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At least 307 records · Page 17

Gamma-ray spectroscopy: The diffuse galactic glow

The goal of this project is the development of a numerical code that provides statistical models of the sky distribution of gamma-ray lines due to the production of radioactive isotopes by ongoing Galactic nucleosynthesis. We are particularly interested in quasi-steady emission from novae, supernovae, and stellar winds, but continuum radiation and transient sources must also be considered. We have made significant progress during the first half period of this project and expect the timely completion of a code that can be applied to Oriented Scintillation Spectrometer Experiment (OSSE) Galactic plane survey data.

Hartmann, Dieter H.↗

Optical monitoring of GRO phase 1 objects

A brief overview is given of the three phases of our Gamma Ray Observatory (GRO) optical monitoring program. The acquisition of simultaneous GRO/optical data on targeted Oriented Scintillation Spectrometer Experiment (OSSE) objects, the collection of noncoincident observations of suspected GRO gamma ray emitters to determine their level of activity, and the optical imaging of burst fields in an attempt to identify the burst source are discussed. Finally, a table of future simultaneous optical observations of Phase 1 GRO targets is presented along with plans for the rapid optical imaging of gamma ray burst fields.

Mcnamara, Bernard↗

Low energy gamma ray emission from the Cygnus OB2 association

According to our newly developed model of gamma-ray emission from chaotic early-type stellar winds, we predict the combined gamma-ray flux from the circumstellar winds of many very luminous early-type stars in the Cyg OB2 association can be detectable by the Energetic Gamma Ray Experiment Telescope (EGRET) (and maybe also by OSSE) on CGRO. Due to different radiation mechanisms, the gamma-ray spectrum from stellar winds can be quite different from that of CYG X-3; this spectral difference and the time-variation of Cyg X-3 flux will help to distinguish the gamma-ray components from different sources in this small region, which is spatially unresolvable by CGRO.

Chen, Wan↗

GRIS observations of the galactic center and the gamma ray galactic diffuse continuum

On two flights in 1988, the Gamma-Ray Imaging Spectrometer (GRIS) discovered the galactic center in a high state (greater than 1 x 10(exp -3) ph/(sq cm sec)) of positron annihilation line emission (511 keV) after nearly a decade of failed attempts to confirm the exciting early results of balloon and satellite instruments. These two flights represented the first flights of a new generation of high resolution germanium spectrometers designed to achieve significantly greater sensitivity for astrophysical observations. During the fall flight, an observation of the galactic plane at 335 degrees longitude was also performed. This observation showed a very low level of 511 keV emission (2 +/- 1 x 10(exp -4) ph/(sq cm sec)), confirming the galactic center origin of the line, and a high level of hard x-ray and gamma-ray continuum emission (1 x 10(exp -4) ph/(sq cm sec keV) at 100 keV), which we attribute to galactic diffuse emission. Improved fits to the spectrum of the galactic center are presented with the proposed diffuse component subtracted. We conclude that our galactic center continuum spectrum is consistent with the sum of the 1E1740.7-2942 spectrum observed by SIGMA/GRANAT and our 1 = 335 degree galactic plane spectrum. The predicted diffuse flux should be easily measurable by the Oriented Scintillation Spectrometer Experiment (OSSE) experiment on the Gamma Ray Observatory (GRO).

Tueller, J.↗

An investigation of the role of current and future remote sensing data systems in numerical meteorology

A flexible system for performing observing system simulation experiments which made contributions to meteorology across all elements of the observing system simulation experiment (OSSE) components was developed. Future work will seek better understanding of the links between satellite-measured radiation and radiative transfer in the clear, cloudy and precipitating atmosphere and investigate how that understanding might be applied to improve the depiction of the initial state and the treatment of physical processes in forecast models of the atmosphere.

