Cornelis A. "Neil" Gehrels (October 3, 1952 – February 6, 2017) was an American astrophysicist specializing in the field of gamma-ray astronomy. He was Chief of the Astroparticle Physics Laboratory at NASA's Goddard Space Flight Center (GSFC) from 1995 until his death, and was best known for his work developing the field from early balloon instruments to space observatories such as the NASA Swift mission, for which he was the principal investigator. Gehrels lead the WFIRST (later named the Nancy Grace Roman Space Telescope) wide-field infrared telescope towards a launch in August 2026. He was a member of the National Academy of Sciences and the American Academy of Arts and Sciences. Gehrels died on February 6, 2017, at the age of 64. On January 10, 2018, NASA announced that Swift had been renamed the Neil Gehrels Swift Observatory, in his honor.
Early life and education Gehrels was born in Lake Geneva, Wisconsin, on October 3, 1952. His father was astronomer Tom Gehrels. He grew up near several telescopes including living at McDonald Observatory before his family settled in Tucson, Arizona, when he was 14, where he attended high school and then the University of Arizona as an undergraduate student. He received bachelor's degrees in music and physics from UofA in 1976. He received his Ph.D. in physics in 1982 from the California Institute of Technology, with Edward C. Stone and Rochus Eugen Vogt as advisers. He took a postdoctoral position at NASA's Goddard Space Flight Center in 1981 working with Bonnard J. Teegarden. In 1982, he became permanent at GSFC as an astrophysicist. In 1980 while in graduate school, he married fellow graduate student Ellen Williams, who is a professor of physics at the University of Maryland and Director of ARPA-E at the Department of Energy. They have two children, Thomas (born 1987) and Emily (born 1990). Gehrels died of pancreatic cancer at the age of 64 on February 6, 2017.
Career
Gehrels was chief of the Astroparticle Physics Laboratory at NASA's Goddard Space Flight Center starting in 1995. He was the principal investigator for the Swift Gamma-Ray Burst Mission. Other responsibilities include: project scientist for the Compton Gamma Ray Observatory (1991–2000), Mission scientist for Mission INTEGRAL, deputy project scientist for the Fermi Gamma-ray Space Telescope and project scientist for the Nancy Grace Roman Space Telescope, then known as the Wide Field Infrared Survey Telescope (WFIRST). He was also a professor at the University of Maryland College of Computer, Mathematical, and Natural Sciences at University of Maryland and adjunct professor of astronomy and astrophysics at Pennsylvania State University. His research focused on transient objects in the universe such as gamma-ray bursts (GRBs), supernova and active galaxy flares. He worked to develop gamma-ray astrophysics from a field of experiments detecting a few objects to a full astronomical discipline with thousands of sources in many classes. He was elected chair of the Astronomy Section of the National Academy of Sciences in 2013. He published over 600 academic articles, which have been cited 40,000 times for an h-index of 97. His most cited works include papers on the discovery of the origin of short gamma-ray bursts, the Swift satellite, Poisson statistics, observations of gamma-ray bursts at the edge of the visible universe, discovery of a relativistic tidal disruption event, and finding of two classes of active galactic nuclei (AGN) in the gamma-ray band.
Research
Overview Gehrels was an astrophysicist with broad interests. He worked predominantly in the area of high-energy astrophysics, studying explosive objects in the universe such as supernovae, gamma-ray bursts, gravitational wave transients and tidal disruption events (stars torn apart when they approach too close to a massive black hole). He was also a laboratory physicist developing instrumentation for space observation.
Graduate studies Gehrels's graduate schooling was at Caltech, working with advisors Rochus Vogt (1976–1979) and Edward C. Stone (1979–1981). The early period was spent performing laboratory and accelerator calibrations of the cosmic ray instrument on Voyager. The later years saw analysis of data from the instrument during the fly-bys of Jupiter (1979 for both Voyager 1 and 2). The elemental abundance of MeV particles in the Jovian magnetosphere was expected to be dominated by hydrogen and helium, so it was a surprise to find oxygen and sulfur dominant. This discovery was found to be related to the Voyager discovery of volcanoes on the moon Io spewing oxygen and sulfur into the magnetosphere. The Gehrels and Stone paper describing the measurement made a prediction, later confirmed, that the auroral emissions on Jupiter are cause by precipitating oxygen and sulfur ions.
Balloon Observations of SN 1987A As a postdoc and then permanent scientist at Goddard, Gehrels worked on the GRIS balloon payload for high resolution (germanium detector) spectroscopy of gamma-ray sources. He first was involved in the proposal to NASA for the instrument, specializing in the design and techniques for background reduction. The payload was nearing completion when Supernova 1987A was discovered. The construction was quickly finished and the instrument flown to observe the gamma-ray line emission from Co-56 decay. The GRIS detection of a broadened and redshift 847 keV line was some of the first strong evidence for mixing and asymmetries in the ejecta, now recognized as signature characteristics of supernova explosions.
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