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NASA NTRS1991

Some aspects of the scientific significance of high energy gamma ray astrophysics

The attraction of high energy gamma-ray astronomy lies in this radiation relating directly to those processes in astrophysical situations which deviate most from thermo-dynamic equilibrium. Some examples of these phenomena which are known to or expected to emit gamma rays are cosmic rays as they interact in intergalactic space, the high energy particles in the magnetic fields of neutron stars, the death of a black hole, the explosion and residual of a supernova, lumps of Weakly Interacting Massive Particles, energetic solar particles interacting near the sun, and very high energy particles in the extreme conditions associated with active galaxies. Although the intensities are known to be low as seen near the earth, a partially compensating characteristic is that the very penetrating nature of high energy gamma rays increases the probability that they can escape from their origin and reach the solar system.

Fichtel, Carl E.↗

NASA NTRS1991

The large-scale surface brightness distribution of the x ray background

The x-ray background (XRB) and the microwave background are the dominant isotropic radiation fields available for measurement. There was extensive work on trying to determine the physical origin of the background. That is, whether it is due to a superposition of numerous faint well-known sources, such as active galaxies, an early unidentified population of AGN at high redshift, a new population of objects, or to truly diffuse processes or to a superposition of these. However, while of great intrinsic interest, these studies were not aimed at using the XRB to provide the cosmological information that was gleaned from the microwave background. An alternate approach is presented, which uses the available information on the large, greater than 5 deg, scale distribution of the sky flux to see if the XRB can provide such constraints.

Mushotzky, Richard↗

NASA NTRS1991

An astrophysics data program investigation of a synoptic study of quasar continua

A summary of the program is presented. The major product of the program, an atlas of quasar energy distributions, is presented in the appendices along with papers written as a result of this research. The topics covered in the papers include: (1) accurate galactic N(sub h) values toward quasars and active galactic nuclei (AGN); (2) weak bump quasars; (3) millimeter measurements of hard x ray selected active galaxies- implications for the nature of the continuous spectrum; (3) persistence and change in the soft x ray spectrum of the quasar PG1211+143; (4) the soft x ray excess in einstein quasar spectra; and (5) EXOSAT x ray spectra of quasars.

Elvis, Martin↗

NASA NTRS1992

The Compton Observatory Science Workshop

The Compton Observatory Science Workshop was held in Annapolis, Maryland on September 23-25, 1991. The primary purpose of the workshop was to provide a forum for the exchange of ideas and information among scientists with interests in various areas of high energy astrophysics, with emphasis on the scientific capabilities of the Compton Observatory. Early scientific results, as well as reports on in-flight instrument performance and calibrations are presented. Guest investigator data products, analysis techniques, and associated software were discussed. Scientific topics covered included active galaxies, cosmic gamma ray bursts, solar physics, pulsars, novae, supernovae, galactic binary sources, and diffuse galactic and extragalactic emission.

Shrader, Chris R.↗

NASA NTRS1992

Scientific objectives and first results from COMPTEL

The imaging Compton telescope (COMPTEL) is the first imaging telescope in space to explore the MeV gamma ray range. At present it is performing a complete sky survey. In later phases of the mission, selected celestial objects will be studied in more detail. Targets of special interest in the COMPTEL energy range are radio pulsars, X-ray binaries, novae, supernova remnants, molecular clouds, and the interstellar medium within the Milky Way, as well as the nuclei of active galaxies, supernovae, and the diffuse cosmic background radiation in extragalactic space. The first four months of operation demonstrated that COMPTEL basically performs as expected. The Crab is clearly seen at its proper position in the first images of the anticenter region of the Galaxy. The Crab pulsar lightcurve was measured with unprecedented accuracy. The quasar 3C273 was seen for the first time at MeV-energies. Several cosmic bursts within the COMPTEL field of view could be located to an accuracy of about 1 degree. On June 9, 11, and 15, 1991 COMPTEL observed gamma ray (continuum and line) emission from three solar flares. Neutrons were also detected from the June 9 flare. At the present state of analysis, COMPTEL achieves the prelaunch predictions of its sensitivity within a factor of 2. Based on the present performance of COMPTEL, the team is confident that COMPTEL will fulfill its primary mission of surveying and exploring the MeV sky.

