Sep 17, 1999; Portsmouth, NH; United States|EGRET Team Papers to be Published in the Proceedings of the Fifth Compton Symposium
    Format: text
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  • 8
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    The Third EGRET Catalog of High-Energy Gamma-Ray Sources (1998)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-10
    Beschreibung: The third catalog of high-energy gamma-ray sources detected by the EGRET telescope on the Compton Gamma Ray Observatory includes data from 1991 April 22 to 1995 October 3 (Cycles 1, 2, 3, and 4 of the mission). In addition to including more data than the second EGRET catalog and its supplement, this catalog uses completely reprocessed data (to correct a number of mostly minimal errors and problems). The 271 sources (E greater than 100 MeV) in the catalog include the single 1991 solar flare bright enough to be detected as a source, the Large Magellanic Cloud, five pulsars, one probable radio galaxy detection (Cen A), and 66 high-confidence identifications of blazars (BL Lac objects, flat-spectrum radio quasars, or unidentified flat-spectrum radio sources). In addition, 27 lower-confidence potential blazar identifications are noted. Finally, the catalog contains 170 sources not yet identified firmly with known objects, although potential identifications have been suggested for a number of those. A figure is presented that gives approximate upper limits for gamma-ray sources at any point in the sky, as well as information about sources listed in the second catalog and its supplement which do not appear in this catalog.
    Schlagwort(e): Space Radiation
    Format: application/pdf
    Standort Signatur Erwartet Verfügbarkeit
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  • 9
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    Gamma Radiation from PSR B1055-52 (1999)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-10
    Beschreibung: The telescopes on the Compton Gamma Ray Observatory (CGRO) have observed PSR B1055-52 a number of times between 1991 and 1998. From these data, a more detailed picture of the gamma radiation from this source has been developed, showing several characteristics which distinguish this pulsar: the light curve is complex; there is no detectable unpulsed emission; the energy spectrum is flat, with no evidence of a sharp high-energy cutoff up to greater than 4 GeV. Comparisons of the gamma-ray data with observations at longer wavelengths show that no two of the known gamma-ray pulsars have quite the same characteristics; this diversity makes interpretation in terms of theoretical models difficult.
    Schlagwort(e): Space Radiation
    Format: application/pdf
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  • 10
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    The Third EGRET Catalog of High-Energy Gamma-Ray Sources (1998)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-10
    Beschreibung: The third catalog of high-energy gamma-ray sources detected by the EGRET telescope on the Compton Gamma Ray Observatory includes data from 1991 April 22 to 1995 October 3 (Cycles 1, 2, 3, and 4 of the mission). In addition to including more data than the second EGRET catalog (Thompson et al. 1995) and its supplement (Thompson et al. 1996), this catalog uses completely reprocessed data (to correct a number of mostly minimal errors and problems). The 271 sources (E greater than 100 MeV) in the catalog include the single 1991 solar flare bright enough to be detected as a source, the Large Magellanic Cloud, five pulsars, one probable radio galaxy detection (Cen A), and 66 high-confidence identifications of blazars (BL Lac objects, flat-spectrum radio quasars, or unidentified flat-spectrum radio sources). In addition, 27 lower-confidence potential blazar identifications are noted. Finally, the catalog contains 170 sources not yet identified firmly with known objects, although potential identifications have been suggested for a number of those. A figure is presented that gives approximate upper limits for gamma-ray sources at any point in the sky, as well as information about sources listed in the second catalog and its supplement which do not appear in this catalog.
    Schlagwort(e): Space Radiation
    Format: application/pdf
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