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NASA NTRS1989

Radioactivity observed in scintillation counters during the HEAO-1 mission

Results are reported from an analysis of radioactivity induced in the NaI medium-energy detector of the hard X-ray and low-energy gamma-ray experiment during the HEAO-1 satellite mission (1977-1978). Consideration is given to the instrument characteristics, the origin and variability of background, and the separation of cosmic-ray activity from the internal activity due to South Atlantic Anomaly trapped protons. Energy spectra and tables listing the nuclide identifications are provided.

Gruber, D. E.↗
NASA NTRS1991

Plastic scintillator block as photon beam monitor for EGRET calibration

The EGRET (Energetic Gamma Ray Experiment Telescope) detector has been calibrated at SLAC (Stanford Linear Accelerator) and, to a lesser degree, at the MIT Bates Linear Accelerator Center. To monitor the photon beams for the calibration, a plastic scintillator block, 5 cm x 5 cm in cross section, 15 cm in length, and viewed by a single photomultiplier tube, was used for the entire beam energy range of 15 MeV to 10 GeV. The design operation, and method of analysis of the beam intensity are presented. A mathematical framework has been developed to treat the general case of a beam with multiphoton beam pulses and with a background component. A procedure to deal with the fluctuations of the beam intensity over a data-taking period was also developed. The photon beam monitor is physically sturdy, electronically steady, simple to construct, and easy to operate. Its major merits lie in its sheer simplicity of construction and operation and in the wide energy range it can cover.

Lin, Y. C.↗
NASA NTRS1992

The EGRET data products

We describe the Energetic Gamma Ray Experiment Telescope (EGRET) data products which we anticipate will suffice for virtually all guest and archival investigations. The production process, content, availability, format, and the associated software of each product is described. Supplied here is sufficient detail for each researcher to do analysis which is not supported by extant software.

Mattox, J. R.↗
NASA NTRS1992

Expected level of self-Compton scattering in radio loud quasars

Radio-loud quasars usually contain parsec-scale nonthermal jets. The most compact emission region ('the core'), and perhaps some of the moving 'knots', are expected to be efficient producers of inverse Compton scattered X-rays and gamma-rays since many of the synchrotron photons will upscatter before escaping. Through multifrequency flux density observations and Very Long Baseline Interferometry (VLBI) measurements of angular sizes, one can predict the flux density of this self-Compton high-energy emission. It is not always the case that the brightest synchrotron sources are also the brightest X-ray and gamma-ray sources. Perhaps a better predictor of high-energy brightness is the ratio of hard X-ray to high-frequency radio emission. Using the synchrotron self-Compton relations, we predict the gamma-ray fluxes of several sources we expect to be detected by the Energetic Gamma Ray Experiment Telescope (EGRET). More accurate predictions will be made when we complete a program of contemporaneous radio-submillimeter and X-ray observations during the course of the EGRET all-sky survey.

Bloom, Steven D.↗
NASA NTRS1992

Gamma rays from clusters of galaxies

The possibility that Coma-type radio halos are powered by secondary electrons produced during the decay of energetic charge pions is examined. These pions are generated by the collision of cosmic ray protons with thermal ions in the intracluster medium. A calculation of both the cosmic ray concentration and the spectrum needed to power the halo in the Coma cluster is presented. The gamma-ray flux is predicted and it is shown that this model can be tested with the Gamma Ray Observatory Energetic Gamma Ray Experiment Telescope.

Vestrand, W. Thomas↗
NASA NTRS1992

Cosmic ray albedo gamma rays from the quiet sun

We estimate the flux of gamma-rays that result from collisions of high energy galactic cosmic rays with the solar atmosphere. An important aspect of our model is the propagation of cosmic rays through the magnetic fields of the inner solar systems. We use diffusion to model propagation down to the bottom of the corona. Below the corona we trace particle orbits through the photospheric fields to determine the location of cosmic ray interactions in the solar atmosphere and evolve the resultant cascades. For our nominal choice of parameters, we predict an integrated flux of gamma rays (at 1 AU) of F(E(sub gamma) greater than 100 MeV) approximately = 5 x 10(exp -8)/sq cm sec. This can be an order of magnitude above the galactic background and should be observable by the Energetic Gamma Ray experiment telescope (EGRET).

Seckel, D.↗
NASA NTRS1992

Very high energy gamma ray extension of GRO observations

This has been an exiciting year for high energy gamma-ray astronomy, both from space and from ground-based observatories. It has been a particularly active period for the Whipple Observatory gamma-ray group. In phase 1 of the Compton Gamma Ray Observatory (GRO), there has not been too much opportunity for overlapping observations with the Energetic Gamma Ray Experiment Telescope (EGRET) and the other GRO telescopes; however, significant progress was made in the development of data analysis techniques and in improving the sensitivity of the technique which will have direct application in correlative observations in phase 2. Progress made during the period 1 Jul. 1991 - 31 Dec. 1991 is presented.

