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Several long-period radio transients have recently been discovered, with strongly polarized coherent radio pulses appearing on timescales between tens to thousands of seconds 1 , 2 . In some cases, the radio pulses have been interpreted as coming from rotating neutron stars with extremely strong magnetic fields, known as magnetars; the origin of other, occasionally periodic and less-well-sampled radio transients is still debated 3 . Coherent periodic radio emission is usually explained by rotating dipolar magnetic fields and pair-production mechanisms, but such models do not easily predict radio emission from such slowly rotating neutron stars and maintain it for extended times. On the other hand, highly magnetic isolated white dwarfs would be expected to have long spin periodicities, but periodic coherent radio emission has not yet been directly detected from these sources. Here we report observations of a long-period (21 min) radio transient, which we have labelled GPM J1839–10. The pulses vary in brightness by two orders of magnitude, last between 30 and 300 s and have quasiperiodic substructure. The observations prompted a search of radio archives and we found that the source has been repeating since at least 1988. The archival data enabled constraint of the period derivative to <3.6 × 10 −13  s s −1 , which is at the very limit of any classical theoretical model that predicts dipolar radio emission from an isolated neutron star. The discovery of a long-period radio transient, GPM J1839–10, prompted a search of radio archives, thereby finding that this source has been repeating since at least 1988.

The high-frequency radio sky is bursting with synchrotron transients from massive stellar explosions and accretion events, but the low-frequency radio sky has, so far, been quiet beyond the Galactic pulsar population and the long-term scintillation of active galactic nuclei. The low-frequency band, however, is sensitive to exotic coherent and polarized radio-emission processes, such as electron-cyclotron maser emission from flaring M dwarfs 1 , stellar magnetospheric plasma interactions with exoplanets 2 and a population of steep-spectrum pulsars 3 , making Galactic-plane searches a prospect for blind-transient discovery. Here we report an analysis of archival low-frequency radio data that reveals a periodic, low-frequency radio transient. We find that the source pulses every 18.18 min, an unusual periodicity that has, to our knowledge, not been observed previously. The emission is highly linearly polarized, bright, persists for 30–60 s on each occurrence and is visible across a broad frequency range. At times, the pulses comprise short-duration (<0.5 s) bursts; at others, a smoother profile is observed. These profiles evolve on timescales of hours. By measuring the dispersion of the radio pulses with respect to frequency, we have localized the source to within our own Galaxy and suggest that it could be an ultra-long-period magnetar. Analysis of archival low-frequency radio data from the Murchison Widefield Array reveals a periodic transient with an unusual periodicity of 18.18 min, the source of which is localized to our Galaxy and could be an ultra-long-period magnetar.

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called ‘coronal holes’. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating 1 , 2 and interchange reconnection 3 – 5 . The coronal magnetic field near the solar surface is structured on scales associated with ‘supergranulation’ convection cells, whereby descending flows create intense fields. The energy density in these ‘network’ magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft 6 that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic ‘switchbacks’ 7 , 8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts. Measurements of fast solar wind streams from the Parker Solar Probe spacecraft provide strong evidence for the interchange reconnection mechanism being responsible for accelerating the fast solar wind.

White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries 1 , 2 . If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge 3 . Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf 4 . In the latter case, the white dwarf remnant is expected to be highly magnetized 5 , 6 because of the strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum 7 . Here we report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibits these properties, but to an extreme: a rotation period of 6.94 minutes, a magnetic field ranging between 600 megagauss and 900 megagauss over its surface, and a stellar radius of 2140 − 230 + 160 kilometres, only slightly larger than the radius of the Moon. Such a small radius implies that the star’s mass is close to the maximum white dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7 is likely to be cooling through the Urca processes (neutrino emission from electron capture on sodium) because of the high densities reached in its core. A binary star merger has produced a white dwarf with a spin period of under 7 minutes, a magnetic field of 600 to 900 million gauss and a radius only slightly larger than that of our Moon.

About ten per cent of ‘massive’ stars (those of more than 1.5 solar masses) have strong, large-scale surface magnetic fields 1 – 3 . It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields 4 , 5 , and the predicted fraction of merged massive stars is also about ten per cent 6 , 7 . The merger hypothesis is further supported by a lack of magnetic stars in close binaries 8 , 9 , which is as expected if mergers produce magnetic stars. Here we report three-dimensional magnetohydrodynamical simulations of the coalescence of two massive stars and follow the evolution of the merged product. Strong magnetic fields are produced in the simulations, and the merged star rejuvenates such that it appears younger and bluer than other coeval stars. This can explain the properties of the magnetic ‘blue straggler’ star τ Sco in the Upper Scorpius association that has an observationally inferred, apparent age of less than five million years, which is less than half the age of its birth association 10 . Such massive blue straggler stars seem likely to be progenitors of magnetars, perhaps giving rise to some of the enigmatic fast radio bursts observed 11 , and their supernovae may be affected by their strong magnetic fields 12 . Simulated mergers of two massive stars provide a solution to the long-standing puzzle of the origin of strong magnetic fields in a subset of massive stars.

