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  • Astronomy  (10)
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  • 1
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    GBM Observations of Terrestrial Gamma-Ray Flashes (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-19
    Description: The TGF detection rate of Gamma-ray Burst Monitor (GBM) has been increased twice since launch. The most recent improvement is from a new operating mode in which data for individual photons are down-linked for selected portions of the orbit, enabling a more sensitive ground-based search for TGFs. The new search has increased the TGF detection rate and is finding TGFs more than five times fainter than the TGFs of the previous GBM sample. We summarize the properties of the original GBM TGF sample and compare to the less intense TGFs now being detected. In addition to gamma-ray TGFs, GBM is observing distant TGFs from the propagation of charged particles along geomagnetic field lines. Strong 511 keV annihilation lines have been observed, demonstrating that both electrons and positrons are present in the particle beams. Spectral fits to these electron/positron TGFs will be shown.
    Keywords: Astronomy
    Type: M11-0072 , 2010 American Geophysical Union (AGU) Fall Meeting; Dec 13, 2010 - Dec 17, 2010; San Francisco, CA; United States
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  • 2
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    SGR J1550-5418 Bursts Detected with the Fermi Gamma-Ray Burst Monitor during Its Most Prolific Activity (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: We have performed detailed temporal and time-integrated spectral analysis of 286 bursts from SGR J1550-5418 detected with the Fermi Gamma-ray Burst Monitor (GBM) in 2009 January, resulting in the largest uniform sample of temporal and spectral properties of SGR J1550-5418 bursts. We have used the combination of broadband and high time-resolution data provided with GBM to perform statistical studies for the source properties.We determine the durations, emission times, duty cycles, and rise times for all bursts, and find that they are typical of SGR bursts. We explore various models in our spectral analysis, and conclude that the spectra of SGR J15505418 bursts in the 8-200 keV band are equally well described by optically thin thermal bremsstrahlung (OTTB), a power law (PL) with an exponential cutoff (Comptonized model), and two blackbody (BB) functions (BB+BB). In the spectral fits with the Comptonized model, we find a mean PL index of -0.92, close to the OTTB index of -1. We show that there is an anti-correlation between the Comptonized E(sub peak) and the burst fluence and average flux. For the BB+BBfits, we find that the fluences and emission areas of the two BB functions are correlated. The low-temperature BB has an emission area comparable to the neutron star surface area, independent of the temperature, while the high temperature BB has a much smaller area and shows an anti-correlation between emission area and temperature.We compare the properties of these bursts with bursts observed from other SGR sources during extreme activations, and discuss the implications of our results in the context of magnetar burst models.
    Keywords: Astronomy
    Type: GSFC-E-DAA-TN9469 , Astrophysical Journal; 749; 2; 122
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  • 3
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    The First Pulse of the Extremely Bright GRB 130427A: A Test Lab for Synchrotron Shocks (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever observed. The initial pulse up to 2.5 s is possibly the brightest well-isolated pulse observed to date. A fine time resolution spectral analysis shows power-law decays of the peak energy from the onset of the pulse, consistent with models of internal synchrotron shock pulses. However, a strongly correlated power-law behavior is observed between the luminosity and the spectral peak energy that is inconsistent with curvature effects arising in the relativistic outflow. It is difficult for any of the existing models to account for all of the observed spectral and temporal behaviors simultaneously.
    Keywords: Astronomy
    Type: GSFC-E-DAA-TN23274
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  • 4
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    Fermi GBM Observations of Terrestrial Gamma Flashes (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-19
    Description: In its first two years of operation, the Fermi Gamma Ray Burst Monitor (GBM) has observed more than 77 Terrestrial Gamma Flashes (TGFs). The thick Bismuth Germanate (BGO) detectors are excellent for TGF spectroscopy, having a high probability of recording the full energy of an incident photon, spanning a broad energy range from 150 keV to 40 MeV, and recording a large number of photons per TGF. Correlations between GBM TGF triggers and lightning sferics detected with the World-Wide Lightning Location Network indicate that TGFs and lightning are simultaneous to within tens of microseconds. The energy spectra of some TGFs have strong 511 keV positron annihilation lines, indicating that these TGFs contain a large fraction of positrons
    Keywords: Astronomy
    Type: M10-0872 , Scineghe2010 - 8th Workshop on Science with the New Generation of High Energy Gamma-Ray Experiments: Gamma-Ray Astrophysics in the Multimessenger Context; Sep 08, 2010 - Sep 10, 2010; Trieste, Italy; Italy
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  • 5
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    When a Standard Candle Flickers: Crab Nebula Variations in Hard X-rays (2011)
    In:  CASI
    Details
    Publication Date: 2019-07-19
    Description: In the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM) since August 2008, a ~7% (70 mcrab) decline was observed in the overall Crab Nebula flux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ~15-50 keV band with four other instruments: Swift/BAT, the RXTE/PCA, INTEGRAL/IBIS, and INTEGRAL/SPI. A similar decline is also observed in the ~3-15 keV data from the RXTE/PCA and in the 50-100 keV band with GBM, Swift/BAT, INTEGRAL/IBIS, and INTEGRAL/SPI. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ~3 year timescale are also seen independently with the PCA, BAT, IBIS, and SPI from 2005 to 2008, with a flux minimum in April 2007. As of April 2011, the Crab nebula flux has stopped declining and may be beginning to increase. We will present updated results on our multi-instrument study of long-term Crab nebula variations.
