Allan S. Jacobson

HEAO 3 discovery of Al-26 in the interstellar medium

A new analysis technique was used with HEAO 3 satellite gamma ray data to quantify the galactic 1809 keV emission. A method of bins was applied in summing for specific scan directions, followed by fitting the source and background components. The 1809 keV line of Al-26 was significant because of the recent discovery that Al-26 was overabundant in meteoritic material and implanted during solar system formation. The HEAO 3 flux indicated a current decay of 3 solar masses of Al-26 per year in the interstellar medium, possibly occurring in red giants and massive main sequence stars. The ratio of the observed Al-26 to the Al-27 flux was similar to that measured in meteorites, which suggests that the ratio is normal for the galactic medium and that no supernova triggered the protosolar nebular collapse.

Diffuse galactic gamma-ray line emission from nucleosynthetic Fe-60, Al-26, and Na-22 - Preliminary limits from HEAO 3

Data obtained during a 2 week period in the fall of 1979 with the HEAO 3 gamma-ray spectroscopy experiment have been searched for diffuse galactic plane gamma-ray lie emission expected to result from the decay of nucleosynthetic /sup 60/Fe, /sup 26/Al, and /sup 22/Na. With the possible exception of the 1809 keV line from /sup 26/Al decay for which a 2.6 sigma cosmic excess of (6.0 +- 2.3) x 10/sup -4/ photons cm/sup -2/ s/sup -1/ rad/sup -1/ was measured, no positive detection was made. However, new limits ranging from 1.8 to 11 times 10/sup -4/ photons cm/sup -2/ s/sup -1/ rad/sup -1/, at the 3 sigma level of confidence, have been placed on diffuse emission in these lines from the vicinity of the galactic center (-30/sup 0/

Variable positron annihilation radiation from the galactic center region

The HEAO 3 Cosmic Gamma-Ray Spectrometer performed the first high spectral resolution survey of the entire sky at gamma-ray energies from 50 keV to 10 MeV. Studies of 511 keV positron annihilation radiation from the vicinity of the galactic center are reported here, based on data which were recorded during 1979 September/October and 1980 March/ April. The 1979 fall data show unshifted, narrow 511 keV line emission of intensity (1.85 ± 0.21) X 10^(-3) photons cm^(-2) s^(-1), consistent with earlier measurements. The 1980 spring measurement showed a statistically significant reduction in 511 keV emission from this region, thus requiring that a significant fraction of the flux originate in one or more compact sources of size ≾ 10^(18) cm. While distribution of sources within ~ 22° (at 90% confidence level) of the direction of the galactic center are allowed by the observations, the data rule out most extended models for positron production, such as by cosmic ray interaction in the interstellar medium or by distributions of many supernovae, novae, or pulsars. The data are well satisfied by assuming that the emission originates in a single compact source at the galactic center.

Long-term gamma-ray spectral variability of Cygnus X-1

Data from HEAO 3 observations of 0.05-10-MeV gamma-ray emission from Cyg X-1 during two 90-d periods (in fall 1979 and spring 1980) are compiled in tables and graphs and analyzed statistically to determine the temporal and spectral variability. It is found that a steady increase in 100-keV emission is accompanied by a decrease (and eventual disappearance) of MeV emission. The mechanisms which could theoretically be responsible for these phenomena are discussed. 28 references.

The HEAO 3 gamma-ray spectrometer

Abstract The Third High Energy Astronomy Observatory (HEAO 3), successfully launched into low earth orbit on 20 September 1979, carries a large high resolution gamma-ray spectrometer designed for cosmic nuclear spectroscopy. This Gamma-Ray Spectrometer (the HEAO C-1 experiment) consists of a cluster of four coaxial high purity germanium detectors, each with a volume of approximately 100 cm 3 . Surrounding the germanium detectors is a 6.6 cm thick CsI shield operating in active anticoincidence with the central detectors and defining a field of view of about 30° fwhm. An initial energy resolution of 3 keV fwhm at 1.46 MeV was achieved for each detector. All valid events in the germanium detectors are individually analyzed by an 8192 channel pulse area analyzer and transmitted at a maximum rate of 15.6 events/s for each detector. During a 6 month mission, the experiment will perform a complete sky survey for narrow cosmic gamma-ray line emission to the sensitivity level of about 10 −4 photons/cm 2 s over an operating energy range of 0.05–10 MeV.

