Riccardo Giacconi (1931–2018)

Harvey Tananbaum, Ethan J. Schreier, and Wallace Tucker

PNAS June 25, 2019 116 (26) 12587-12589

Figure: Riccardo Giacconi at the rostrum of the Chandra Symposium held in Huntsville, Alabama on September 16, 2003. Image courtesy of NASA/Marshall Space Flight Center.

Riccardo Giacconi, the “Father of X-ray Astronomy,” Nobel laureate, and one of the most influential figures in astrophysics over the past 60 years, died on December 9, 2018, at the age of 87. With a career spanning the electromagnetic spectrum, Riccardo opened up new windows for observing the universe and revolutionized “big astronomy.” Many in the astronomy community continue to base their research on data from observatories that he conceived, built, and/or directed.

Riccardo’s outstanding scientific capabilities were well matched by his extraordinary leadership and management skills. He had a deep belief in a scientific approach to problem solving and to establishing systematic processes. He insisted that instruments and observatories be built to answer driving scientific questions. Another key to his success was the legendary dedication and drive of the research teams that he assembled, which could be traced directly to Riccardo’s deep commitment to establishing an environment of intellectual honesty and trust.

Born in Genoa, Italy, on October 6, 1931, Riccardo received his doctorate in 1954 from the University of Milan. At that time, the only practical way to study high-energy nuclear reactions was through the detection and analysis of the interaction of high-energy cosmic rays, primarily protons, with atomic nuclei in the atmosphere. For his thesis research, he spent about two years at the Testa Grigio Observatory (elevation: 3,500 meters) in the Italian Alps and obtained 80 high-energy cosmic ray detection events. Although he learned much about the conception, design, and building of detectors, Riccardo was frustrated by the lack of “action.”

A Fulbright fellowship brought Riccardo to the United States, where he worked at Indiana and Princeton before joining American Science and Engineering (AS&E) in Cambridge, Massachusetts, in September 1959. Shortly thereafter, AS&E Board Chair and Massachusetts Institute of Technology Professor Bruno Rossi suggested at a party that Riccardo look into the possibility of developing a program to search for sources of cosmic X-rays. Riccardo’s thesis research experience and the failure of other groups to detect X-rays from beyond the Solar System led him to consider ways to concentrate weak X-ray signals (i.e., how to build an X-ray telescope). Drawing from previous considerations of potential X-ray microscopes by Hans Wolter (2), Riccardo quickly concluded that a parabolic mirror could focus X-rays impinging at near-grazing incidence angles, while Rossi added the notion of nesting surfaces to increase the collecting area (1). Riccardo undertook a research program using two reflecting surfaces to design and build a fully imaging X-ray telescope.

Meanwhile, in work with Frank Paolini and Herbert Gursky, he developed detectors with a much wider field of view and about 50 times more sensitivity than ones flown previously. On June 12, 1962, their rocket payload detected a strong source in the direction of the constellation Scorpius, which they named Scorpius X-1. They also discovered an all-pervasive X-ray background radiation.

Riccardo moved quickly to exploit this new window for exploring the universe, proposing with Gursky a bold five-year X-ray astronomy program that included more rocket flights, an X-ray satellite to survey the entire sky, and eventually, an X-ray telescope. Applying his strong conviction and remarkable persistence, Riccardo persuaded NASA to support the initial phases of the program.

With funding from NASA, Riccardo’s group developed and operated the first X-ray satellite, Uhuru, launched on December 12, 1970. Riccardo continued to refine what he termed a science systems engineering approach. Engineers and scientists worked side by side to establish requirements, develop a design, construct and test the hardware, and plan the operations for the satellite. Even before launch, the team (which the authors of this retrospective joined in the late 1960s) also developed software to analyze the data, which enabled us to rapidly rearrange the observing schedule and satellite configuration to follow-up and exploit discoveries.

On a personal level, although junior scientists, we three and several colleagues received assignments that challenged us to our limits while providing opportunities to develop technical, management, scientific, and communications skills. Riccardo met weekly with the Uhuru science group. In these often-stormy sessions, there was wide-open give and take. Ideas were floated with abandon and shot down remorselessly. There was respect for one and all, but no one was sacrosanct, and everyone, including Riccardo, had to defend his ideas based on logic and scientific merit.

While the all-sky catalog of more than 300 newly discovered X-ray sources achieved one of the primary mission goals, the most significant result from Uhuru was the revelation that luminous X-ray sources (such as Scorpius X-1) were powered by accretion onto compact stars in binary systems. One of those stars, Cygnus X-1, provided the first compelling evidence for the existence of black holes.

The merging of X-ray astronomy into the mainstream of astronomy was accelerated in the late 1970s by the launch of the Einstein Observatory, with Riccardo as Principal Investigator. By then, he had moved with a core group to the Harvard–Smithsonian Center for Astrophysics (CfA). The Einstein Observatory’s imaging capabilities revealed that essentially all types of astronomical objects from nearby stars to distant quasars radiate X-rays. Riccardo also initiated a guest observer program, enabling all astronomers to use Einstein Observatory. This model, new at the time, has now been widely adopted.

In 1976, cognizant of the limited lifetime projected for Einstein Observatory, Riccardo and Harvey Tananbaum proposed its successor, the Chandra X-ray Observatory. Launched in 1999 and now in its 20th year of operation, the Chandra X-ray Observatory remains without peer in the X-ray band for its subarc-second angular resolution and its sensitivity for studying objects as diverse as exoplanets, neutron stars, black holes, clusters of galaxies, dark matter, and dark energy. Although Riccardo would move on to new challenges, he remained involved with the Chandra X-ray Observatory, pursuing the soft X-ray background radiation discovered in 1962. This research shows that the X-ray background is primarily produced by the accreting supermassive black holes in galaxies distributed near and far across the sky.

In 1981, Riccardo became the first director of the Space Telescope Science Institute (STScI) and professor of astrophysics at Johns Hopkins University. The scientific community had insisted that the scientific operations of a large, unique, and expensive new facility—the first major international optical observatory in space, later to be christened Hubble—be managed by the community itself (3). Recognizing the need to transfer the scientific operations philosophy from his X-ray group to the optical community, Riccardo recruited Ethan Schreier to oversee the Hubble operations and data system. Riccardo proceeded to build an entire new institute from scratch, assembling a core staff with expertise in the operational, engineering, and scientific disciplines necessary to operate Hubble. Innovations introduced by Riccardo and his staff for Hubble included a formal data archive with a funded data analysis program, the distribution and archiving of calibrated data, an artificial intelligence-based planning and scheduling system, reserved time for large and “key” programs, and freely distributed portable data analysis software.

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Riccardo was elected to the National Academy of Sciences in 1971 before his 40th birthday. He received the 1980 Franklin Institute Elliott Cresson Medal, the 1981 American Astronomical Society Dannie Heineman Prize for Astrophysics and Henry Norris Russell Lectureship, the 1981 Astronomical Society of the Pacific Bruce Medal, the 1982 Gold Medal of the Royal Astronomical Society, the 1987 Wolf Prize in Physics, and the 2003 National Medal of Science. In 2002, he shared the Nobel Prize for Physics (with astrophysicists Raymond Davis Jr. and Masatoshi Koshiba, who were honored for research on cosmic neutrinos) “for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources.”

In summing up his career, Riccardo said: “I am grateful to live in this heroic era of astronomy and to have been able to participate and contribute to its evolution”

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