Diak, George R.↗

An investigation of the role of current and future remote sensing data systems in numerical meteorology

The goals of this research endeavor have been to develop a flexible and relatively complete framework for the investigation of current and future satellite data sources in numerical meteorology. In order to realistically model how satellite information might be used for these purposes, it is necessary that Observing System Simulation Experiments (OSSEs) be as complete as possible. It is therefore desirable that these experiments simulate in entirety the sequence of steps involved in bringing satellite information from the radiance level through product retrieval to a realistic analysis and forecast sequence. In this project we have worked to make this sequence realistic by synthesizing raw satellite data from surrogate atmospheres, deriving satellite products from these data and subsequently producing analyses and forecasts using the retrieved products. The accomplishments made in 1991 are presented. The emphasis was on examining atmospheric soundings and microphysical products which we expect to produce with the launch of the Advanced Microwave Sounding Unit (AMSU), slated for flight in mid 1994.

Diak, George R.↗

The Co-57 abundance in SN 1987A

Astrophysical implications of the detection by OSSE of Co-57 gamma radiation from SN 1987A are discussed. By burying the alpha-rich-freezeout portion at deeper gamma depths than in published models, it is shown that it remains barely possible that the bolometric luminosity during days 1200-1800 could derive from Co-57 power without requiring 57/56 production ratios greater than twice solar. Alternative mechanisms which may contribute to the bolometric power at late times are proposed.

Clayton, Donald D.↗

Diffuse galactic annihilation radiation

The study reports observations of positron annihilation radiation from the inner region of the Galaxy which show that there are two components of the radiation: a steady, diffuse Galactic component and a variable component from discrete, presumably compact sources. The existence of the variable component is supported by the ensemble of all narrow FOV 511 keV line observations, including recent detections with OSSE. The fit of this ensemble to a time-independent source distribution can be excluded at the approximately 3-sigma level. The same ensemble, combined with the broad FOV SMM observations of Galactic 511 keV line emission, sets constraints on the Galactic distribution of the diffuse component.

Ramaty, R.↗

Active galactic nuclei make the cosmic X-ray background

While it has long been known that active galactic nuclei (AGNs) make up a significant part of the X-ray background near 1 keV, it has been generally thought that the typical AGN X-ray spectrum did not fit the higher energy background spectrum. More recent data (from Ginga, SIGMA, and OSSE) show that the X-ray spectra of radio-quiet AGNs do in fact resemble the X-ray background spectrum. These data are well described (from 2 to 100 keV) by a model in which the X-ray photons are made by thermal Comptonization, and a fraction (around a half) of the X-rays are reflected by a cool surface. Using the observed evolution of X-ray-bright AGNs, we show that the known AGN population, radiating the spectrum observed at low redshift and integrated in redshift back to z about 4, is capable of producing the entire X-ray background from 2 to 100 keV.

Zdziarski, Andrzej A.↗

The EUVE bright source list

Initial results for bright extreme ultraviolet sources discovered during the EUVE all-sky and deep ecliptic surveys have been published as a Bright Source List (BSL) and released to the astronomical community with a recent NASA research announcement (NRA 93-OSS-02, Appendix F). This paper describes the data processing software, the EUVE survey data set, and the production of the BSL at the Center for EUV Astrophysics. The contents, format, and selection criteria for sources, the data processing strategy, some problems encountered, and a summary of the BSL results are presented.

Stroozas, B.↗

Calculation of geopotential and temperature fields from an array of nearly continuous wind observations

The accuracy is studied of temperature estimates derived from the divergence equation when wind observations of various spatial and temporal resolutions and accuracies are available. The basic data set used is the high resolution model data set used by Kuo and Anthes (1984a) in observing systems simulation experiments (OSSE) designed to estimate the errors in heat and moisture budgets (Kuo and Anthes, 1984b) calculated from the AVE-SESAME-1979 spatial observational network. This model data set is modified in ways to simulate wind observations that appear feasible from an operational regional network of wind profilers.