The central regions of galaxy clusters are permeated by magnetic fields and filled with relativistic electrons 1 . When clusters merge, the magnetic fields are amplified and relativistic electrons are re-accelerated by turbulence in the intracluster medium 2 , 3 . These electrons reach energies of 1–10 GeV and, in the presence of magnetic fields, produce diffuse radio halos 4 that typically cover an area of  around 1 Mpc 2 . Here we report observations of four clusters whose radio halos are embedded in much more extended, diffuse radio emission, filling a volume 30 times larger than that of radio halos. The emissivity in these larger features is about 20 times lower than the emissivity in radio halos. We conclude that relativistic electrons and magnetic fields extend far beyond radio halos, and that the physical conditions in the outer regions of the clusters are quite different from those in the radio halos. For four galaxy clusters it is shown that the radio halo emission is embedded in a much larger emission that extends over 2–3 Mpc, filling the volume of the clusters.

Solar eruptions are spectacular magnetic explosions in the Sun’s corona, and how they are initiated remains unclear. Prevailing theories often rely on special magnetic topologies that may not generally exist in the pre-eruption source region of corona. Here, using fully three-dimensional magnetohydrodynamic simulations with high accuracy, we show that solar eruptions can be initiated in a single bipolar configuration with no additional special topology. Through photospheric shearing motion alone, an electric current sheet forms in the highly sheared core field of the magnetic arcade during its quasi-static evolution. Once magnetic reconnection sets in, the whole arcade is expelled impulsively, forming a fast-expanding twisted flux rope with a highly turbulent reconnecting region underneath. The simplicity and efficacy of this scenario argue strongly for its fundamental importance in the initiation of solar eruptions.Fully three-dimensional magnetohydrodynamic simulations show that solar eruptions can be simply and efficiently initiated in a single bipolar configuration through photospheric shearing motion alone, without the need of any additional special topology.

A red giant star is an evolved low- or intermediate-mass star that has exhausted its central hydrogen content, leaving a helium core and a hydrogen-burning shell. Oscillations of stars can be observed as periodic dimmings and brightenings in the optical light curves. In red giant stars, non-radial acoustic waves couple to gravity waves and give rise to mixed modes, which behave as pressure modes in the envelope and gravity modes in the core. These modes have previously been used to measure the internal rotation of red giants 1 , 2 , leading to the conclusion that purely hydrodynamical processes of angular momentum transport from the core are too inefficient 3 . Magnetic fields could produce the additional required transport 4 – 6 . However, owing to the lack of direct measurements of magnetic fields in stellar interiors, little is currently known about their properties. Asteroseismology can provide direct detection of magnetic fields because, like rotation, the fields induce shifts in the oscillation mode frequencies 7 – 12 . Here we report the measurement of magnetic fields in the cores of three red giant stars observed with the Kepler 13 satellite. The fields induce shifts that break the symmetry of dipole mode multiplets. We thus measure field strengths ranging from about 30 kilogauss to about 100 kilogauss in the vicinity of the hydrogen-burning shell and place constraints on the field topology. Magnetic fields of 30 to 100 kilogauss are measured in the cores of three giant red stars observed with the Kepler satellite.

Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales 1 – 3 . This variability seems mostly random, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare 4 in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet 5 , plasma instabilities 6 , 7 or orbital motion in an accretion disc 7 , 8 . Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref.  9 ). BL Lac, the prototype of a subclass of blazars 10 , is powered by a 1.7 × 10 8  M Sun (ref.  11 ) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref.  12 )). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities 6 near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image. Analysis of the optical and γ-ray flux monitoring of the blazar BL Lacertae during its outburst in 2020 shows the existence of quasi-periodic oscillations in the relativistic jet with cycles as short as 13 h.

The origin of magnetic fields in white dwarfs remains a fundamental unresolved problem in stellar astrophysics. In particular, the very different fractions of strongly (more than about a megagauss) magnetic white dwarfs in evolutionarily linked populations of close white dwarf binary stars cannot be reproduced by any scenario suggested so far. Strongly magnetic white dwarfs are absent among detached white dwarf binary stars that are younger than approximately a billion years. In contrast, of cataclysmic variables (semi-detached binary star systems that contain a white dwarf) in which the white dwarf accretes from a low-mass star companion, more than a third host a strongly magnetic white dwarf1. Here we present binary star evolutionary models that include the spin evolution of accreting white dwarfs and crystallization of their cores, as well as magnetic field interactions between the stars. We show that a crystallization- and rotation-driven dynamo similar to those working in planets and low-mass stars2 can generate strong magnetic fields in the white dwarfs in cataclysmic variables, which explains their large fraction among the observed population. When the magnetic field generated in the white dwarf connects with that of the secondary star in the binary system, synchronization torques and reduced angular momentum loss cause the binary to detach for a relatively short period of time. The few known strongly magnetic white dwarfs in detached binaries, such as AR Scorpii3, are in this detached phase.The complex evolutionary dance of the strongly magnetic white dwarf in a compact binary system can be effectively modelled by considering spin evolution, core crystallization and a rotation-driven dynamo similar to that in planets and low-mass stars.