Astrophysicist Penetrated Many Of the Mysteries of the Cosmos

Astrophysicist Elihu Boldt, 77, who died Sept. 12 after an apparent heart attack, made contributions to understanding the sources and composition of high-energy particles that dart around the universe at the speed of light.

In 1965, he started the program in X-ray astronomy at NASA's Goddard Space Flight Center in Greenbelt, and devoted much of his life to exploring highly intricate mysteries in space.

As a doctoral student at the Massachusetts Institute of Technology in the 1950s, he worked under the guidance of Bruno B. Rossi, whose investigations into the nature and origin of cosmic rays were laying the foundations of high-energy particle physics.

"The cosmic ray world was overflowing with people. It was the ultimate candy store after World War II," said astro-particle physicist Frank B. McDonald, who hired Boldt at Goddard in 1964. "Most every research university had a cosmic ray group. People had not worried about X-ray astronomy. People did not realize what a giant, violent universe was out there making all this stuff."

Influenced by Rossi, Boldt veered toward the study of cosmic X-rays. This was a counterintuitive move, McDonald said, because "theorists had predicted in the 1950s that X-rays would not be prominent in astrophysics. They were thought to originate mainly from stars, and stars were not hot enough to give off great fluxes you could measure over great distances."

But Boldt had predicted -- correctly, to some degree -- that cosmic rays would interact with interstellar gas and produce X-rays. By 1965, he began sending high-altitude balloons aloft from Australia and New Mexico. Fitted with instruments, the balloons traveled 120,000 feet into the heavens and saw X-rays emanating from the galactic center. He later used satellites and rockets to make his studies.

Boldt was especially interested in the "X-ray background," one of the great mysteries of astronomy. While the night sky is dark except for the stars, the sky is in fact bright with the glow of X-ray emissions that are invisible to the human eye. The glow is actually coming from point sources that are very numerous and distant.

In 1977, Boldt was a principal investigator for an experiment on HEAO-1, the first High Energy Astrophysics Observatory, which NASA launched to collect information on X-ray and gamma-ray cosmic radiation.

Jean Swank, a Goddard astrophysicist, said Boldt was able to make accurate measurements of the flux of the X-ray background using a clever design. The X-ray detectors on board had two different fields of view, which allowed one to distinguish between cosmic X-rays and the noise signal produced by local energetic particles in the Earth's orbit.

HEAO-2, sent into orbit a year after HEAO-1, on which Boldt was a co-investigator, brought back images of the X-ray sky in which black holes in distant galaxies could be seen contributing to the X-ray background, Swank said.

The currently Chandra observatory -- the X-ray counterpart to the Hubble Space Telescope -- has probed the X-ray background even more deeply, and almost all of it has been shown to come from distant galaxies, most of it from black holes.

Elihu Aaron Boldt was born July 15, 1931, in New Brunswick, N.J., to a civil engineer and a schoolteacher. His father was involved in the construction and maintenance of New York City's subway and bridges. He died of renal failure when the younger Boldt was 5.