Kuo, Ying-Hwa↗

The Telecommunications and Data Acquisition Report

This quarterly publication provides archival reports on developments in programs managed by JPL's Office of Telecommunications and Data Acquisition (TDA). In space communications, radio navigation, radio science, and ground-based radio and radar astronomy, it reports on activities of the Deep Space Network (DSN) in planning, supporting research and technology, implementation, and operations. Also included are standards activity at JPL for space data and information systems and reimbursable DSN work performed for other space agencies through NASA. The preceding work is all performed for NASA's Office of Space Communications (OSC). The TDA Office also performs work funded by another NASA program office through and with the cooperation of OSC. This is the Orbital Debris Radar Program with tile Office of Space Systems Development. The TDA Office is directly involved in several tasks that directly support the Office of Space Science (OSS), with OSC funding DSN operational support. In radio science, The TDA Progress Report describes the spacecraft radio science program conducted using tile DSN. For the High-Resolution Microwave Survey (HRMS), the report covers implementation and operations for searching the microwave spectrum. In solar system radar, it reports on the uses of the Goldstone Solar System Radar for scientific exploration of the planets, their rings and satellites, asteroids, and comets. In radio astronomy, the areas off support include spectroscopy, very long baseline interferometry, and astrometry.

Edward C Posner↗

Workshop on advanced technologies for planetary instruments

NASA's robotic solar system exploration program requires a new generation of science instruments. Design concepts are now judged against stringent mass, power, and size constraints--yet future instruments must be highly capable, reliable, and, in some applications, they must operate for many years. The most important single constraint, however, is cost: new instruments must be developed in a tightly controlled design-to-cost environment. Technical innovation is the key to success and will enable the sophisticated measurements needed for future scientific exploration. As a fundamental benefit, the incorporation of breakthrough technologies in planetary flight hardware will contribute to U.S. industrial competitiveness and will strengthen the U.S. technology base. The Workshop on Advanced Technologies for Planetary Instruments was conceived to address these challenges, to provide an open forum in which the NASA and DoD space communities could become better acquainted at the working level, and to assess future collaborative efforts. Over 300 space scientists and engineers participated in the two-and-a-half-day meeting held April 28-30, 1993, in Fairfax, Virginia. It was jointly sponsored by NASA's Solar System Exploration Division (SSED), within the Office of Space Science (OSS); NASA's Office of Advanced Concepts and Technology (OACT); DoD's Strategic Defense Initiative Organization (SDIO), now called the Ballistic Missile Defense Organization (BMDO); and the Lunar and Planetary Institute (LPI). The meeting included invited oral and contributed poster presentations, working group sessions in four sub-disciplines, and a wrap-up panel discussion. On the first day, the planetary science community described instrumentation needed for missions that may go into development during the next 5 to 10 years. Most of the second day was set aside for the DoD community to inform their counterparts in planetary science about their interests and capabilities, and to describe the BMDO technology base, flight programs, and future directions. The working group sessions and the panel discussion synthesized technical and programmatic issues from all the presentations, with a specific goal of assessing the applicability of BMDO technologies to science instrumentation for planetary exploration.

Appleby, J.↗

Gamma-ray emission from globular clusters. Shock high energy emission from the Be-Star/Pulsar System PSR 1259-63. Echoes in x-ray novae

This grant covers work on the Compton phase 3 investigation, 'Shock High Energy Emission from the Be- Star/Pulsar System PSR 1259-63' and cycle 4 investigations 'Diffuse Gamma-Ray Emission at High Latitudes' and 'Echoes in X-Ray Novae'. Work under the investigation 'Diffuse Gamma-Ray Emission at High Latitudes' has lead to the publication of a paper (attached), describing gamma-ray emissivity variations in the northern galactic hemisphere. Using archival EGRET data, we have found a large irregular region of enhanced gamma-ray emissivity at energies greater 100 MeV. This is the first observation of local structure in the gamma-ray emissivity. Work under the investigation 'Echoes in X-Ray Novae' is proceeding with analysis of data from OSSE from the transient source GRO J1655-40. The outburst of this source last fall triggered this Target of Opportunity investigation. Preliminary spectral analysis shows emission out to 600 keV and a pure power low spectrum with no evidence of an exponential cutoff. Work is complete on the analysis of BATSE data from the Be-Star/Pulsar Sustem PSR 1259-63.

