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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Active galaxies and the diffuse gamma-ray background

Active galaxies are shown to account for the observed gamma ray background radiation if a steepening of the spectra above about 100 keV is present. An analytical model is discussed in which protons undergo Fermi acceleration at a shock in a spherical accretion flow onto a massive black hole. Relativistic protons with power law spectra, nuclear interactions producing gamma rays from neutal pion decay and electrons from pion-mu meson-electron decay, with a power law spectrum above several hundred MeV, synchrotron and inverse Compton losses steepening the electron spectrum, a photon spectrum close to the pion gamma spectrum and a high-energy gamma ray spectrum steepened by photon-photon pair production interactions with X rays are covered in the model. Comparisons are made with HEAO 2 data on active galaxies, which have estimated luminosities and radii consistent with the compactness necessary for producing the steepening predicted by the model. The active galaxies spectra would be described by a spherical accretion-shock model.

Kazanas, D.↗
NASA NTRS1994

The optical-ultraviolet-gamma-ray spectrum of 3C 279

We have obtained spectrosocpy of the violently variable quasar 3C 279, simultaneous with gamma-ray observations, in 1992 April. Our combined optical (McDonald Observatory and Cerro Tololo InterAmerican Observatory (CTIO) and ultraviolet (HST) observations, made when the source was faint, show a very steep power-law continuum (F(sub nu) is proportional to nu(exp -1.95), and strong broad emission lines. This is the first time that the broad ultraviolet lines of this object have been measured, and we note several unusual properties of the spectrum. In particular, the profiles of C IV lambda 1549 and Mg II lambda 2798 are asymmetric, with very strong red wings, in contrast to the symmetic profiles of Ly alpha C III lambda 1909, and possible H-beta. The observed asymmetry cannot be explained by a simple outflow associted with the eruption of the source. In addition, the C IV lambda 1549/Ly-alpha and C III lambda 1909/Ly-alpha line intensity ratios are the largest we have observed in out Hubble Space Telescope (HST) sample of more than 30 radio-loud quasars, even though the C III llambda 1909/C IV lambda 1549 ratio is quite typical. 3C 279 was observed in the gamma-ray region by EGRET (Energetic Gamma-Ray Experiment Telescope) at the same time as our optical-ultraviolet observations. The extrapolated ultraviolet continuum falls nine orders of magnitude below the gamma-ray point and we show that this, combined with the optical UV continuum slope, is enough to rule out several synchtoyotron-self-Compton models suggested to explain the multiwavelength spectra of blazars.

Netzer, Hagai↗
NASA NTRS1994

Gamma-ray and millimeter-wave emissions from the 1991 June X-class solar flares

We have studied the spectacular 1991 June X-class flares using gamma-ray data from the Charged Particle Detectors (CPDs) of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) and 80 GHz millimeter data from Nobeyama, Japan. The CPDs were the only CGRO instrument that did not saturate during the extremely intense 1991 June 4 flare. We have shown that for this flare the CPDs respond to MeV photons, most of which are due to bremsstrahlung produced by relativistic electrons at the Sun. We have further shown that the gamma-ray and millimeter observations agree numerically if the 80 GHz radiation is gyrosynchrotron radiation produced by trapped electrons and the gamma rays are thick-target bremsstrahlung due to electrons precipitating out of the trap. The requirement that the trapping time obtained from the numerical comparison be consistent with the observed time profiles implies a magnetic field between about 200 and 300 G and an electron spectral index between about 3 to 5. By comparing the CPD observations with both the 80 GHz data and nuclear line data from the Energetic Gamma Ray Experiment Telescope (EGRET) and the Oriented Scintillation Spectroscopy Experiment (OSSE) on CGRO for the flares of June 4, 6, 9, and 11, we found that the ratio of the CPD counts to both the millimeter flux densities and the nuclear line fluences decreases with decreasing flare heliocentric angle. All of these flares were produced in the same active region. We interpreted this result in terms of a loop model in which the gyrosynchrotron emission is produced in the coronal portion of the loop where the electrons are kept isotropic by pitch angle scattering due to plasma turbulence, while the bremsstrahlung is produced by precipitating electrons that interact anisotropically. We found that the trapping time in the coronal portion is time dependent, reaching a minimum of about 10 s at the peak of the CPD count rate. We suggested the damping of the turbulence as a possible reason for the variation of the trapping time. turbulence as a possible reason for the variation of the trapping time.

