Johannes A. Van Paradijs (1946–99) Sources, and Found Them All to Be About 16 Km, Typical of a Neutron Star
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news and views Obituary ‘standard candle’. From this he was able to measure the diameters of several burst Johannes A. van Paradijs (1946–99) sources, and found them all to be about 16 km, typical of a neutron star. Among his Jan van Paradijs, who died in Amsterdam other discoveries were the demonstration AM on 2 November, was one of the world’s that X-ray-burst sources are members of foremost astrophysicists. He will probably binary systems, and the first spatially be remembered most for the discovery, in resolved spectroscopic mapping of an . AMSTERD 1997, of the first optical afterglow of a accretion disk that forms between two UNIV g-ray burst, which established the stars in a low-mass binary. extragalactic nature of these bursts and In 1978, while working at the European solved what had for some 25 years been Southern Observatory in Chile, together one of the greatest problems of with Holger Pedersen from Copenhagen, astrophysics. Because of this first optical van Paradijs discovered the first optical sighting, which has since been followed by flash from an X-ray burst, by a dozen others, we now know that g-ray simultaneously combining telescope and bursts occur in very distant galaxies and satellite data. Such simultaneous represent the largest explosions of energy observations of unpredictable, brief events in the Universe since the Big Bang. require much organization and a flair for Born into a bricklayer’s family in logistics. There can be no doubt that van Haarlem, The Netherlands, van Paradijs Paradijs’ expertise in this area gave him a was the eldest of seven children. Thanks to head start over other groups when in 1996 the intervention of the headmaster of his the BeppoSAX satellite began to provide Roman Catholic primary school, his High-energy accurate positions of g-ray bursts. It was parents sent him to a Haarlem secondary therefore no coincidence that, together school that prepares students for higher astrophysicist with his students Paul Groot and Titus education. In 1963 he began studying who hunted big Galama, he was the first to detect the mathematics, physics and astronomy at the astronomical game optical afterglow from a g-ray burst and to University of Amsterdam. He excelled in all establish its extragalactic nature. three fields and graduated in 1970. During Jan van Paradijs married the Greek those years he played top-league basketball In 1975 van Paradijs succeeded in astrophysicist Chryssa Kouveliotou in for his home town Haarlem. He also took measuring the first ever mass of a neutron 1992. She works at NASA’s Marshall Space many university classes in philosophy, and star: Vela X-1. This neutron star is about Flight Center in Huntsville, Alabama, clear logic and consistent reasoning twice as heavy as the Sun, but has a where from 1993 Jan was a part-time became second nature to him in all his diameter of only about 20 km. He realized physics professor at the University of activities. Later, he would teach these the great importance of these accurate Alabama. He divided his time between qualities, both in words and by example, to mass determinations for fundamental Huntsville and Amsterdam, where he was students and colleagues. physics. Together with radius also a professor. The marriage was a very Van Paradijs carried out his PhD measurements of neutron stars — a topic happy one, each partner strongly research on the structure and chemical he addressed in later studies of X-ray burst supporting the other in research as well as composition of cool giant stars under the sources — they are the only way to study in personal life. He was co-author on her guidance of David Koelbloed. The nuclear matter at densities far beyond 1998 paper reporting the discovery of knowledge he gained in stellar what can be reached in the laboratory. magnetars — neutron stars with giant spectroscopy and photometry, as well as in From 1977 to 1979 van Paradijs worked magnetic fields of 1015 Gauss — just as she the theory of stellar atmospheres, prepared at MIT, where he became interested in X- was co-author on his paper reporting the him well for his later research on X-ray ray bursts — powerful, brief outbursts of discovery of the first g-ray-burst afterglow. binaries and neutron stars. When I came to X-rays that were discovered by a Dutch All in all, in the last eight years they Amsterdam in 1974, he had just become a satellite in 1975. Colleagues at MIT, George published more than 90 papers together. tenured research associate and decided to Clark, Walter Lewin and Paul Joss, were It came as a great shock that at the peak join me in studies of binary X-ray sources, using a small astronomy satellite to of their careers, Jan suddenly appeared to complementing my theoretical work with observe such X-ray bursts and made many be gravely ill. During the past seven careful observations. important discoveries. His close friendship months Chryssa practically suspended her X-ray binaries are the strongest sources with Walter Lewin dated from this time — career to care for him until the end. Van of X-rays in the Galaxy, in which gas flows they produced over 120 papers together, Paradijs guided the research of over 25 from a normal star to a nearby compact out of 400 or so scientific papers van PhD students in Amsterdam and companion, such as a neutron star or black Paradijs produced during his lifetime. Huntsville. He will be remembered by hole. Van Paradijs carried out systematic A trait of van Paradijs’ work was his them all as an outstanding scientist and studies of the orbits and light curves talent for seeing the ‘big picture’ through a teacher, and a fine colleague and friend. (variation of light over time) of the normal multitude of often confusing observational Edward van den Heuvel stars in these systems, with the aim of facts. A prime example was his Edward van den Heuvel is at the Astronomical accurately determining the mass of a demonstration that X-ray burst sources are Institute “Anton Pannekoek” and the Center for companion neutron star. As their name neutron stars and not black holes as had High Energy Astrophysics, University of suggests, neutron stars consist almost been generally thought. He showed that Amsterdam, Kruislaan 403, NL-1098 SJ entirely of neutrons and so are very dense the peak luminosities of many bursts were Amsterdam, The Netherlands. and small. always the same, leading to the idea of a e-mail: [email protected] 742 © 1999 Macmillan Magazines Ltd NATURE | VOL 402 | 16 DECEMBER 1999 | www.nature.com.