    Keywords: Astronomy
    Type: M11-0563 , 2011 meeting of the High Energy Astrophysics Division of the American Astronomical Society; Sep 07, 2011 - Sep 10, 2011; Providence, RI; United States
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  • 6
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    All-Sky Earth Occultation Observations with the Fermi Gamma Ray Burst Monitor (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-19
    Description: Using the Gamma Ray Burst Monitor (GBM) on-board Fermi, we are monitoring the hard X-ray/soft gamma ray sky using the Earth occultation technique. Each time a source in our catalog is occulted by (or exits occultation by) the Earth, we measure its flux using the change in count rates due to the occultation. Currently we are using CTIME data with 8 energy channels spanning 8 keV to 1 MeV for the GBM NaI detectors and spanning 150 keV to 40 MeV for the GBM BGO detectors. Our preliminary catalog consists of galactic X-ray binaries, the Crab Nebula, and active galactic nuclei. New sources are added to our catalog as they become active or upon request. In addition to Earth occultations, we have observed numerous occultations with Fermi's solar panels. We will present early results. Regularly updated results will be found on our website http://gammaray.nsstc.nasa.gov/gbm/science/occultation.
    Keywords: Astronomy
    Type: M10-0126 , M10-0094 , High Energy Astrophysics Division of American Astronomical Society (HEAD) 2010; Mar 01, 2010 - Mar 04, 2010; Big Island, HI; United States
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  • 7
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    Analysis of Line Candidates in Gamma-Ray Bursts Observed by BATSE (1999)
    In:  Other Sources
    Details
    Publication Date: 2019-07-17
    Description: A comprehensive search of BATSE Spectroscopy Detector data from 117 Gamma Ray Bursts (GRBs) has uncovered 13 statistically significant line candidates. The case of a candidate in GRB-930916 is discussed. In the data of SD-2 there appears to be a emission line at 46 keV, however the line is not seen in the data of SD-7. Simulations indicate that the lack of agreement between the results from SD-2 and SD-7 is implausible but not impossible.
    Keywords: Astronomy
    Type: Jan 01, 1999; Amsterdam; Netherlands
    Format: text
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  • 8
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    The Formation of Relativistic Jets from Kerr Black Holes (2003)
    In:  Other Sources
    Details
    Publication Date: 2019-07-18
    Description: We have performed the first fully three-dimensional general relativistic magnetohydrodynamics (GRMHD) simulation for Schwarzschild and Kerr black holes with a free falling corona and thin accretion disk. The initial simulation results with a Schwarzschild metric show that a jet is created as in the previous axisymmetric simulations with mirror symmetry at the equator. However, the time to form the jet is slightly longer than in the 2-D axisymmetric simulation. We expect that the dynamics of jet formation are modified due to the additional freedom in the azimuth dimension without axisymmetry with respect to the Z axis and reflection symmetry respect to the equatorial plane. The jet which is initially formed due to the twisted magnetic fields and shocks becomes a wind at the later time. The wind flows out with a much wider angle than the initial jet. The twisted magnetic fields at the earlier time were untwisted and less pinched. The accretion disk became thicker than the initial condition. Further simulations with initial perturbations will provide insights for accretion dynamics with instabilities such as magneto-rotational instability (MRI) and accretion-eject instability (AEI). These instabilities may contribute to variabilities observed in microquasars and AGN jets.
    Keywords: Astronomy
    Type: Particle Acceleration in Astrophysical Objects; Jun 24, 2003 - Jun 28, 2003; Cracow; Poland
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  • 9
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    All-Sky Earth Occultation Observations with the Fermi Gamma-Ray Burst Monitor (2009)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Using the Gamma Ray Burst Monitor (GBM) on-board Fermi, we are monitoring the hard X-ray/ soft gamma ray sky using the Earth occultation technique. Each time a source in our catalog is occulted by (or exits occultation by) the Earth, we measure its flux using the change in count rates due to the occultation. Currently we are using CTIME data with 8 energy channels spanning 8 keV to 1 MeV for the GBM NaI detectors and spanning 150 keV to 40 MeV for the GBM BGO detectors. Our preliminary catalog consists of galactic X-ray binaries, the Crab Nebula, and active galactic nuclei. In addition, to Earth occultations, we have observed numerous occultations with Fermi's solar panels.
    Keywords: Astronomy
    Type: M10-0041 , M09-0798 , M10-0196 , Fermi Symposium; Nov 02, 2009 - Nov 05, 2009; Washington, DC; United States
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  • 10
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    When a Standard Candle Flickers (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The Crab is the only bright steady source in the X-ray sky. The Crab consists of a pulsar wind nebula, a synchrotron nebula, and a cloud of expanding ejecta. On small scales, the Crab is extremely complex and turbulent. X-ray astronomers have often used the Crab as a standard candle to calibrate instruments, assuming its spectrum and overall flux remains constant over time. Four instruments (Fermi/GBM, RXTE/PCA, Swift/BAT, INTEGRAL/ISGRI) show a approx.5% (50 m Crab) decline in the Crab from 2008-2010. This decline appears to be larger with increasing energy and is not present in the pulsed flux, implying changes in the shock acceleration, electron population or magnetic field in the nebula. The Crab is known to be dynamic on small scales, so it is not too surprising that its total flux varies as well. Caution should be taken when using the Crab for in-orbit calibrations.
    Keywords: Astronomy
    Type: M10-1075 , 8th INTEGRAL Workshop: Restless Gamma-Ray Universe; Sep 27, 2010 - Sep 30, 2010; Dublin, Ireland; Ireland
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