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At least 145 records · Page 8
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NASA NTRS1976

A search of the SAS-2 data for pulsed gamma-ray emission from radio pulsars

Data from the SAS-2 high-energy (above 35 MeV) gamma-ray experiment have been examined for pulsed emission from each of 75 radio pulsars which were viewed by the instrument and which have sufficiently well-defined period and period-derivative information from radio observations to allow for gamma-ray periodicity searches. When gamma-ray arrival times were converted to pulsar phase using the radio reference timing information, two pulsars, PSR 1747-46 and PSR 1818-04, showed positive effects, each with a probability of less than 1 part in 10,000 of being a random fluctuation in the data for that pulsar. These are in addition to PSR 0531+21 and PSR 0833-45, previously reported. The results of this study suggest that gamma-ray astronomy has reached the detection threshold for gamma-ray pulsars and that work in the near future should give important new information on the nature of pulsars.

Ogelman, H.↗
NASA NTRS1983

Observation of an absorption feature in a gamma ray burst spectrum

A gamma ray burst was detected on March 25, 1978 by the High Energy X-ray and Low Energy Gamma Ray Experiment on HEAO-1. The burst spectrum shows an absorption feature at 55 + or - 5 keV with an equivalent width of 13 + or - 3 keV, values commensurate with those of similar features observed by the KONUS experiment. The burst spectrum also is characterized by a hard component extending from about 0.25-6 MeV. This component can be interpreted in terms of a fireball model for gamma ray bursts, which places the distance to the source at 1 kpc. The integrated fluence of the burst between 0.025 and 6 MeV is 1.5 x 10 to the -5th ergs/sq cm. The burst source has been localized to within a degree of RA = 237.5 deg and Dec = 76.2 deg.

Hueter, G. J.↗
NASA NTRS1989

High energy flare physics group summary

The contributions of the High Energy Flare Physics Special Session in the American Astronomical Society Solar Physics Division Meeting are reviewed. Oral and poster papers were presented on observatories and instruments available for the upcoming solar maximum. Among these are the space-based Gamma Ray Observatory, the Solar Flare and Cosmic Burst Gamma Ray Experiment on the Ulysses spacecraft, the Soft X Ray Telescope on the spacecraft Solar-A, and the balloon-based Gamma Ray Imaging Device. Ground based observatories with new capabilities include the BIMA mm-wave interferometer (Univ. of California, Berkeley; Univ. of Illinois; Univ. of Maryland), Owens Valley Radio Observatory and the Very Large Array. The highlights of the various instrument performances are reported and potential data correlations and collaborations are suggested.

Ryan, J. M.↗
NASA NTRS1978

A study of 'onboard' data processing for typical astrophysics payloads

The data, signal flow, and ground processing requirements for a typical set of X-ray and gamma-ray experiments were examined. A set of algorithms for ground processing were derived and these were taken as requirements imposed on 'onboard' spacecraft computers. Five types of CPU microprocessors were evaluated and compared with one another to determine the degree to which the data processing could be performed onboard the spacecraft. These included the NASA Standard Spacecraft Computer (NSSC-1), the Z-80, a Z-80 augmented with an auxiliary fast multiply chip, and an advanced augmented Z-80 with virtually unlimited memory. These CPU were investigated when used in a centralized mode, an experiment dedicated mode, and a hybrid of the first two modes. It was found that the use of onboard processing can significantly reduce downlink data rates for typical payloads, and may lead to greater efficiency of operation. Quantitative data for gamma-ray and X-ray payloads will be presented, and the results of the analysis discussed.

Stout, C. M.↗
NASA NTRS2000

Galactic Diffuse Gamma Ray Emission Is Greater than 10 Gev

AGILE and Gamma-ray Large Area Telescope (GLAST) are the next high-energy gamma-ray telescopes to be flown in space. These instruments will have angular resolution about 5 times better than Energetic Gamma-Ray Experiment Telescope (EGRET) above 10 GeV and much larger field of view. The on-axis effective area of AGILE will be about half that of EGRET, whereas GLAST will have about 6 times greater effective area than EGRET. The capabilities of ground based very high-energy telescopes are also improving, e.g. Whipple, and new telescopes, e.g. Solar Tower Atmospheric Cerenkov Effect Experiment (STACEE), Cerenkov Low Energy Sampling and Timing Experiment (CELESTE), and Mars Advanced Greenhouse Integrated Complex (MAGIC) are expected to have low-energy thresholds and sensitivities that will overlap the GLAST sensitivity above approximately 10 GeV. In anticipation of the results from these new telescopes, our current understanding of the galactic diffuse gamma-ray emission, including the matter and cosmic ray distributions is reviewed. The outstanding questions are discussed and the potential of future observations with these new instruments to resolve these questions is examined.