Schoenfelder, V.↗

NASA NTRS1992

Spectral-luminosity evolution of active galactic nuclei and the cosmic X- and gamma ray background

Coherent electromagnetic dynamo acceleration processes, which act on charge particles within the context of black hole accretion disk scenarios, are generally regarded as the underlying central power source for active galactic nuclei (AGN). If the precursor active galaxies (PAG) for such AGN are formed at high redshift and contain initial seed black holes with mass approximately equal to 10(exp 4) solar masses, then the Eddington limited X-ray radiation emitted during their lifetime will undergo the phenomenon of 'spectral-luminosity evolution'. When accretion disks are first formed at the onset of galaxy formation the accretion rate occurs at very high values of luminosity/size compactness parameter L/R greater than 10(exp 30) erg/cm-sec. In the absence of extended structure, such high values of L/R generate dynamic constraints which suppress coherent, black hole/accretion disk dynamo particle acceleration processes. This inhibits nonthermal radiation processes and causes the spectrum of X-radiation emitted by PAG to be predominantly thermal. A superposition of PAG sources at z is greater than or equal to 6 can account for the residual cosmic X-ray background (CXB) obtained from the total CXB after subtraction of foreground AGN sources associated with present epoch Seyfert galaxies. The manner in which the PAG undergo spectral-luminosity evolution into Seyfert galaxies is investigated.

Leiter, Darryl↗

NASA NTRS1991

Ultraviolet imaging of globular clusters and galaxies

The techniques of UV imaging and its scientific potential are reviewed for studies of advanced stellar evolution, the star formation history and interstellar medium of galaxies, active nuclei, and the evolutionary state of systems observed at high redshift. Early Hubble Space Telescope imaging is expected to emphasize such programs since its image quality is excellent by earlier standards for the UV, despite the presence of spherical aberration.

O'Connell, Robert W.↗

NASA NTRS1992

The near-infrared counterpart of a variable galactic plane radio source

A near-infrared counterpart to the highly variable, unresolved galactic plane radio source GT 0116 + 622 is identified. This source is of particular interest, as it has been previously suggested to be the counterpart of the gamma-ray source Cas gamma-l. The present NIR and red images detect a faint, spatially extended (3 arcsec FWHM), very red object coincident with the radio position. There is complex spatial structure which may be due in part to an unrelated superposed foreground object. Observations on multiple nights show no evidence for flux variability, despite the high amplitude variability on a time-scale of days reported for the radio source. The data are consistent with an interpretation of GT 0116 + 622 as an unusually variable, obscured active galaxy at a distance of several hundred megaparsecs, although more exotic, and in particular galactic, interpretations cannot yet be ruled out. If the object is extragalactic, the previously suggested identification with the gamma-ray source would seem unlikely.

Margon, Bruce↗

NASA NTRS1991

The Gamma Ray Observatory - Viewing the violent universe

An overview is given of the Gamma Ray Observatory (GRO) experimental satellite and the four instruments that permit the study of space phenomena from 0.03 to 30,000 MeV. The GRO comprises: (1) the Burst and Transient Source Experiment; (2) the Oriented Scintillation Spectrometer Experiment; (3) the Imaging Compton Telescope; and (4) the Energetic Gamma-Ray Experiment Telescope. The GRO is expected to provide data regarding the active galaxies, extragalactic diffuse radiation, the Galactic center, matter-antimatter annihilation, and gamma-ray emission from quasars.

Thompson, D. J.↗

NASA NTRS1992

Synchrotron emission from a cosmological jet as a model of gamma-ray bursts

The nonthermal rapidly varying spectra of gamma-ray bursts require relativistic beaming if the source is at a cosmological distance. Limits on the Lorentz factors and magnetic fields of extragalactic jet models of gamma-ray bursts are derived from the synchrotron source function, the position of the fundamental cyclotron resonance, the Thomson optical depth of radiating electrons, and the Thomson optical depth of electron-positron pairs created in photon-photon collisions of Compton upscattered radiation. The last of these constraints is the strongest, and it is satisfied by the observations if the jet's Lorentz factor is greater than 100. The limit from the position of the cyclotron fundamental requires a magnetic field strength below approximately 10 exp 10 G. Jet models producing observable cyclotron lines are allowed. The acceleration region in the burst must be separate from the emission region. The similarity of gamma-ray burst jet models to models of quasars and BL Lac objects suggest that if gamma-ray bursts are cosmological sources, they are a type of active galaxy.

Brainerd, J. J.↗

NASA NTRS1992

The discovery of an X-ray selected, radio-loud quasar at z = 3.9

About 600 weak X-ray sources from a data base of about 100,000 2-4sigma fluctuations derived from Einstein Observatory imaging proportional counter X-ray images are selected on the basis of a flux-limited catalog of radio sources in the northern sky. Optical spectroscopy of only six of these sources resulted in the discovery of a quasar with an emission-line redshift of 3.87. Subsequent VLA observations reveal the source to have a flat-spectrum radio core plus an unresolved radio lobe 2.5 arcsec away with a spectral index of 1.45. The lobe is connected to the core by a weak jet. The integrated flux density of the source at 20 cm is 0.66 Jy. The v magnitude of the quasar is 19.5, and the tentative X-ray sources has a very soft spectrum. The properties of this object are discussed in the context of known high-redshift radio sources and existing X-ray selected quasar samples, and the potential of the present discovery technique for examining the properties of very distant active galaxies is explored.