Ramaty, Reuven↗
NASA NTRS1976

Measurement of the positron-electron reaction which produces a gamma ray pair in the region psi/3684/

An experiment was conducted to measure gamma ray pairs produced in the electron-positron reaction in the region of psi(3684) resonance. The apparatus consisted of two identical spectrometers mounted in a collinear configuration about the beam interaction region. Graphs are presented for the coplanarity angle between reconstructed gamma ray directions, for the scattering and energy deposition of gamma rays in the two crystals used, and for the observed rate of annihilation reactions as a function of center of mass energy.

Hughes, E. B.↗
NASA NTRS1981

Galactic gamma-ray emission from pulsars

Using data from the most recent pulsar surveys, the contribution of pulsars to the gamma-ray flux from the galactic plane is examined. The assumption is made that pulsar gamma rays are produced by curvature radiation from relativistic particles above the polar cap and attenuated by pair production in the strong magnetic and electric fields. If the assumption is made that all pulsars produce gamma rays in this way, their luminosities can be predicted as a function of period and magnetic field strength. The distribution of pulsars in the Galaxy is determined using data on 328 pulsars detected in three surveys. The Z and R distributions are found to be very sensitive to the mean electron density in the galactic plane, as are the total number and birthrate of pulsars in the Galaxy. Longitude profile of pulsar gamma-ray flux are calculated for different values of the mean electron density.

Harding, A. K.↗
NASA NTRS1992

Gamma-ray bursts from colliding neutron stars

Three distance scales to gamma-ray bursters are discussed: about 300 pc, about 2 - 50 kpc, and about 1 Gpc, corresponding to the Galactic disk, Galactic halo, and extragalactic origin. No compelling evidence is found in favor of any of them. The BATSE experiment on GRO should determine the distance scale by determining the angular distribution of very weak bursts. The rate of collisions between the neutron stars is about 0.0001/yr in our Galaxy, and about 10 exp 5/yr within the Hubble distance. The collisions are the final phases of binary orbit decay driven by gravitational radiation, and may produce gamma-ray bursts detectable at extra-galactic distances. If strange stars exist then their collisions must release about 10 exp 49 erg in gamma-rays over about 10 seconds. Such events should be detectable out to about 1 Gpc with the current instruments.

Paczynski, Bohdan↗
NASA NTRS1984

Solar-flare neutrons and gamma-rays

The numbers and spectra of the accelerated protons and nuclei that produce the neutrons and gamma-rays observed in solar flares are derived, and the results are compared with interplanetary observations of flare protons. The two most widely studied flare acceleration mechanisms, stochastic and diffusive shock acceleration, are discussed, and the arguments favoring the thick-target interaction model for neutron and gamma-ray production at the sun are briefly reviewed. The pertinent results of the theory of neutron and gamma-ray production are presented. The number and spectrum of the accelerated particles are derived from observations of nuclear deexcitation lines and the 2.223 MeV line from several flares. The June 21, 1980 and June 3, 1982 flares, from which a wealth of neutron, gamma-ray and energetic-particle data has recently become available, are discussed.

Murphy, R. J.↗
NASA NTRS1979

A look at the universe in gamma rays

The scientific background and objectives, proposed instruments, spacecraft constraints, operations plans and present status and schedule of the Gamma Ray Observatory (GRO), a shuttle-launched free-flyer satellite currently planned by NASA, are reviewed. Objectives for the satellite study of the highest energy electromagnetic radiation in the cosmos discussed include the investigation of the evolution of compact objects, nucleosynthesis, gamma-ray objects whose nature is not yet known, the gamma-ray properties of the Galaxy and other galaxies, cosmological effects and intense gamma-ray bursts. The five instruments considered in the definition study which span six decades in energy are presented and proposed experiments are outlined. Although a spacecraft has not yet been selected, several feasible alternatives have been identified, and a launch in the mid-1980s is planned.

Kniffen, D. A.↗
NASA NTRS2007

Gamma Ray Burst Observations with Swift and GLAST

The Swift and GLAST missions promise a great increase in our understanding of the gamma-ray universe. Swift was launched in November 2004 with a primary objective to study gamma-ray bursts. All instruments are performing well and more than 200 GRBs have been studied in detail. Major advances have already been made in the areas of short bursts, high redshift events and afterglow physics. The GLAST mission is scheduled for launch in fall 2007. It features a large newtechnology instrument for high energy gamma-ray observations. Hundreds of gamma-ray bursts will be detected by the LAT and GBM instruments. The talk will discuss the GRB science available with GLAST and the opportunities for joint Swift and GLAST observations of bursts.