Hunter, Stanley D.↗
NASA NTRS1992

Detection of high-energy gamma-ray emission from the BL Lacertae object Markarian 421 by the Egret telescope on the Compton Observatory

Gamma radiation in the energy range from 50 MeV to well over 1 GeV has been observed from the direction of the BL Lac object Markarian 421 by the Energetic Gamma Ray Experiment Telescope on the Compton Observatory during the period 1991 June 27-July 11. The source flux is weak, but still statistically significant at the level of 10 exp -5; the integrated photon flux above 100 MeV is (1.4 +/- 0.3) x 10 exp -7/sq cm s. The differential photon energy spectrum can be represented by a power law with exponent 1.96 +/- 0.14. This is the first detection of gamma-ray emission from a BL Lac object.

Lin, Y. C.↗
NASA NTRS2004

GLAST: GeV Astronomy in a Multiwavelength Context

The GLAST Large Area Telescope (LAT), successor to Energetic Gamma-ray Experiment Telescope (EGRET) on the Compton Observatory, will play an important role in multiwavelength studies during the second half of this decade. Operating at energies between 20 MeV and greater than 300 GeV with sensitivity 30 or more times greater than that of EGRET, the Large Area Telescope (LAT) will offer good spatial and time resolution over a large (less than 2 sr) field of view. The LAT will bring insight to the whole range of high-energy gamma-ray phenomena, including bursts, active galactic nuclei, pulsars, supernova remnants, diffuse emission and unidentified sources. In essentially all cases, the maximum scientific return will come from coordinated (although not necessarily simultaneous) multiwavelength observations. Particularly with its planned scanning mode of operation, GLAST will have full sky coverage on relatively short time scales. The LAT team looks forward to cooperating with observers at other wavelengths.

Thompson, David J.↗
NASA NTRS1994

COMPTEL gamma-ray observations of the quasars CTA 102 and 3C 454.3

The blazar-type active galactic nuclei CTA 102 (QSO 2230+114) and 3C 454.3 (QSO 2251+158), located about 7 deg apart, were observed by the Compton Gamma Ray Observatory at four epochs in 1992. Both were detected by Energy Gamma Ray Experiment Telescope (EGRET). The combined Compton Telescope (COMPTEL) observations in the 10-30 MeV energy range clearly indicate a source of MeV emission, which is likely due to a contribution from both quasars. These observations strongly suggest that the power-law spectra measured by EGRET above approximately 50 MeV flatten at lower MeV energies. A comparison with observations at other wavelengths shows that the power spectra of CTA 102 and 3C 454.3 peak at MeV energies. This behavior appears to be a common feature of gamma-ray active galactic nuclei (AGN).

Blom, J. J.↗
NASA NTRS1997

New COMPTEL results on pulsar studies at MeV energies

The Compton telescope (COMPTEL) onboard the Compton Gamma Ray Observatory (CGRO) detected the pulsar PSR B1951-32 at MeV energies, and found indications of a signal from PSR B0656+14. In the combined spectra from COMPTEL and the energetic gamma ray experiment telescope (EGRET) onboard CGRO, it can be seen that the maximum luminosities of these objects are reached in the COMPTEL energy range. These spectra can be compared with those from four other pulsars observed in MeV energies with COMPTEL. The spectral properties of five of the six pulsars, Vela, PSR B1509-58, PSR B1951-32 and PSR B0656+14, require breaks and bends at MeV energies. The sixth pulsar, the Crab pulsar, approximately follows a power law flux relation from keV to GeV energies. It is concluded that this spectral behavior may play a role in the discrimination between current gamma ray emission models.

Hermsen, W.↗
NASA NTRS1992

Detection of high-energy gamma radiation from quasar 3C 279 by the EGRET telescope on the Compton Gamma Ray Observatory

Intense gamma radiation has been observed from the direction of the quasar 3C 279 throughout the energy range from 30 MeV to over 5 GeV by the Energetic Gamma Ray Experiment Telescope (EGRET) during the period June 15-28, 1991. Its spectrum is well represented by a photon differential power-law exponent of 2.0 +/- 0.1, with a photon intensity above 100 MeV of (2.8 +/- 0.4) x 10 exp -6/sq cm s. For E is greater than 100 MeV, the 2-sigma upper limits were 1.0 x 10 exp -6/sq cm s in 1973 from the SAS 2 observations and 0.3 x 10 exp -6/sq cm s for the combined 1976, 1978, and 1980 COS B observations. Hence, there has been a large increase in high-energy gamma-ray intensity relative to the earlier times, as there has been in the radio, infrared, optical, and X-ray ranges. This source is the most distant and by far the most luminous gamma-ray source yet detected.

