Present Position: Visiting Senior Research Scientist, Department of Astronomy, University of Maryland, College Park, MD.

Education: B.S., Physics, 1961, Queens College, City University of New York, Flushing, N.Y. Ph.D., Physics, 1966, University of Rochester, Rochester, N.Y.

Experience:

NRC/NRL Post-Doctoral Res. Assoc. in Lab. for Cosmic-Ray Physics, 1966 - 1968.

Research Physicist in NRL/Laboratory for Cosmic-Ray Physics, 1968 - 1973.

Astrophysicist in Space Science Division, NRL, 1973 - 2005.

Senior Staff Scientist, Astrophysics Div., NASA Headquarters, May 1989 - May 1991.

Fellowships and awards:

Phi Beta Kappa (Queens College)

NRL-Sigma Xi Pure Science Award 1992

Bruno Rossi Prize HEAD/American Astronomical Society 1992

Fellow American Physical Society (1997)

Fellow American Astronomical Society (2020)

Society Memberships:

American Physical Society (Astrophysics Division); American Astronomical Society (HEAD, SPD); American Geophysical Union; Sigma Xi,AAAS

 

 

Gerald Share started his research career at the University of Rochester under the guidance of his thesis adviser, Prof. Everett Hafner. His thesis covered the development, calibration, flight and analysis of a balloon-borne experiment to make pioneering studies of gamma radiation above 10 MeV in the upper atmosphere. Only upper limits were obtained on radiation from celestial sources.

After receiving his PhD in 1967, he accepted a NAS/NRC post-doctoral appointment in the Laboratory for Cosmic Ray Physics under the direction of Dr. Maurice Shapiro. He worked with Mr. Nathan Seeman, Dr. Robert Kinzer, and Mr. Carl Noggle to develop a balloon-borne experiment to detect gamma-radiation above 10 MeV with ~2 angular resolution and 20% energy resolution. The instrument incorporated a stack of nuclear emulsions and used gas proportional, plastic scintillation and Cerenkov detectors, and a wide gap spark chamber. This array enabled electron positron pairs to be tracked back into the emulsion where the precision measurements were made. The experiment was first flown in 1969 and observed the Crab Nebula. It provided the first evidence for the detection of the pulsar at gamma-ray energies. The experiment also was flown from the Southern Hemisphere in 1971 over Argentina and in 1975 over Brazil in order to view the Galactic Center region and to reduce the background from the atmosphere. These observations provided measurements of atmospheric background and limits on the celestial diffuse radiation, and revealed a narrow band of gamma-ray emission along the Galactic Plane.

In 1974 Dr. Share and Dr. Kinzer transferred into the Space Science Division and began working in the Hard X-ray and Gamma-ray Branch headed by Dr. James Kurfess. He worked on early balloon data obtained by Dr. Kurfess and established the phase of hard X-rays with other pulsed radiation from the Crab. Dr. Share then began studies of the newly discovered cosmic gamma-ray bursts using data from a solar hard X-ray spectrometer on OSO-6. These measurements and those made in a joint study with Dr. Sharad Kane using data from OGO-5 demonstrated that these gamma-ray bursts were also detected at energies down to 10 keV.

He also worked with Dr. Kurfess and others in the Branch on proposals for a gamma-ray detectors on NASA's Solar Maximum Mission (SMM) and Compton Gamma Ray Observatory. As part of this work he established the ability to separate neutron and gamma-ray emission detected by NaI. With the launch of NASA's first High Energy Astronomy Observatory in 1977, Dr. Share became involved in observations of soft X-rays with the NRL's large area X-ray instrument. This work led to the identification of GX339-4 as a black hole candidate and to the realization that many X-ray sources were associated with B-type stars with emission lines.

