Letters in 2021

The Voyager 1 spacecraft is now probing interstellar space beyond the heliopause. Here, measurements from the Plasma Wave System reveal au-scale density fluctuations that trace interstellar turbulence without the need for solar shock-generated plasma oscillation events.

Fast-moving pulsars and neutron stars in general may have received a kinetic ‘kick’ from an asymmetric element in the supernova explosion that formed them. Here, the spin axis of a pulsar is determined to lie along the three-dimensional direction of the pulsar’s motion, providing a challenging constraint on supernova explosion modelling.

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.

Two sub-arcsec double quasars at z > 2 are discovered from a targeted search with a novel astrometric technique. They could be the long-sought kpc-scale dual supermassive black holes or sub-arcsec gravitationally lensed quasar images.

Gravitational lensing is detected in the light curve of a gamma-ray burst. The inferred lens mass, while dependent on the unknown lens redshift, provides evidence for an intermediate-mass black hole at a false alarm probability of 0.6% with trial factors.

Following HAWC observations of the Cygnus Cocoon, massive star-forming regions can now be considered to be sources of very-high-energy (TeV to PeV) Galactic cosmic rays.

Using a new infrared spectropolarimeter at Palomar Observatory, the geometry of a number of supernova ejecta have been assessed, revealing a potential similarity in geometry between SN 2018hna and SN 1987A.

Gamma-ray emission up to and above 100 TeV is detected from the supernova remnant G106.3+2.7. The emission above 10 TeV is associated with a molecular cloud rather than the pulsar PSR J2229+6114, favouring a hadronic origin via the π⁰ decay caused by accelerated relativistic protons.

Outbursts from accreting pulsars encode much information on mass accretion in X-ray binary systems. Measuring optical as well as X-ray pulsations can constrain models and, indeed, point to particle acceleration taking place during accretion.

Tidal-evolution modelling, combined with new geophysical constraints of Mars and viscoelastic laboratory measurements, suggests that the two Martian moons have a common progenitor that was disrupted between 1 and 2.7 billion years ago.

A recent association of a tidal disruption event with neutrino emission can be explained by an expanding cocoon from a relativistic jet providing an external target of backscattered X-rays for the production of neutrinos via proton–photon interactions.

In April 2020, the Konus-Wind instrument registered two X-ray bursts temporally coincident with two radio bursts from the Galactic magnetar SGR 1935+2154. The unusual spectral hardness of the X-ray bursts may be an indicator of fast-radio-burst-like radio emission from magnetars.

Insight-HXMT detected a double-peaked X-ray burst from Galactic magnetar SGR J1935+2154, consistent with two fast radio bursts (FRBs) observed from the same object within seconds. This coincidence suggests a common physical origin, and gives insight into the mechanism behind the origin of FRBs.

Detections of lithium (and in one case, potassium) in the atmospheres of four old white dwarfs suggest that they have accreted fragments of planets; specifically, planetary crusts. One white dwarf evolved from an intermediate-mass progenitor, indicating that rocky planets form even around short-lived B-type stars.

The formation mechanism of the most common type of planet in the Galaxy, those with masses between those of the Earth and Neptune, is far from clear. However, simulations of disk fragmentation presented here, which incorporate a spiral-driven dynamo, produce protoplanets of the right mass and longevity.

Stars in the Tucana II ultrafaint dwarf galaxy observed out to nine half-light radii reveal the presence of an extended dark matter halo with a total mass of >10⁷ solar masses, consistent with a generalized Navarro–Frenk–White density profile and suggestive of past strong bursty feedback or an early galactic merger.

Sticking coefficients quantify the readiness of gas molecules to freeze onto the surface of dust grains. Here, laboratory measurements of the sticking coefficients of water and carbon dioxide onto realistic bare and icy dust grain analogues provide key data for the study of snowlines in protoplanetary disks, for example.

Three-dimensional hydrodynamic simulations show that head-on galaxy collisions can suppress black hole fuelling by stripping the torus-shaped gas surrounding the massive black hole. Galaxy collisions could therefore either switch off or turn on nuclear activity, depending on the collision orbit.

The protocluster LAGER-z7OD1 is discovered at a redshift of 6.93, identified by an overdensity of 6 times the average galaxy density and 16 spectroscopically confirmed members. It shows an elongated shape, indicative of a past merger, and its intergalactic medium is almost fully ionized.

Studies of iron meteorites show that volatile nitrogen originated in three isotopically distinct reservoirs in the early Solar System: the nebular gas, sampled by the Sun and Jupiter, and two others related to organic molecules and dust in the inner and outer Solar System, from which growing protoplanets incorporated nitrogen.