Hartman, R. C.↗
NASA NTRS1985

Spectra and positions of galactic gamma-ray sources

The UCSD/MIT Hard X-Ray and Low Energy Gamma-Ray Experiment aboard HEAO-1 scanned the galactic center region during three epochs in 1977 and 1978 from 13 to 180 keV. The results are presented from the scanning epoch of 1978 September. Twenty-two known 2 to 10 keV source positions were necessary for an acceptable fit to the data. The spectra of the 16 strongest, least confused sources are all consistent with power laws with photon spectral indices ranging from 2.1 to 7.2. Acceptable fits to thermal bremsstrahlung models are also possible for most sources. No one source in this survey can be extrapolated to higher energy to match the intensity of the gamma-ray continuum as measured by HEAO-1 large field of view detectors, which implies that the continuum is a composite of contributions from a number of sources.

Knight, F. K.↗
NASA NTRS1993

EGRET detection of high-energy gamma radiation from the OVV quasar 3C 454.3

High-energy gamma radiation has been observed from the optically violent variable quasar 3C 454.3 (PKS 2251 + 158) by the Energetic Gamma-Ray Experiment Telescope on the Compton Observatory. During the 1992 January-February observation, the emission showed a power-law photon spectrum with an exponent of -2.18 +/- 0.08. The flux density (greater than 100 MeV) was observed to vary within the range (0.4-1.4) x 10 exp -6 photon/sq cm s on a time scale of less than a week. Lower sensitivity observations during 1992 April and May also detected emission within that range, but with lower statistical significance. An earlier low-sensitivity exposure during 1991 August gave a 95 percent confidence upper limit of 0.26 x 10 exp 6 photon/sq cm s. The similarity of the gamma-ray emission of 3C 454.3 to that of 3C 279 parallels the similarity of these two objects at lower frequencies.

Hartman, R. C.↗
NASA NTRS1994

EGRET observations of the Vela pulsar, PSR0833-45

During the all sky survey (May 1991 to November 1992) of the Compton Gamma Ray Observatory, the Vela pulsar PSR0833-45 was in the field of view of the Energetic Gamma Ray Experiment Telescope (EGRET) in ten separate viewing periods. The pulsar was detected in each one. The average intensity from 100 MeV to 2GeV was (7.8 +/- 1.0) x 10(exp -6) photons /sq cm/s, which indicates that the pulsar in the years 1991/92 was in a state comparable to the low fluxes observed in 1977-1980. No significant changes in intensity were detected during the EGRET observations. The total spectrum of PSR0833-45 measured by EGRET can be described by a power-law with index -1.70 +/- 0.02 over the range 30 MeV to 2 GeV. The extrapolation of this spectrum into the 3-30 MeV range agrees with the observations by COMPTEL. Above 2-4 GeV EGRET detects a strong spectral break. The lightcurves obtained show a familiar structure in the phase histogram: two peaks separated by 0.424 +/- 0.002 in phase with considerable emission in the phase interval between the peaks. The first gamma ray peak maximum trails the single radio peak maximum by 10.54 +/- 0.09 ms (= 0.118 +/- 0.001 in phase). The widths of the emission peaks (FWHM) are 2.7 ms for the first peak (0.03 phase) and 4.1 ms for the second peak (0.05 phase). The widths are approximately constant below a GeV, but show a tendency to become narrower at higher energies. The statistics available for the Vela observations allow for a division of the lightcurve into eight phase intervals. The emission peak cores (central FWHM) with leading and trailing wings and two interval regions were defined and spectra were derived for all parts of the lightcurve. The energy spectra for the eight phase intervals show significant differences: the first peak (approximately E(exp -1.81 +/- 0.04)) is somewhat softer than the second peak (approximately E(exp -1.71 +/- 0.03)); the wings attached to each peak show softer spectra than the core of the peaks; the interval emission has the hardest spectrum (approximately E(exp -1.52 +/- 0.03)).