Gehrels, Neil↗
NASA NTRS2016

Search for Gamma-Ray Emission from the Coma Cluster with Six Years of Fermi-LAT Data

We present results from gamma-ray observations of the Coma cluster incorporating six years of Fermi-LAT data and the newly released 'Pass 8' event-level analysis. Our analysis of the region reveals low-significance residual structures within the virial radius of the cluster that are too faint for a detailed investigation with the current data. Using a likelihood approach that is free of assumptions on the spectral shape we derive upper limits on the gamma-ray flux that is expected from energetic particle interactions in the cluster. We also consider a benchmark spatial and spectral template motivated by models in which the observed radio halo is mostly emission by secondary electrons. In this case, the median expected and observed upper limits for the flux above 100 MeV are 1.7 x 10(exp -9) ph cm(exp -2) s(exp -1) and 5.2 x 10(exp -9) ph cm(exp -2) s(exp -1) respectively (the latter corresponds to residual emission at the level of 1.8sigma). These bounds are comparable to or higher than predicted levels of hadronic gamma-ray emission in cosmic-ray (CR) models with or without reacceleration of secondary electrons, although direct comparisons are sensitive to assumptions regarding the origin and propagation mode of CRs and magnetic field properties. The minimal expected gamma-ray flux from radio and star-forming galaxies within the Coma cluster is roughly an order of magnitude below the median sensitivity of our analysis.

galaxies: clusters: individual (Coma) – gamma ra↗
NASA NTRS1989

Scientific considerations in the design of the Mars Observer gamma-ray spectrometer

Cosmic-ray primary and secondary particles induce characteristic gamma-ray and neutron emissions from condensed bodies in our solar system. These characteristic emissions can be used to obtain qualitative and quantitative elemental analyses of planetary surfaces from orbital altitudes. Remote sensing gamma-ray spectroscopy has been successfully used to obtain elemental composition of the Moon and Mars during United States Apollo 15 and 16 missions and the Soviet Luna and Mars missions. A remote sensing gamma-ray and neutron spectrometer will be included aboard the United States Mars Observer Mission. If proper care is not taken in the design of the spectrometer and choice of materials in the construction of the detector system ahd spacecraft, the sensitivity of these remote sensing spectrometers can be greatly degraded. A discussion of these design and material selection problems is presented.

Arnold, J. R.↗
NASA NTRS1987

The imaging of extra-galactic low-energy gamma-ray sources prospects, techniques, and instrumentation

Improved understanding of the nature of active galaxies will require detailed observations of 10 to 20 sources, while understanding of their gamma ray luminosity function and its evolution will require the detection of approximately 100 sources. Instruments capable of detecting 100 active galaxies at low-energy, gamma ray energies are achievable. The angular resolution of an instrument, as well as its sensitivity, can limit the number of sources it can observe. An investigation of the angular resolution requirements for future low-energy gamma ray instruments is presented. It was found that the strictest requirements arise not from the need to resolve detectable sources, but from the need to control the level of direction-to-direction fluctuations in the diffuse background level. It was concluded that gamma-ray instruments capable of detecting 100 active galaxies must have sub-degree angular resolution. The use of the coded aperture imaging technique is proposed as a method of achieving accurate control of systematic errors and fine angular resolution without unduly increasing the time needed to conduct full sky surveys. An analysis of coded aperture imaging is presented for instruments that employ masks based on hexagonal uniformly redundant arrays. An instrument, the gamma-ray imaging payload, was built that employs these imaging techniques. The design and testing of the instrument is described in detail. Preliminary results from a balloon flight of the instruments are shown, demonstrating its imaging performance.

