R. L. Kinzer

OSSE Mapping of Galactic 511 keV Positron Annihilation Line Emission

The Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Galactic plane and Galactic center region have been combined with scanning observations by the Transient Gamma-Ray Spectrometer (TGRS) and Solar Maximum Mission (SMM) instruments to produce maps of the Galactic narrow 511 keV positron annihilation line radiation. Two different mapping methods, singular value decomposition and maximum entropy, have been applied to the data. In both cases, the resulting maps show evidence for three distinct features: (1) a central bulge, (2) emission in the Galactic plane, and (3) an enhancement or extension of emission at positive latitudes above the Galactic center. Modeling of the data confirmed the existence of these features. The derived distribution is found to be in good qualitative agreement with nearly all of the historical observations of narrow 511 keV line emission from the Galactic center region. No evidence of time variability is found. Various possible production mechanisms for the observed positrons, including the positive-latitude enhancement, are presented. It is found that supernovae are capable of producing positrons at the required rate to account for the intensity and morphology of the observed 511 keV line emission.

The gamma ray spectrometer for the Solar Maximum Mission

The Solar Maximum Mission Gamma Ray Experiment (SMM GRE) utilizes an actively shielded, multicrystal scintillation spectrometer to measure the flux of solar gamma rays. The instrument provides a 476-channel pulse height spectrum (with energy resolution of ∼7% at 662 keV) every 16.38 s over the energy range 0.3–9 MeV. Higher time resolution (2 s) is available in three windows between 3.5 and 6.5 MeV to study prompt gamma ray line emission at 4.4 and 6.1 MeV. Gamma ray spectral analysis can be extended to ≳15 MeV on command. Photons in the energy band from 300–350 keV are recorded with a time resolution of 64 ms. A high energy configuration also gives the spectrum of photons in the energy range from 10–100 MeV and the flux of neutrons ≳20 MeV. Both have a time resolution of 2 s. Auxiliary X-ray detectors will provide spectra with 1-sec time resolution over the energy range of 10–140 keV. The instrument is designed to measure the intensity, energy, and Doppler shift of narrow gamma ray lines as well as the intensity of extremely broadened lines and the photon continuum. The main objective is to use this time and spectral information from both nuclear gamma ray lines and the photon continuum in a direct study of the dynamics of the solar flare/particle acceleration phenomena.

OSSE Observations of Blazars

Abstract : Results are reported on observations obtained with the Oriented Scintillation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory (CGRO) of 17 active galactic nuclei known to exhibit blazar properties at other wavelengths. These observations span the period from 1991 June through 1994 May. Of the 33 high-confidence EGRET detections of blazars during CGRO observing phases 1 and 2 (1991 May 16 - 1993 Sept. 7), OSSE has observed 8 and detected 5, namely 3C 273, 3C 279, PKS 0528+134, CTA 102, and 3C 454.3. Additionally, OSSE has detected the BL LAC sources H 1517+65.6 and PKS 2155-304, which were not detected with EGRET. Variablility in the energy band 50 -150 keV is observed for all of the detected AGN. The OSSE blazar sources are all well described by simple power law models with photon number indices,,, varying from 1.0 to 2.1 among sources. When combined with available, although not necessarily contrmporaneous, COMPTEL and EGRET observations, 4 out of 5 detected blazars show clear evidence for spectral breaks between the hard X-ray and medium energy gamma-ray bands. The exception is the combined OSSE/EGRET data for 3C 279 during 1991 October, where a simple power-law with, approximately equal 1.9 works equally well. Gamma-ray evidence for beaming in CTA 102, PKS 0528+134, and 3C 454.3 is presented.

Accelerated Particle Composition and Energetics and Ambient Abundances from Gamma-Ray Spectroscopy of the 1991 June 4 Solar Flare

The Oriented Scintillation Spectrometer Experiment (OSSE) on board the Compton Gamma Ray Observatory observed the 1991 June 4 X12+ solar flare, one of the most intense nuclear gamma-ray line flares observed to date. Using these OSSE observations, we have derived time profiles of the various components of gamma-ray emission and obtained information about the accelerated particle spectra and composition and about the ambient plasma at the flare site. The main results are (1) the nuclear reactions associated with the impulsive phase of the flare continued for at least 2 hours and resulted from ions that were probably continuously accelerated rather than impulsively accelerated and trapped; (2) the total energy in these accelerated ions exceeded the energy in >0.1 MeV electrons; (3) the accelerated α/proton ratio was closer to 0.5 than to 0.1; (4) there is evidence for a decrease of the accelerated heavy ion-to-proton ratio as the flare progressed; (5) there is evidence for a temporal change in the composition of the flare plasma; (6) the ratio of electron bremsstrahlung to the flux in narrow γ-ray lines decreased as the flare progressed; (7) the high-energy (>16 MeV) component of the electron spectrum was much more impulsive than the lower energy ~MeV component; (8) a model-dependent upper limit of 2.3 × 10-5 was obtained for the photospheric 3He/H abundance ratio; and (9) energetic ions may have been present for several hours prior to and following the impulsive phase of the flare.