A High Resolution Measurement of the 2.223 MeV Neutron Capture Line in a Solar Flare

An intense solar flare lasting 40 s was observed by the HEAO 3 γ-ray spectrometer on 1979 November 9 at 3:05 UT. The flare was observed in four high-resolution germanium detectors as well as in five CsI shield detectors over an energy range of 100 keV to above 5 MeV. Of particular interest is a line feature at 2.2248 ± 0.0010 MeV. The precise energy measurement provides unambiguous evidence that this is the ^1H(n,γ)^2H line resulting from neutron capture on hydrogen. An upper limit of 5 keV is found for the natural line width. The time dependence of the neutron capture line is discussed as well as the overall characteristics of the November 9 flare.

A possible line feature at 73 keV from the Crab Nebula

Evidence is reported for a possible line feature at 73 keV from the Crab Nebula. The experiment was conducted with a balloon-borne high-resolution gamma-ray spectrometer on June 10, 1974, over Palestine, Texas. The intensity and the width of the line derived from the fitting of these data are approximately 0.0038 photon per (sq cm-sec) and less than 4.9 keV FWHM, respectively. The line is superposed on a power-law continuum of 11.2 E to the -2.16 photons per (sq cm-keV) in the energy range from 53 to 300 keV, which is consistent with other measurements of the Crab Nebula spectrum.

HEAO 3 observations of the Crab pulsar

The Crab pulsar (PSR 0531+21) is the only pulsar which has been observed throughout almost the entire electromagnetic spectrum from 0.5 keV to over 2000 GeV. In general, the emission from the Crab nebula and its pulsar has been remarkably constant in time (except for the pulsations). However, several recent observations, especially of gamma-ray line emission, indicate possible time variations. The present investigation is concerned with the data obtained with the HEAO C-1 experiment. The data have been studied with the hope of finding information regarding a number of questions, taking into account the existence of line emission from the pulsar, the variation of the shape of the light curve with time, and the spectral variations as a function of pulsar phase. The spectrum of the total phase-averaged pulsed emission is found to be consistent with a single power-law spectrum from 50 keV to 10 MeV.

Gamma-ray observations of Co-56 in SN 1987A

During observations of SN 1987A 286 days after the supernova explosion using a JPL high-resolution gamma-ray spectrometer, a line feature was found in the net supernova spectrum at an energy of 1240.8 + or - 1.7 keV, with an intrinsic width of 8.2 + or - 3.4 keV FWHM, and a net flux of 0.0021 + or - 0.0007 photons/sq cm per s. This feature is interpreted as the 1238 keV line from the decay of Co-56 in the supernova remnant. The blueshift and intrinsic broadening of the 1238 keV line are consistent with models incorporating mixing of the radioactive cobalt in the expanding eject. 26 references.

The gamma-ray spectrum of the galactic center region

The Galactic center region was observed with the HEAO 3 High Resolution Gamma-Ray Spectrometer during the fall of 1979 and the spring of 1980. Between these epochs there was observed (1) a statistically significant decrease in the high-energy (511 keV to about 3 MeV) luminosity, (2) a decrease in the positron annihilation line intensity, reported previousy, and (3) a low positronium annihilation fraction f = 0.38 + or 0.19 during the fall of 1979. If positrons are generated by photon-photon collisions of high-energy photons, then the absence of a detected flux above 511 keV in the spring of 1980 may indicate a time delay between positron production and annihilation. 26 references.