Donald Charles Backer, discoverer of millisecond pulsars, died of a heart attack on 25 July 2010 near his home in Berkeley, California.
Born in Plainfield, New Jersey, on 9 November 1943, Don graduated from Cornell University in 1966 with a BS in engineering physics. His career was shaped by the dual aspect of that training: technical expertise and intense curiosity about the physics of the universe. He was also an intercollegiate swimmer. He then went to the University of Manchester, UK, for an MSc degree. While there, he became caught up in the excitement surrounding the discovery of pulsars, the rotating neutron-star remnants of supernova explosions, at nearby Cambridge University. Returning to Cornell in 1968, he began working on pulsars under the guidance of Frank Drake using the then-new Arecibo 1000-foot telescope. His PhD was awarded in 1971 for a thesis entitled “Radio Intensity Fluctuations in Pulsars,” in which he described the phenomenon of missing pulses and drifting subpulses in pulsar emission.
Don was a postdoctoral fellow at the National Radio Astronomy Observatory (1971–73) and NASA’s Goddard Space Flight Center (1973–75), where he developed a lifelong interest in very long baseline interferometry (VLBI). In 1975 he moved to the University of California, Berkeley; he remained there for the rest of his career, first as a research astronomer with the Radio Astronomy Laboratory (RAL) and after 1989 as a professor of astronomy. He served as chair of the astronomy department for six years and director of RAL from 2008 until his death. At that time, he was overseeing the development of three radio facilities spanning a frequency range from 0.1 to 300 GHz and involving widely different technologies.
In the early 1980s, Don became fascinated with understanding the nature of the peculiar radio source 4C21.53, originally dubbed a “scintar.” Its scintillation, or “twinkling,” caused by the intervening ionized interstellar medium, suggested it was a very compact object, and its steep spectrum at decameter length scales suggested it was a pulsar, but no pulses in the normal range of repetition rate were found. With exemplary determination over several years, he and his colleagues used the Arecibo telescope to isolate the source and finally uncover a pulsar having the unprecedentedly high repetition rate of 642 Hz (a 1.558-ms period)—the first millisecond pulsar, known thereafter as PSR 1937+21. Don’s colleagues vividly recall his jubilation at solving the mystery of the fast pulsar.
Millisecond pulsars are thought to be old neutron stars that have been “recycled,” or spun up, late in life by accretion from a companion object. About 100 millisecond pulsars are now known. The steadiness of their pulses makes them extremely accurate clocks, with fractional stability of 1 part in 1015. Don was among the earliest proponents of setting up pulsar timing arrays, which, with ever-improving timing accuracy, should be able to detect gravitational waves from coalescing objects such as black holes or the stochastic primordial background.
Another of Don’s achievements was to push the development of VLBI to ever-higher frequencies. He brought the 25-meter telescope at Berkeley’s Hat Creek Radio Observatory into the first organized VLBI network of six US telescopes in 1976. He served the organization as executive officer and wrote the first user manual, which helped make VLBI accessible to nonexperts. The network’s success led to the construction of the Very Long Baseline Array. Don worked with various VLBI arrays for more than a decade to characterize and measure the angular dimensions of the radio source known as Sagittarius A*, formed in the accretion envelope of the black hole in the center of our galaxy.
In the mid 2000s, Don turned his attention to one of the grand challenges of modern cosmology, the detection of fluctuations in neutral-hydrogen distribution during the reionization epoch by imaging the redshifted radiation of the 21-cm line (z = 6–10) in three dimensions. Aided mainly by graduate students, he built the Precision Array to Probe the Epoch of Reionization. Independent arrays set up in West Virginia and South Africa have begun pushing down the limits of the spatial fluctuations in the radiation distribution.
Don constantly strove to break down barriers and bring people together. He frequently encouraged his fellow radio astronomers to eschew the technical parochialism of their field and think broadly in studying the universe. Ironically, he was one of the most technically accomplished radio astronomers in the field. For his pioneering work, Don was awarded the Jansky Lectureship of the National Radio Astronomy Observatory in 2003.
Don had a heightened sense of obligation to the community and those around him. He worked hard to help protect parts of the radio band from manmade interference. He served several terms on the National Research Council’s Committee on Radio Frequencies and Committee on the Scientific Use of the Spectrum.
An inspiring teacher and mentor, Don was generous with his time, patient, and absorbed by the challenges of solving problems and understanding nature. His twinkling blue eyes, disheveled hair, laid-back attitude, and quick laugh set him apart. He understood deeply the human condition, and his comments showed a wry wit, sometimes with an acerbic edge when discussing administrative problems. A careful listener, he spoke with a slow, thoughtful cadence that listeners sometimes mistook for inattention.
Don was devoted to his family, and their outings were always adventures. Don thought of any mishap—a flat tire or lighting a fire at a wet campsite—as a challenge to his ingenuity.
Don’s remarkable career trajectory was characterized by increasing energy and leadership. The three main areas he pioneered—imaging the supermassive black hole in our galaxy, the precise timing of pulsars in search of gravitational waves, and the study of the epoch of reionization—remain frontiers of astrophysical research.
