The Shaw Prize in Astronomy 2021 is awarded in equal shares to Victoria M Kaspi, Professor of Physics and Director of McGill Space Institute, McGill University, Canada and Chryssa Kouveliotou, Professor and Chair, Department of Physics at George Washington University, USA for their contributions to our understanding of magnetars, a class of highly magnetized neutron stars that are linked to a wide range of spectacular, transient astrophysical phenomena. Through the development of new and precise observational techniques, they confirmed the existence of neutron stars with ultra-strong magnetic fields and characterized their physical properties. Their work has established magnetars as a new and important class of astrophysical objects.
Neutron stars are the ultra-compact remnants of stellar explosions. Most are rapidly rotating with periods of milli-seconds to seconds, and emit powerful beams of electromagnetic radiation (observed as pulsars). As such they are accurate “cosmic clocks” that enable tests of fundamental physics in the presence of a gravitational field many billion times stronger than that on Earth. As a result, the Nobel Prize in Physics has been awarded twice for work on pulsars (1974 and 1993).
Pulsars also have strong magnetic fields, since the magnetic field lines in the progenitor star are “frozen in” in the stellar remnant as it collapses to become a neutron star. These magnetic fields funnel jets of particles along the magnetic poles, but classical radio pulsars are powered mainly by rotational energy and slowly spin down over their lifetimes.
The research of Kaspi and Kouveliotou was motivated by the theoretical prediction (Duncan & Thompson 1992) that neutron stars with extreme magnetic fields up to a thousand times stronger than those in regular pulsars could form if dynamo action would be efficient during the first few seconds after gravitational collapse in the core of the supernova. Such objects (termed “magnetars”) would be powered by their large reservoirs of magnetic energy, not rotation, and were predicted to produce highly-energetic bursts of gamma-rays through generation of highly energetic ionized particle pairs at their centres.
From observations of a class of X-ray/γ-ray sources called “soft gamma-ray repeaters” (SGRs) Chryssa Kouveliotou and her colleagues in 1998/99 established the existence of magnetars and provided a stunning confirmation of the magnetar model. By developing new techniques for pulse timing at X-ray wavelengths and applying these to data from the Rossi X-ray timing satellite (RXTE), Kouveliotou in 1998 was able to detect X-ray pulses with a period of 7.5 seconds within the persistent X-ray emission of SGR 1806-20. She then measured a spin-down rate for the pulsar, and derived both the pulsar age and the dipolar magnetic field strength — which lay within the range of values predicted for magnetars, close to 1014 gauss (1010 T). The spin-down measurements were extremely challenging because of the faintness of the pulsed signal and the need to correct the rotation phase across multiple epochs.
Victoria Kaspi showed that a second class of rare X-ray emitting pulsars, the “anomalous X-ray pulsars” (AXPs), were also magnetars (Gavriil et al. 2002). Kaspi took the techniques used by radio astronomers to maintain phase coherence in pulsar timing and adapted them to work in the much more challenging X-ray domain. This allowed her to make extremely accurate timing measurements of X-ray pulsars with full phase coherence across intervals of months to years, and hence to measure spin-down rates far smaller than those seen in SGR 1806-20. Kaspi has also made fundamental contributions to the characterization of magnetars as a population, through the elucidation of their physical properties and their relationship to the classical radio pulsars (Olausen & Kaspi 2014). Her work has cemented the recognition of magnetars as a distinct source class. Today, magnetars are routinely invoked to explain the physics underlying a diverse range of astrophysical transients including γ-ray bursts, superluminous supernovae and nascent neutron stars.
Magnetars probe extreme physical conditions inaccessible on Earth, such as strong gravity, ultra-nuclear densities and the strongest magnetic fields in the Universe. In this high energy environment particle-anti-particle pairs are created from the vacuum, and unique tests of general relativity and quantum electrodynamics become possible. In 2020/2021, the first associations of a Galactic magnetar with milli-second duration outbursts of radio emission, so called Fast Radio Bursts (FRBs), were established (CHIME/FRB et al. 2020, Younes et al. 2021). These results may suggest that “flaring” magnetars are the central engines of at least some of the spectacular extragalactic FRBs. Future studies will undoubtedly shed further light on these exciting discoveries.
The Shaw Prize 2021 recognizes the seminal contributions of Victoria M Kaspi and Chryssa Kouveliotou to the understanding of the enigmatic properties of magnetars, pulsars and γ-ray bursts.
Astronomy Selection Committee
The Shaw Prize
1 June 2021 Hong Kong