Magnetars are the strongest magnets in the present universe and the combination of extreme magnetic field, gravity and density makes them unique laboratories to probe current physical theories (from quantum electrodynamics to general relativity) in the strong field limit. Magnetars are observed as peculiar, burst–active x-ray pulsars, the anomalous x-ray pulsars (AXPs) and the soft gamma repeaters (SGRs); the latter emitted also three ‘giant flares’, extremely powerful events during which luminosities can reach up to 10 ⁴⁷ erg s ⁻¹ for about one second. The last five years have witnessed an explosion in magnetar research which has led, among other things, to the discovery of transient, or ‘outbursting’, and ‘low-field’ magnetars. Substantial progress has been made also on the theoretical side. Quite detailed models for explaining the magnetars’ persistent x-ray emission, the properties of the bursts, the flux evolution in transient sources have been developed and confronted with observations. New insight on neutron star asteroseismology has been gained through improved models of magnetar oscillations. The long-debated issue of magnetic field decay in neutron stars has been addressed, and its importance recognized in relation to the evolution of magnetars and to the links among magnetars and other families of isolated neutron stars. The aim of this paper is to present a comprehensive overview in which the observational results are discussed in the light of the most up-to-date theoretical models and their implications. This addresses not only the particular case of magnetar sources, but the more fundamental issue of how physics in strong magnetic fields can be constrained by the observations of these unique sources.

The Imaging X-Ray Polarimetry Explorer¹ (IXPE) is a NASA small explorer mission selected in early 2017. IXPE will be a pathfinder mission opening a new window on the x-ray sky by enabling polarimetry measurements on essentially all classes of the brightest cosmic x-ray sources. Launched on 2021 December 9, with a 2-year baseline mission, IXPE will perform a study of dozens of sources in its first year, with follow-on more detailed observations of selected targets in year two.

Spinning up an extragalactic neutron star: Ultraluminous x-ray sources (ULXs) are strange objects in other galaxies that cannot be explained by conventional accretion onto stellar-mass objects. This has led to exotic interpretations, such as the long-sought intermediate-mass black holes. Israel et al. observed a ULX in the nearby galaxy NGC 5907 and found that it is instead a neutron star. The spinning neutron star is accreting material so fast that its spin period is quickly accelerating. The only way that it can consume enough material to explain these properties is if it has a strong multipolar magnetic field. Science , this issue p. 817

Odd Magnetar: Magnetars are neutron stars that are widely thought to be powered by extremely high magnetic fields. Using data from three different x-ray observatories, Rea et al. (p. 944 , published online 14 October) show that a previously known magnetar has a magnetic field that is much smaller than those of other magnetars. A strong magnetic field is thus not a necessary requirement for a neutron star to show magnetar-like behavior, implying that this population is wider than was previously thought.