Weekes, Trevor C.↗
NASA NTRS1992

Quasars, blazars, and gamma rays

The paper discusses the extragalactic sources that have been discovered with the Energetic Gamma Ray Experiment Telescope. All of the sources demonstrate evidence of blazar properties at other wavelengths, including high optical polarization, extreme optical variability, flat-spectrum radio emission associated with a compact core, and apparent superluminal motion. These properties are believed to be produced by those few rare extragalactic quasars and radio galaxies that are favorably aligned to make it possible to observe almost directly down a relativistically outflowing jet of matter expelled from a supermassive black hole.

Dermer, Charles D.↗
NASA NTRS1993

Detection of high-energy gamma rays from quasar PKS 0528 + 134 by EGRET on the Compton Gamma Ray Observatory

The first several pointing directions of the Compton Gamma Ray Observatory, launched on 1991 April 5, were toward the Galactic anticenter. In addition to the known gamma-ray sources, Crab and Geminga, high-energy gamma-ray emission was observed from the quasar PKS 0528 + 134 by the Energetic Gamma Ray Experiment Telescope (EGRET). A redshift measurement, reported here, of 2.07 confirms the identification of this object as a quasar. The differential photon spectrum is well represented by a power law with an exponent of 2.56 +/- 0.09 and a photon intensity above 100 MeV of (8.4 +/- 1.0) x 10 exp -7 photons sq cm/s. There is evidence for time variability on a time scale of a few days.

Hunter, S. D.↗
NASA NTRS1992

EGRET observations of 3 C 273

The quasar 3C 273 was detected by the Compton Observatory Satellite (COS-B) in the 1970's. Energetic Gamma Ray Experiment Telescope (EGRET) observations of this sky region in Jun. and Oct. 1991 revealed a flux from 3C 273 lower than that measured by COS-B. The flux observed by EGRET in the June period is approximately 0.0000003/sq cm(exp -2) sec(exp -1) for energies greater than 100 MeV. During the Oct. observation it appears to be even lower. For the first observation a preliminary spectrum was derived which was a photon index of 2.4.

Vonmontigny, C.↗
NASA NTRS1994

Very High Energy Gamma Ray Extension of GRO Observations

The membership, progress, and invited talks, publications, and proceedings made by the Whipple Gamma Ray Collaboration is reported for june 1990 through May 1994. Progress was made in the following areas: the May 1994 Markarian Flare at Whipple and EGRET (Energetic Gamma Ray Experiment Telescope) energies; AGN's (Active Galactic Nuclei); bursts; supernova remnants; and simulations and energy spectra.

Weekes, Trevor C.↗
NASA NTRS1994

Optical studies of gamma-ray bursts. 2: The superbowl burst GRB 930131

We report Charge Couple Devices (CCD) observations of the central region of the Compton Telescope (COMPTEL) error box for the 31 Jan 1993 gamma-ray burst. One optical transient was found. Its distance from the IPN track and Energy Gamma Ray Experiment Telescope (EGRET) error box make the association of this object with GRB 930131 doubtful. We present new CCD observations of two objects identified by Schaefer et al. (1993) that are located within a few arcminutes of the interplanetary network (IPN) that were reported to be possibly variable. We also report observations on the ROSAT All-sky Survey Extreme Ultraviolet (EUV) source noted by those investigators and the RS CVn star HD 106255. The ROSAT All-Sky Survey EUV source is suggested to arise from a faint, but relatively rich cluster of galaxies. HD 106255 is located approximately 7 min from the IPN. Our CCD results show that this star was bluer than previously reported, but these observations were obtained significantly after the occurrence of GRB 930131. It is not possible to estimate the color of HD 106255 when the burst occurred.

Mcnamara, Bernard↗
NASA NTRS1994

Relativistic particle transport in hot accretion disks

Accretion disks around rapidly rotating black holes provide one of the few plausible models for the production of intense radiation in Acitve Galactic Nuclei (AGNs) above energies of several hundred MeV. The rapid rotation of the hole increases the binding energy per nucleon in the last stable orbit relative to the Schwarzschild case, and naturally leads to ion temperatures in the range 10(exp 12) - 10(exp 13) K for sub-Eddington accretion rates. The protons in the hot inner region of a steady, two-temperature disk form a reservoir of energy that is sufficient to power the observed Energetic Gamma Ray Experiment Telescope (EGRET) outbursts if the black hole mass is 10(exp 10) solar mass. Moreover, the accretion timescale for the inner region is comparable to the observed transient timescale of approximately 1 week. Hence EGRET outbursts may be driven by instabilities in hot, two-temperature disks around supermassive black holes. In this paper we discuss turbulent (stochastic) acceleration in hot disks as a possible source of GeV particles and radiation. We constrain the model by assuming the turbulence is powered by a collective instability that drains energy from the hot protons. We also provide some ideas concerning new, high-energy Penrose processes that produce GeV emission be directly tapping the rotational energy of Kerr black holes.

