Fifty years since Franco Pacini's prediction

Elena Amato and Niccolò Bucciantini, INAF Osservatorio Astrofisico di Arcetri

Just over 50 years ago, on November 11th, 1967, Franco Pacini, a young Italian researcher, who had graduated from Rome University only 3 years earlier and was working at the time at Cornell University, published an article suggesting, for the first time, that the non-thermal radiation from the Crab Nebula, and the accelerated expansion of the same nebula, were to be attributed to a continuous injection of energy by a neutron star. Despite neutron stars had been theorized in 1932 by Lev Landau, and that already in 1934 Walter Baade and Fritz Zwicky had proposed their possible formation in coincidence with supernova explosions, after more than 30 years they still remained among the most elusive objects of the universe, to the point that few believed in their real existence and, even among these, the majority would bet that these stars, given their extremely small sizes, would never be directly observed. It had been known for a long time that the Crab Nebula was born from the supernova explosion occurred in 1054. Pacini suggested that, during the collapse of the core of the progenitor star, a neutron star had formed with a very rapid rotation, and a very strong magnetic field. The combination of these two properties would have transformed the star into a rotating mega-magnet, which would emit a large amount of low-frequency electromagnetic waves. Such waves would have been easily absorbed by the stellar material expelled during the supernova explosion, transferring to it a large part of the rotational energy of the neutron star. This transfer would have involved, on the one hand, the acceleration of the material itself, and on the other the emission of synchrotron radiation, due to high energy particles. Although Pacini was not aware of it, since the news was still confidential and would only be made public on February 24th 1968, in the same year 1967, on August 6th, Jocelyn Bell, who was at the time a young PhD student in astronomy, had discovered a fast pulsing radio signal. It was the first pulsar, soon after identified with a magnetized and rapidly rotating neutron star. Later in the year 1968, Pacini’s thesis found definitive confirmation in the observation of a pulsar right at the center of the Crab Nebula. This intuition by Franco Pacini starts an entire sector in the field of high energy astrophysics, the study of pulsars and their wind nebulae. Over the years, first Pacini first, and then his collaborators, have refined the initial idea developing a model that today is universally considered canonical.  Today more than 2000 pulsars are known, and many of them are at the center of supernova remains, to which they transfer their energy, as foreseen by Pacini, producing some of the most spectacular nebulae observed.

To know more: Franco Pacini on Polvere di stelle: The cultural heritage of italian astronomy