Detection of galactic Al-26 gamma radiation by the SMM spectrometer

The gamma-ray spectrometer on the Solar Maximum Mission Satellite has detected a line near 1.81 MeV with a significance of >5 sigma in each of 3 yr when the Galactic center traversed the broad aperture of the instrument in 1980, 1981, and 1982. There was no significant variation in intensity from year to year. The Galactic center/anticenter intensity ratio is >2.5, and the center of the emission is consistent with the location of the Galactic center. The distribution could not be measured well enough to distinguish between candidate sources, e.g., novae, supernovae, red giants, and massive stars. For an assumed source distribution which follows the >100 MeV Galactic gamma radiation, the total flux measured in the direction of the Galactic center is (4.0 +- 0.4) x 10/sup -4/ ..gamma..(cm/sup 2/ s rad)/sup -1/. The measured energy of the line is 1.804 +- 0.004 MeV. These measurements are consistent with the detection of a narrow ..gamma..-ray line from interstellar /sup 26/Al by HEAO 3 in 1979/1980.

Oriented Scintillation Spectrometer Experiment observations of Co-57 in SN 1987A

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory has observed SN 1987A for two 2 week periods during the first 9 months of the mission. Evidence for gamma-ray line and continuum emission from Co-57 is observed with an intensity of about 10 exp -4 gamma/sq cm/s. This photon flux between 50 and 136 keV is demonstrated by Monte Carlo calculations to be independent of the radial distribution of Co-57 for models of low optical depth, viz., models having photoelectric absorption losses of 122 keV photons no greater than several percent. For such models the observed Co-57 flux indicates that the ratio Ni-57/Ni-56 produced in the explosion was about 1.5 times the solar system ratio of Fe-57/Fe-56. When compared with nearly contemporaneous bolometric estimates of the luminosity for SN 1987A, our observations imply that Co-57 radioactivity does not account for most of the current luminosity of the supernova remnant in low optical depth models. We suggest alternatives, including a large optical depth model that is able to provide the SN 1987A luminosity and is consistent with the OSSE flux. It requires a 57/56 production ratio about twice solar.

Positron Annihilation Radiation from the Inner Galaxy

The OSSE instrument on the Compton Gamma Ray Observatory (CGRO) was used to measure the one-dimensional latitudinal and longitudinal distributions of positron annihilation radiation (annihilation line and positronium continuum components) along the inner Galactic ridge. Intensity measurements near the Galactic center (differential relative to background fields offset by 9°-12°) show a symmetrical and spatially narrow bulge around the center (effective FWHMs of 49 ± 07 in Galactic latitude and 63 ± 15 in longitude) with significant disk contributions to at least 35° in longitude. The latitude width of the Galactic plane emission away from the Galactic center is only weakly constrained in the current analysis. The longitudinal distribution is well fitted by either a model comprising a narrow (~63 FWHM ) Gaussian bulge plus ~35° FWHM Gaussian and CO-like disk components, or by a center-truncated R1/4 spheroid plus exponential disk model. For the latter model, the observed narrow latitudinal distribution near the Galactic center suggests that the bulge is ellipsoidal, although the fits are consistent with a spherical bulge as well. The positronium fraction estimate from the Galactic center direction is 0.93 ± 0.04. This, combined with the existing narrow line width measurements for the 511 keV annihilation line, suggests that the bulk of the positron annihilation from the Galactic center direction occurs after positronium formation in a warm environment. The inner Galaxy total annihilation radiation flux intensities of ~10.6 × 10-3 to ~13.7 × 10-3 cm-2 s-1 implied by the fitted R1/4 ellipsoid plus exponential disk model (narrow and broad disks, respectively) are in agreement with intensities implied by the 511 keV line flux observed by the broad-field SMM instrument. The double-Gaussian plus CO disk model gives similar agreement if a broad (~12° FWHM) disk component is assumed.

The soft gamma-ray spectrum of A0535+26: Detection of an absorption feature at 110 keV by OSSE

Abstract : We present soft gamma-ray observations by the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (GRO) of the transient X-ray binary pulsar A0535+26. The observations were made 1994 February 8{17, immediately prior to the peak of a giant outburst. The phase- averaged spectrum is complex and cannot be described by a single-component model. We find that structure in the spectrum above 100 keV can best be modeled by an absorption feature near 110 keV, which we interpret as the signature of cyclotron resonant scattering. Because of OSSEs 45-keV threshold, we are unable to make a definitive statement on the presence of a 55-keV absorption line; however we can conclude that if this line does exist, it must have a smaller optical depth than the line at 110 keV. A first harmonic (= fundamental) cyclotron resonance at 110 keV corresponds to a magnetic field strength at the surface of the neutron star of ~1 x 10(exp 13) G (~5 x 10(exp 12) G if the first harmonic is at 55 keV).

SMM detection of diffuse Galactic 511 keV annihilation radiation

Observations of the 511 keV annihilation line from the vicinity of the Galactic center from October to February for 1980/1981, 1981/1982, 1982/1983, 1984/1985, and 1985/1986 are presented. The measurements were made with the gamma-ray spectrometer on the SMM. The design of the instrument and some of its properties used in the analysis are described, and the methods used for accumulating, fitting, and analyzing the data are outlined. It is shown how the Galactic 511 keV line was separated from the intense and variable background observed in orbit. The SMM observations are compared with previous measurements of annihilation radiation from the Galactic center region, and the astrophysical implications are discussed. It is argued that most of the measurements made to date suggest the presence of an extended Galactic source of annihilation radiation. 37 references.

Supernovae and Positron Annihilation Radiation

Radioactive nuclei, especially those created in SN explosion, have long been suggested to be important contributors of galactic positrons. In this paper we describe the findings of three independent OSSE/SMM/TGRS studies of positron annihilation radiation, demonstrating that the three studies are largely in agreement as to the distribution of galactic annihilation radiation. We then assess the predicted yields and distributions of SN-synthesized radionuclei, determining that they are marginally compatible with the findings of the annihilation radiation studies.