As a result of the Branch's efforts to build a gamma-ray spectrometer on SMM, Dr. Share became responsible for collaborative studies with the University of New Hampshire using their experiment. This excellent experiment, developed by Prof. Edward Chupp and Dr. David Forrest, operated from the 1980 to 1989 and made several discoveries in celestial and solar physics. It was made up of seven 3 in. diam. x 3 in. NaI detectors, well stabilized and calibrated. Dr. Share focused his early efforts on the study of celestial radiation and cosmic gamma-ray bursts. Working with Drs. Steve Matz and Pat Nolan, Dr. Share showed that gamma-ray burst spectra typically extended up to energies above 1 MeV and did not exhibit any lines. One of the bursts was observed up to 100 MeV. The lack of emission lines conflicted with measurements made by the Soviet Konus experiments that suggested the presence of broad lines that could be associated with positron annihilation. A later study by Messina and Share set emission line limits in bursts covering the full 9-year mission and the full energy range of the instrument. In the years since those measurements no emission lines have been detected by recently flown instruments.

Drs. Kinzer and Share developed techniques for using the solar pointed SMM spectrometer for detecting lines and continua from celestial sources. This used various methods for eliminating backgrounds and depended on the slow scan of the detector along the ecliptic plane. As a result of these efforts SMM confirmed the discovery by HEAO-3 of a diffuse glow in the ²⁶Al line along the galactic plane. This emission can come from supernovae, novae, Wolf-Rayet and other giant stars. Dr. Share next turned his attention to the study of the galactic positron-electron annihilation line. At that time it was believed to be both variable and likely to be associated with a discrete source, possibly a black hole near the galactic center. The SMM observations showed that annihilation source was relatively constant over an eight-year time scale and was likely due to diffuse galactic emission from several sources including supernovae, novae, and other positron emitters. Dr. Barry Geldzahler worked with Dr. Share on SMM observations of SS433. This source was reported by HEAO-3 to emit red and blue shifted emission lines. SMM set upper limits on these lines that were inconsistent with the HEAO-3 observations. The HEAO-3 measurements were subsequently retracted.

In 1987 the brightest and nearest supernova to occur in 500 years occurred in the Large Magellanic Cloud. This gave scientists the opportunity to detect the gamma-ray lines from the Ni-Co-Fe decay chain that would definitively show that nucleo-synthesis of these elements occurred in supernovae. Because it was a type II supernova, the lines were not expected to be detectable for about 2 years. However, hard X-rays were detected from the supernova by the MIR detectors only 9 months after the explosion. Dr. Steve Matz worked with Dr. Share and reported the first detection of gamma-ray lines about a month later. Dr. Mark Leising continued this work with Dr. Share and showed the full nuclear line spectrum and how it evolved in time. This early detection of line and continuum indicated that radioactive material in the nebula was not uniformly layered. Drs. Matz and Share also set constraining limits on gamma-ray lines from a nearby type I supernova in Centaurus A, while Leising and Share set limits on nucleosynthetic lines from ²²Na, ⁴⁴Ti, and ⁶⁰Fe from novae.

Dr. Michael Harris then worked with Dr. Share to make additional celestial observations. He constructed the first gamma-ray spectrum of radiation from the galactic plane and set limits on variable emission lines from a variety of celestial sources. None of these lines have subsequently been detected. Harris and Share then set upper limits on the ⁷Be and neutron capture lines from compact celestial sources. They also set upper limits on galactic de-excitation lines from ¹²C and ¹⁶O. This study also did not confirm the reported broad emission lines from Orion based on measurements from the Compton Gamma-Ray Observatory Comptel experiment. A subsequent search from these lines was performed by Murphy and Share using OSSE data and set even more constraining limits. The Comptel investigators subsequently determined that the Orion observation was spurious. Harris and Share completed the SMM celestial studies by showing that there is no new class of cosmic gamma-ray bursts that are primarily visible at energies above 1 MeV (limited to bursts with duration >2 s).