Kaaret, Philip↗

Hard X-rays from SN 1993J

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Observatory observed SN 1993J during three intervals centered approximately 12, 30, and 108 days after its outburst. Hard X-ray emission was detected in the first two of these intervals. No emission was seen in the third observation or in two earlier observations in 1991 and 1992. The coincidence of the observed excess with the outburst of SN 1993J and the consistency of the spectra and time evolution with those seen at lower energies by ROSAT and ASCA (Astro-D) argue that the observed emission is indeed from SN 1993J. It is probably due to the interaction of the fast supernova ejecta with circumstellar material. The luminosity, 5 x 10(exp 40) ergs/sec (50-150 keV) in the first interval, is significantly larger than predicted. Extrapolating the spectrum to a few keV accounts for most or all of the observed emission at low energy. The observed high temperature, 10(exp 9) K, is easily obtained in the shocked circumstellar matter, but a surprisingly high density is required there to give the observed luminosity, and little or no additional X-ray emission from denser shocked supernova ejecta is allowed. The hard emission might also be explained in terms of the shocked supernova ejecta itself with unexpectedly high temperature.

Leising, M. D.↗

Gamma-ray properties of active galactic nuclei

Recent observations by the Compton Observatory (CGRO) have increased our knowledge about the gamma-ray emission from Active Galactic Nuclei (AGN) considerably. The three most important findings of CGRO with respect to AGNs are: first, no Seyfert 1 galaxy has been found to show emission above 500 keV. The by far strongest Seyfert 1 galaxy NGC 4151 shows a spectrum which falls off exponentially with an e-folding energy of 39 keV between 65 and 500 keV. OSSE so far has detected or has indications of detections for seven additional Seyfert 1 galaxies, which, however, all show very weak hard X-ray emission compared to NGC 4151. No annihilation feature has been seen from any Seyfert galaxy to this date. Second, the radio galaxy Cen A shows a power-law energy spectrum from hard X-ray energies of about 150 keV to at least 3 MeV. It has not been seen at EGRET-energies. Third, a new class of AGN was discovered at energies above 100 MeV by EGRET. The power of these objects in gamma-rays can dominate the luminosity in other spectral ranges. These objects are associated with extragalactic sources that have blazar properties. The high-energy gamma-ray emission is probably produced in relativistically outflowing jets. At hard X-ray energies the objects are rather weak. Spectral breaks at MeV energies were found by COMPTEL for three of these objects (3C 273, 3C 279, and PKS 0528+134).

Schoenfelder, V.↗

Gamma-ray bright quasars and BL Lacertae objects: EGRET data

The Compton Gamma-Ray Observatory (CGRO) Instruments, in particular, EGRET, have recently discovered new, gamma-ray bright active galactic nuclei (AGNs)(fluxes approximately 0.5-3 times as bright as 3C 273). Yet current sample sizes are modest (for example, EGRET as of 1993 January has detected 17 quasars and five BL Lac objects), and the sources probe only a small range in flux. Only by using a large, unbiased sample of AGNs can we answer key questions about source evolution, contributions to the gamma-ray background, and the different gamma-ray properties of quasars and BL Lac objects. Therefore, we need gamma-ray data on a much larger number of AGNs. We are currently pursuing two approaches toward remedying this situation: first, new (phase 2) OSSE, COMPTEL, and EGRET observations of X-ray selected AGNs predicted to be gamma-ray bright on simple assumptions; even an upper limit can set constraints on continuum models (e.g., pair production, synchrotron self-Compton); second, searches for radio-selected AGNs in wide-field EGRET data obtained during the pointings. For these AGN, we expect a large number of faint sources (i.e., mainly low signal-to-noise ratio detections and upper limits), a continuous progression down to the limiting EGRET sensitivity. Although special techniques must be employed to deal carefully with low-significance sources, the end product will be a much larger sample than currently available on which to base deductions about the gamma-ray properties of AGNs. Here we report the first results of this program. We find an upper limit for 3C 382 and demonstrate, using the 3C 382 field, how serendipitous blazars can systematically searched for in existing EGRET public (Phase 1) data. Preliminary results indicate that synchrotron self-Compton processes dominate.

Schachter, Jonathan↗

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↗