Pulsars are among the most remarkable and exotic objects in the heavens: rapidly spinning neutron stars with a mass similar to the Sun’s, diameters of only a few tens of kilometers and magnetic fields a million million times stronger than the field we experience at the surface of the Earth. Pulsars emit beams of radio waves from their magnetic poles, which sweep across the Earth as the pulsars spin; these periodic pulses can be detected by Earth-based telescopes. On August 24 2001, during a search for pulsars in nearby galaxies, a telescope at the Parkes Observatory in Australia recorded a brief but unusually strong burst of radio emission. This event lay undetected in the archives of that search for more than half a decade, until it was found by David Narkevic, a student working with Duncan Lorimer and Maura McLaughlin at West Virginia University on a search for single pulses from pulsars with strongly variable radio emission. The arrival time of the burst was dispersed, that is, it changed with frequency, a characteristic of astrophysical signals such as those from pulsars that have propagated through the plasma of ionized gas that fills much of our Milky Way and other galaxies. However, the dependence of arrival time on frequency was far larger than for any known pulsar. Lorimer, McLaughlin, Bailes and their collaborators recognized that this dependence implied that the mysterious source – now known as the “Lorimer burst” – lay far outside our own Milky Way, roughly 100,000 times further than the typical pulsar and so far away that the burst was emitted over a billion years ago. They also showed that despite this enormous distance and the correspondingly huge energy requirements, the object emitting the burst had to be very small – the finite speed of light and the short duration of the signal implied that the burst must have come from a region smaller than the Earth.
The properties of the Lorimer burst were so extreme that many astronomers suspected that it was caused by Earth-based radio interference. It was only five years later, as more such fast radio bursts (FRBs) were discovered on different telescopes, that they were recognized to be a real cosmic phenomenon. By now almost a thousand FRBs have been detected, still only a small fraction of the estimated rate of over a thousand per day across the whole sky. Most of these detections have been made by new telescopes and instruments designed specifically to look for FRBs, and an armada of even more ambitious instruments are being designed or under construction.
FRBs are the only extragalactic sources that have short enough timescales to be used to measure dispersion in the arrival time. This dispersion contains a large contribution from plasma in the intergalactic medium along the line of sight to the burst. As Lorimer and his collaborators pointed out, FRBs that are found in galaxies with measured distances offer a unique probe of the spatial distribution of ionized matter in the diffuse intergalactic medium outside galaxies, which contains most of the baryonic or normal matter in the Universe and is one of the major remaining unknowns in our inventory of the contents of the Universe. Recent preliminary estimates, which should improve rapidly in the future, suggest that the density of matter in intergalactic space is consistent with values estimated indirectly from the cosmic microwave background and the primordial abundances of the elements.
With future FRB discoveries we may be able to probe issues in fundamental physics, such as the validity of Einstein’s equivalence principle, possible new forms of exotic matter, the expansion rate of the Universe, and the origin of intergalactic magnetic fields.
The nature of FRBs remains unknown. Probably, like pulsars, they are associated with neutron stars, whose large rotational energies and strong magnetic fields make them plausible candidates for the progenitors of FRBs. The diversity of FRBs – some repeat but most do not, some are associated with young stellar populations and others with old, some but not all show polarized light, etc. – suggests that more than one mechanism may be responsible. Even more exotic possibilities involving evaporating black holes or new forms of matter have been discussed. A much larger sample of FRBs, with well characterized burst properties and robustly identified galaxy hosts, is needed to differentiate between the dozens of proposed progenitor theories.
The award of the Shaw Prize is also intended to recognize the other collaborators in this research, David Narkevic and Fronefield Crawford, as well as the investigators who collected the original data for other purposes and the team who developed the novel wide-field multibeam receiver for the Parkes Telescope that made the discovery possible. The story of the discovery of FRBs highlights the importance of the stewardship of archived astronomical databases and of supporting archival research that exploits these databases.
The discovery paper is
D. R. Lorimer, M. Bailes, M. A. McLaughlin, D. J. Narkevic, F. Crawford, “A bright millisecond radio burst of extragalactic origin”, Science, volume 318, p. 777-780, 2 November 2007
12 November 2023 Hong Kong