Kanbach, G.↗
NASA NTRS1985

Burst and Transient Source Experiment (BATSE) for the Gamma Ray Observatory (GRO)

The Burst and Transient Source Experiment (BATSE) on the Gamma Ray Observatory (GRO) is expected to provide new and better observational data on bursts to test current and future models of burst sources. These data will include: (1) the celestial distribution of hundreds of burst sources over the life of the mission, (2) burst locations within several degrees, within 2 days after their occurrence, (3) observations of weaker bursts and better observations of short timescale fluctuations and spectral variations, (4) observations by a single experiment over a much larger energy range than previously available, and (5) more sensitive measurements of the spectral features which have been observed in many bursts. This paper briefly describes the GRO mission, the BATSE instrumentation and the burst observational capabilities.

Fishman, G. J.↗
NASA NTRS1994

EGRET observations of the Vela pulsar PSR 0833-45

During the all sky survey (May 1991 - Nov. 1992) of the Compton Gamma Ray Observatory the Vela pulsar PSR0833-45 was in the field of view of the Energetic Gamma Ray Experiment Telescope (EGRET) in ten separate viewing periods. The pulsar was detected in each one. The average intensity from 100 MeV to 2 GeV was (7.8 +/- 1.0) x 10 (exp -6) photons cm(exp -2) s(exp -1), which indicates that the pulsar in the years 1991/92 was in a state comparable to the low fluxes observed in 1977-1980. No significant changes in intensity were detected during the EGRET observations. The total spectrum of PSR0833-45 measured by EGRET can be described by a power law with index -(1.70 +/- 0.02) over the range 30 MeV to 2 GeV. The extrapolation of this spectrum into the 3 to 30 MeV range agrees with the observations by COMPTEL. Above 2 to 4 GeV EGRET detects a strong spectral break. The lightcurves obtained show a familar structure in the phase histogram: two peaks separated by 0.424 +/- 0.002 in phase with considerable emission in the phase interval between the peaks. The first gamma ray peak maximum trails the single radio peak maximum by 10.54 +/- 0.09 ms (= 0.118 +/- 0.001 in phase). The widths of the emission peaks (FWHM) are 2.7 ms for the first peak (0.03 phase) and 4.1 ms for the second peak (0.05 phase). The widths are approximately constant below a GeV, but show a tendency to become narrower at higher energies. On Jul. 20 1991 a glitch of the Vela period was registered in monitor radio observations. No significant differences between the pre- and post-glitch gamma ray lightcurves were found. The statistics available for the Vela observations allow for a division of the lightcurve into eight phase intervals. The emission peak cores (central FWHM) with leading and trailing wings and two interval regions were defined and spectra were derived for all parts of the lightcurve. The energy spectra for the eight phase intervals show significant differences: the first peak (approximately E(exp -1.81 +/- 0.04)) is somewhat softer than the second peak (approximately E(exp -1.71 +/- 0.03)); the wings attached to each peak show softer spectra than the code of the peaks; the interval emission has the hardest spectrum (approximately E(exp -1.52 +/- 0.03)).

Kanbach, G.↗
NASA NTRS2013

Methods and Results of a Search for Gravitational Waves Associated with Gamma-Ray Bursts Using the GEO 600, LIGO, and Virgo Detectors

In this paper we report on a search for short-duration gravitational wave bursts in the frequency range 64 Hz-1792 Hz associated with gamma-ray bursts (GRBs), using data from GEO600 and one of the LIGO or Virgo detectors. We introduce the method of a linear search grid to analyze GRB events with large sky localization uncertainties such as the localizations provided by the Fermi Gamma-ray Burst Monitor (GBM). Coherent searches for gravitational waves (GWs) can be computationally intensive when the GRB sky position is not well-localized, due to the corrections required for the difference in arrival time between detectors. Using a linear search grid we are able to reduce the computational cost of the analysis by a factor of O(10) for GBM events. Furthermore, we demonstrate that our analysis pipeline can improve upon the sky localization of GRBs detected by the GBM, if a high-frequency GW signal is observed in coincidence. We use the linear search grid method in a search for GWs associated with 129 GRBs observed satellite-based gamma-ray experiments between 2006 and 2011. The GRBs in our sample had not been previously analyzed for GW counterparts. A fraction of our GRB events are analyzed using data from GEO600 while the detector was using squeezed-light states to improve its sensitivity; this is the first search for GWs using data from a squeezed-light interferometric observatory. We find no evidence for GW signals, either with any individual GRB in this sample or with the population as a whole. For each GRB we place lower bounds on the distance to the progenitor, assuming a fixed GW emission energy of 10(exp -2)Stellar Mass sq c, with a median exclusion distance of 0.8 Mpc for emission at 500 Hz and 0.3 Mpc at 1 kHz. The reduced computational cost associated with a linear search grid will enable rapid searches for GWs associated with Fermi GBM events in the Advanced detector era.

methods↗