Becker, Peter A.↗
NASA NTRS1995

Celestial gamma ray study

This report documents the research activities performed by Stanford University investigators as part of the data reduction effort and overall support of the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Observatory. This report is arranged chronologically, with each subsection detailing activities during roughly a one year period of time, beginning in June 1991.

Michelson, Peter F.↗
NASA NTRS1997

Few arc-minute and keV resolutions with the TIGRE Compton telescope

The tracking and imaging gamma ray experiment (TIGRE) Compton telescope concept can provide an angular resolution of a few arcmin, an energy resolution of a few keV and high sensitivity, while providing the wide field of view necessary for surveying and monitoring observations. Silicon and CdZnTe strip detectors are used to detect Compton pair events and determine their incident directions and energies. Above 400 keV, Compton recoil electrons are tracked through successive layers of thin silicon strip detectors. Compton scattered photons are detected with CdZnTe strip detectors. Pair electrons and positrons are tracked to provide high sensitivity observations in the 10 to 100 MeV range. Polarization studies are performed with large angle Compton scatter events. The TIGRE concept and development status are described.

Zych, A.↗
NASA NTRS1998

EGRET High Energy Capability and Multiwavelength Flare Studies and Solar Flare Proton Spectra

The accomplishments of the participation in the Compton Gamma Ray Observatory Guest investigator program is summarized in this report. The work involved the study of Energetic Gamma Ray Experiment Telescope (EGRET)/Total Absorption Shower Counter(TASC) flare data. The specific accomplishments were the use of the accelerator neutron measurements obtained at the University of New Hampshire to verify the TASC response function and to modify the TASC fitting program to include a high energy neutron contribution, and to determine a high energy neutron contribution to the emissions from the 1991 June 11, solar flare. The next step in the analysis of this event was doing fits to the TASC energy-loss spectra as a function of time. A significant hardening of the solar proton spectrum over time was found for the flare. Further data was obtained from the Yohkoh HXT time histories and images for the 1991 October 27 flare. The results to date demonstrate that the TASC spectral analysis contributes crucial information on the particle spectrum interacting at the Sun. The report includes a paper accepted for publication, a draft of a paper to be delivered at the 26th International Cosmic Ray Conference and an abstract of a paper to be presented at the Meeting of the American Physical Society.

Chupp, Edward L.↗
NASA NTRS1998

X-Ray Emission from the Unidentified Gamma-Ray Transient GRO J1838-0415

The gamma-ray transient GRO J1838-04 is one of the most enigmatic sources in the sky. Despite its closeness to the Galactic plane, it showed a strong gamma-ray flare in 1995 typical of blazars. However, no blazar is detected in its approx. 1 deg. error box, and a Galactic source is suspected. Other time variable gamma-ray sources are known in the Galactic disk, and GRO J1838-04 may belong to a new class of sources (isolated young pulsars, the only proven Galactic gamma-ray sources, do not show variability). The Advanced Satellite for Cosmology and Astrophysics (ASCA) observed the centroid of the Energetic Gamma Ray Experiment Telescope (EGRET) error box of GRO J1838-04 in April 1997. Five weak X-ray sources are detected, one of which appears to be diffuse (3 arcmins). No known radio source is coincident with the X-ray sources. A preliminary report of our results appeared and a more systematic report is being written . The fact that no prominent X-ray source appears in the field, excludes an X-ray transient source with strong persistent emission is a possible counterpart of GRO JI838- 04 (such as superluminal transients). To uncover the nature of GRO J1838-04 requires more study of the weak X-ray sources in its error box, and a long-time scale monitoring of their variability.

Tavani, Marco↗
NASA NTRS1998

X-Ray Emission from the Unidentified Gamma Ray Transient GRO J1838-0415 and X-Ray Localization of Gamma-Ray Transients near the Galactic Plane

The investigation was focused on studying the residual X-ray emission from possible counterparts of enigmatic gamma-ray transients near the Galactic plane. These sources (discovered by our group at Columbia) are highly variable gamma-ray sources with no radio-loud spectrally flat blazar in their approx 1 deg. diameter error box. BSAX Narrow Field Instruments pointed the gamma-ray error box of the prominent non-blazar Energetic Gamma Ray Experiment Telescope (EGRET) source GRO 1838-04 twice on April 17, 1997, and on April 20, 1997. Most of the EGRET error box was imaged and several faint X-ray sources were detected. BSAX Wide Field Camera data were also obtained for a variety of sources near the Galactic plane, including GRO J1838-04.

Tavani, Marco↗