Finger, Mark Harold↗
NASA NTRS2011

Latest Progress on the Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS)

We present the latest progress on building the balloon-borne Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument, including testing and calibration of the detectors and development of the imaging and aspect systems. A continental-US test flight is slated for fall 2012. GRIPS will provide a near-optimal combination of high-resolution imaging, spectroscopy, and polarimetry of solar-flare gamma-ray/hard X-ray emissions from approx.20. keV to > approx.10 MeV. The spectrometer/polarimeter consists of sixteen 3D position-sensitive germanium detectors (3D-GeDs), where each energy deposition is individually recorded with an energy resolution of a few keV FWHM and a spatial resolution to within <0.1 cu mm. Imaging is accomplished by a single multi-pitch rotating modulator (MPRM), a 2.5-cm thick tungsten-alloy grid with pitches that range quasi-continuously from 1 to 13 mm. With the MPRM situated 8 meters from the spectrometer, this instrument will provide excellent image quality and unparalleled angular resolution at gamma-ray energies (12.5 arcsec FWHM), sufficient to separate the 2.2 MeV footpoint sources for almost all flares. Polarimetry is accomplished by analyzing the anisotropy of reconstructed Compton scattering in the 3D-GeDs (i.e. as an active scatterer), with an estimated minimum detectable polarization of a few percent at 150-650 keV in an X-class flare. GRIPS will address questions relevant to particle acceleration and energy release that ha-.e been raised by recent solar flare observations, such as: What causes the spatial separation between energetic electron producing hard X-rays and energetic ions producing gamma-ray lines? How anisotropic are the accelerated electrons, and why do relativistic electron dominate in the corona? How does the composition of accelerated and ambient material vary with space and time, and why?

Shih, Albert Y.↗
NASA NTRS1992

The BATSE experiment on the Compton Gamma Ray Observatory: Status and some early results

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (GRO) is a sensitive all-sky detector system. It consists of eight uncollimated detectors at the corners of the spacecraft which have a total energy range of 15 keV to 100 MeV. The primary objective of BATSE is the detection, location, and study of gamma ray bursts and other transient sources. The instrement also has considerable capability for the study of pulsars, solar flares, and other discrete high energy sources. The experiment is now in full operation, detecting about one gamma ray burst per day. A brief description of the on-orbit performance of BATSE is presented, along with examples of early results from some of the gamma ray bursts.

Fishman, G. J.↗
NASA NTRS1994

COMPTEL observations of Ti-44 gamma-ray line emission from Cas A

The Compton Telescope (COMPTEL) telescope aboard the Compton Gamma-Ray Observatory (CGRO) is capable of imaging gamma-ray line sources in the MeV region with a sensitivity of the order 10(exp -5) photons/(sq cm s). During two observations periods in July 1992 and February 1993 the Galactic plane in the region of the young supernova remnant Cas A was observed, showing evidence for line emission at 1.16 MeV from the decay of Ti-44 at a significance level of approximately 4 sigma. This is the first time a supernova remnant has been detected in the gamma-ray line from Ti-44 decay. Adopting a distance of 2.8 kpc to the Cas A remnant, the measured line flux (7.0 +/- 1.7) x 10(exp -5) photons/(sq cm s), can be translated into a Ti-44 mass ejected during the Cas A supernova explosion, between (1.4 +/- 0.4) x 10(exp -4) solar mass and (3.2 +/- 0.8) x 10(exp -4) solar mass, depending on the precise value of the Ti-44 mean life time and on the precise date of the event. Implications of this result for supernova nucleosynthesis models are discussed.

Iyudin, A. F.↗
NASA NTRS1991

A model of unpulsed very high energy gamma rays from the Crab Nebula and pulsar

The angular resolution of gamma-ray detectors does not allow one to separate the nebula from the pulsar in the Crab. It is generally assumed that the steady emission of gamma rays comes from the nebula. Using the 'outer magnetospheric gap' model, an alternative mechanism in which the steady emission of gamma rays could come from a compact region, a couple of light cylinder radii beyond the pulsar.

Kwok, P. W.↗
NASA NTRS1982

Frequency of gamma-ray bursts greater than 3 x 10 to the -6th erg/sq cm

Results are presented for gamma-ray burst observations with the Large Area Sky Survey instrument on HEAO-1. It is noted that during the time HEAO-1 was operational, the instrument detected at least 12 confirmed gamma-ray bursts with intensities greater than 3 microerg/sq cm and identified six additional unconfirmed bursts. The rates of gamma-ray bursts are estimated to be about 125 per yr for intensities greater than 3 microerg/sq cm and approximately 50 per yr for intensities greater than 10 microerg/yr. The data are shown to yield a log N - log S relation for gamma-ray bursts that is consistent with a spherical distribution of sources within the Galaxy and having a broad range of intrinsic luminosities.

Share, G. H.↗