Early in the SMM mission, Dr. Erich Rieger discovered transient annihilation line events and other high-energy transients. Working with Dr. Keith Marlowe, Dr. Share identified the source of these events as emissions from orbiting Soviet nuclear reactors. The annihilation line was detected by SMM when it passed through clouds of positrons stored in the earth's magnetic field. These events could be detected even while the SMM and reactor borne satellites were separated by several thousand km. SMM also recorded the gamma-ray spectrum emitted by the reactors when they passed within about 800 km and identified several emission features.

In the early 1990's, Dr. Share focused his research on the study of nuclear line radiation from solar flares. That started a long term collaboration with Dr. Ron Murphy.  This study utilized data from both SMM and the Compton Observatory OSSE experiments. Along with Tom Vestrand, they published a catalog of high-energy flares observed by the SMM spectrometer. Using 19 of these flares with strong nuclear line emission, Share and Murphy made some key spectral measurements that produced some fundamental discoveries about particle acceleration, transport and energetics in flares and the ambient abundance of the corona and chromosphere. The framework for these discoveries was developed in the theories and models of Drs. Reuven Ramaty, Benz Kozlovsky, Natalie Mandzhavidze and Ron Murphy.

These discoveries include: 1) enhancement in the concentration of low FIP elements where accelerated particles interact, 2) a new line ratio for deriving the spectra of accelerated particles < 10 MeV, 3) energies in accelerated ions that exceed those in electrons for some flares, 4) a highly variable ion to electron ratio during flares, 5) concentration of ³He in flare-accelerated particles enhanced by a factor of >1000 over its photospheric value, 6) an accelerated alpha/p ratio > 0.1 in several flares and evidence for high ambient ⁴He in some flares, 7) measurements of the positronium fraction and a temperature-broadened 511 keV line width, 8) new information on the directionality of electrons, protons, and heavy ions and/or on the homogeneity of the interaction region, and 9) the spectrum of broadened gamma-ray lines emitted by accelerated heavy ions that indicates Fe enhancements consistent with those observed in solar energetic particles.

 

Dr. Share continued his solar flare research using data from NASA’s RHESSI satellite launched in 2002. One of the most exciting findings of this recent research was the observation of a rapid change in the width of the positron-electron annihilation line during the 2003 October 28 solar flare.  In the first 10 min of the RHESSI observation the line had a width consistent with production in a medium at temperatures >10⁵ K. In two minutes time the width changed suggesting evolution to a standard active region atmosphere.

 

Dr. Share retired from the Naval Research Laboratory in July 2005 and has continued his solar research as a Visiting Senior Research Scientist in the Astronomy Department of the University of Maryland.

 

With the launch of the Fermi gamma-ray satellite, Dr. Share organized a solar science team to optimize the analysis of the Large Area Telescope (LAT) and Gamma Burst Monitor (GBM) for studying solar quiescent and flare emission.  He focused his research activities on the study of what is called late phase gamma-ray emission (LPGRE).  This emission is primarily observed at energies above 100 MeV and has a spectrum consistent with gamma-rays from neutral pion decays.  These pions are produced by interaction of >300 MeV gamma rays in the solar atmosphere.  Because the time profile of this emission is distinct, and typically delayed, from the flare hard X-ray and nuclear line gamma-ray emission, it is likely that is originates in a different acceleration process than the flare.  The evidence suggests that the emission comes from protons accelerated by the shock preceding the coronal mass ejection (CME) which find magnetic field lines returning to the Sun and impact the Sun.  This work resulted in the publication of a detailed catalog and analysis of 30 solar LPGRE events in 2018.

 

Since 2019 Dr. Share has returned to his earlier solar flare nuclear gamma-ray studies.  Working with Dr. Ron Murphy at NRL, he has made detailed spectroscopic and timing analyses of 19 flares observed by SMM from 1980 to 1989 and from a flare observed in 2002 by RHESSI.  These spectroscopic studies enable the determination of the ambient elemental abundance in the solar atmosphere where the accelerated ions interact, as well as the composition of these ions.