Elihu Aaron Boldt passed away 12 September 2008 from a heart attack, at age 77. He had retired from NASA/Goddard Space Flight Center (GSFC) in 2004, with a variety of health problems, including dialysis, but was continuing to investigate the possible connection between ultra high-energy cosmic rays and quasars, in collaboration with visiting scientists. He continued to enjoy the activities of the X-ray astrophysics group that he had founded, enthusiastically encouraging talented young scientists to pursue new experimental and theoretical approaches. He is survived by his wife Yvette and their three children.
Elihu was born 15 July 1931 in New Brunswick, N.J., to a civil engineer and a schoolteacher. His father, who was involved in the construction and maintenance of New York City's subway and bridges, died of renal failure when Elihu was a child. Elihu had the experience of a variety of jobs growing up, including maintaining the Rutgers University greenhouses during the summers. While botany interested him as a youth, he commented later that physics and astronomy had quite supplanted it.
On a full scholarship to MIT, he obtained a B.S. in 1953. He was awarded his Ph.D. in physics in 1958 with his thesis on the Lambda-Hyperon. He was a Research Associate with his thesis advisor David Caldwell during his graduate years, conducting bubble chamber experiments at the Brookhaven Cosmotron. He authored a spate of papers on the decay modes of the Lambda and on interactions of K mesons. In 1958 alone, he was first author of four papers in Physical Review and Physical Review Letters. A few years after his Ph.D., Elihu was using cosmic rays as his source of accelerated particles. In a 2009 retrospective at GSFC, researcher Thomas Cline noted, “Proximity to Bruno Rossi [at MIT] had undoubtedly prompted Elihu to think deeply about cosmic radiation and its relationships to, and influence on, other natural phenomena. This dedication would last him a lifetime.”
Elihu was Assistant Professor at Rutgers University from 1958 to 1964, during which he spent two years as a guest staff member at Ecole Polytechnique in France. Also during that time, at a conference in Israel, he met Yvette Benharroch, whom he married in 1971.
In 1964 Elihu was invited by Frank McDonald to start an X-ray astronomy group in the Laboratory for High Energy Physics (LHEA) at the Goddard Space Flight Center (GSFC), and turned from cosmic rays to X-rays. Elihu’s studies of cosmic rays led to interest in the possibilities of their producing some of the X-rays seen in the first rocket flights that found extra-solar X-rays, and he remained interested in the X-ray background throughout his career. By 1965, the group, Elihu, Peter Serlemitsos, and Stephen Holt, who were to continue working together for many years, along with others at the LHEA, began flying high altitude balloons and then rockets from New Mexico and Australia to study the X-ray sources in the Galactic Center, in Cygnus, and the Crab.
They led the development and testing of instruments that later flew on spacecraft missions. The multi-wire proportional counters flew on many balloon and rocket flights and then on Orbiting Solar Observatory (OSO-8, 1975), High Energy Astronomy Observatory ( HEAO-1, 1977), and later Rossi X-ray Timing Explorer (RXTE, 1995). Elihu was the principal investigator on the A2 experiment on HEAO-1. He was head of the X-Ray group until 1995 and encouraged the group’s developments of light-weight mirrors and higher resolution non-dispersive spectroscopy. He was also an Adjunct Professor at the University of Maryland College Park.
Elihu served on many committees during his career: the NASA Scientific Working Group for Advanced X-ray Astrophysics Facility (AXAF, which became the Chandra Observatory), executive committees for the APS Division of Cosmic Physics and the AAS High Energy Astrophysics Division, and on NSF and NASA review panels. He was honored at GSFC with its highest scientific awards — the Outstanding Scientific Achievement and the Lindsay Memorial Award — and was one of the first elected Senior Fellows at GSFC. Over 20 post-doctoral research associates and a comparable number of graduate students benefited from his guidance, many becoming scientific leaders themselves, both inside and outside of Goddard.
The early rocket flights found that the sky was bright in X-rays, with an approximately isotropic emission plus a galactic ridge. While it was clear that point sources would contribute to both, the possibility of diffuse emission was being considered theoretically. Elihu’s HEAO-1 experiment was designed especially to measure accurately the spectrum of the average emission on degree scales. While the spectrum (from analysis led by Frank Marshall) accurately fit that of bremsstrahlung from hot gas, the later X-ray telescope missions, starting with HEAO-2, measured the fluxes and spectra of active galactic nuclei of increasingly large redshift and found significant evolution in their luminosities and spectra. It now appears that the background can be entirely accounted for by the evolving sources. But the HEAO-1 measurement of the average X-ray background has been the criterion for constraints on what is needed to make the sum.
Elihu was always interested in the underlying physics producing the X-ray sources. He pointed out that evidence for cyclotron resonance could be seen in the Her X-1 pulsar spectrum and that the stellar black hole source Cyg X-1 was unique among the early discovered sources in its high amplitude of millisecond time-scale variability, both from a 1973 rocket flight. While measuring the spectrum of the X-ray background, the HEAO-1 experiment also determined the spectra of active galactic nuclei and clusters of galaxies that should be part of it. His interests were not confined to X-rays, but included the Balmer recombination lines from the interstellar medium, charge exchange from the solar wind, and the possible origin of ultra high-energy cosmic rays in supermassive black hole quasar remnants. He encouraged and helped younger researchers to pursue these questions. His abiding interest in the physical universe and his ability to make simple derivations were greatly appreciated.