Becker, R. H.↗

NASA NTRS1992

Goddard Visiting Scientist Program for the Space and Earth Sciences Directorate

A visiting scientist program was conducted in the space and earth sciences at GSFC. Research was performed in the following areas: astronomical observations; broadband x-ray spectral variability; ground-based spectroscopic and photometric studies; Seyfert galaxies; active galactic nuclei (AGN); massive stellar black holes; the differential microwave radiometer (DMR) onboard the cosmic background explorer (COBE); atmospheric models; and airborne and ground based radar observations. The specific research efforts are detailed by tasks.

Kerr, Frank↗

NASA NTRS1991

Spectral-luminosity evolution of active galactic nuclei and the cosmic X-ray background

Black hole accretion disk dynamo processes are generally regarded as the central power source for AGN (Rees, 1984). If the precursor active galaxies for such AGN are formed at redshift z greater than about 4 and contain initial central seed black holes about 10 exp 6 solar masses, then the Eddington limited X-ray radiation emitted during their lifetime will undergo the phenomenon of 'spectral-luminosity evolution'. When accretion disks are first formed at the onset of galaxy formation, the accretion rate occurs at high values of luminosity/size compactness parameter L/R greater than 10 exp 30 erg/cm-sec. Such high values of L/R generate dynamic constraints which suppress nonthermal black hole accretion disk dynamo processes in favor of thermal processes. This causes the spectrum of X-radiation emitted by early AGN to be predominantly thermal. A superposition of such thermal, comptonized PAG sources can account for the residual cosmic X-ray background and can act as a source of X-ray heating of the intergalactic medium for z greater than about 4.

Leiter, Darryl↗

NASA NTRS1993

Overview of the first results from EGRET

An overview of the early results of EGRET, one of four instruments carried on the Compton Gamma Ray Observatory, is presented with regard to observations of pulsars, bursts, galactic diffuse radiation, and active galaxies. The June 11, 1992 solar flare was observed in gamma rays with energies from the threshold of detection to energies over 1 GeV. Detailed time histories and energy spectra were obtained for the Crab and Vela pulsars, with one of the Vela pulses being quite sharp, approximately 2 percent of the total period. Several gamma-ray bursts were recorded. The one on May 3, 1991 was seen in EGRET's spark chamber system, which made it possible to determine a position. The quasars 3C 279, which has a redshift of 0.538, was seen in high energy gamma rays in both June and October 1992.

Fichtel, C. E.↗

NASA NTRS1992

Testing the AGN paradigm for our home Galaxy

Although there are some flaws in each of the methods to constrain the black hole mass, which are described in the above paper, all of them unambiguously indicate that the mass of a putative black hole at the Galactic center is much smaller than a widely quoted value, about 10 exp 6 M(solar). Possibly, the mass of the black hole, if any, does not exceed 100 M(solar) or so. Further work is needed in order to distinguish between such a modest-mass black hole and its potential rival such as a very massive star of about the same mass. Anyway, the Galactic nucleus seems to be a tracer of a past star burst rather than a remnant of a Seyfert galaxy activity.

Ozernoy, Leonid↗

NASA NTRS1992

Astrophysical bags - A new paradigm for active galactic nuclei?

Active galaxies are believed to consist of a compact nucleus, the standard model for which is a massive black hole or a cluster of black holes. A different paradigm is considered here, deriving from quark confinement theory in QCD. It is an 'astrophysical bag', modelled after the 'hadron bags' of particle physics which have already been studied in astrophysics as quark stars. Another interpretation of the cosmological constant in general relativity, and possibly a new quasar redshift formula, are introduced. As a highly-energetic object, this model may resolve the baryonic matter problem for fuelling AGN accretion processes which black hole paradigms cannot account for. Here, baryons, cosmic rays, and neutrinos are free.

Wilson, Thomas L.↗

NASA NTRS1992

Spectral-luminosity evolution of active galactic nuclei (AGN)

The origin of the cosmic X-ray and gamma-ray backgrounds is explained via the mechanism of AGN spectral-luminosity evolution. The spectral evolution of precursor active galaxies into AGN, and Newton-Raphson input and output parameters are discussed.

Leiter, Darryl↗

NASA NTRS1993

Is 1308+326 A BL Lacertae object or a quasar?

The active galaxy 1308+326 (z = 0.996) has been classified as a BL Lacertae object because of its nearly featureless optical continuum and high and variable polarization. New VLBI polarization images of 1308+326, however, show both VLBI polarization structure and tentative superluminal motions that are characteristic of quasars rather than BL Lacertae objects. These data strongly suggest that this source is simply a quasar with emission lines of small equivalent width. We discuss the possibility that the optical continuum is enhanced by microlensing in a foreground galaxy, as suggested by Stickel et al.

Gabuzda, D. C.↗