An Essay on the Prize

The 2021 Shaw Prize in Astronomy is awarded in equal parts to Victoria M Kaspi and Chryssa Kouveliotou for their contributions to the understanding of magnetars, a class of highly magnetized neutron stars that are linked to a wide range of spectacular, transient astrophysical phenomena. By developing new and precise observational techniques, they confirmed the existence of this new class 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 ultra-compact remnants of explosions of massive stars, which have exhausted their “fuel” for generating energy through fusion, and then collapse under their own gravity. Most young neutron stars are rapidly rotating with periods of milliseconds to seconds, and many of them 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. The Nobel Prize in Physics was awarded twice for work on pulsars, in 1974 and in 1993.

Pulsars have strong magnetic fields, since the magnetic field lines of the progenitor star are “frozen” into the stellar remnant as it collapses to become a neutron star. These magnetic fields funnel jets of particles along the magnetic poles, but classic radio pulsars are powered mainly by rotational energy and slowly spin down over their lifetime.

The research of Kaspi and Kouveliotou was motivated by the theoretical prediction of Duncan and Thompson in 1992 that neutron stars with magnetic fields up to a thousand times stronger than those in regular pulsars could form if the so-called “dynamo mechanism” for generating magnetic fields were efficient during the first few seconds after gravitational collapse in the core of the progenitor star. Such objects (henceforth termed “magnetars”) would be powered by their large reservoirs of magnetic energy, not rotation, and were predicted to produce highly energetic bursts of γ-rays by generating highly energetic ionized particle pairs at their centres. Magnetars have magnetic field strengths of 1010 Tesla (or 1014 Gauss), one hundred million times stronger than any manufactured magnet, and among the strongest magnetic objects throughout the universe.

In 1998/99, Chryssa Kouveliotou and her colleagues discovered a class of variable X-ray/ γ-ray sources called “soft gamma-ray repeaters” (SGRs) and identified them as magnetars, thus providing a stunning confirmation of the Duncan–Thompson 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 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 magnetic field strength. 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.

In 2002, Victoria Kaspi showed that a second class of rare X-ray-emitting pulsars, the “anomalous X-ray pulsars” (AXPs), were also magnetars. 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 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 by elucidating their physical properties and their relationship to classic radio pulsars. 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, super-luminous supernovae, and nascent neutron stars.

In 2020, a so-called “fast radio burst” was found to likely be associated with a magnetar, possibly linking magnetars to another spectacular and mysterious observational phenomenon.

The Shaw Prize 2021 recognizes the seminal contributions of Vicky Kaspi and Chryssa Kouveliotou to the understanding of the enigmatic properties of magnetars and pulsars.

28 October 2021 Hong Kong