Brian D. Metzger

Magnetars became favored engines of fast radio bursts (FRBs) following the discovery of a luminous radio burst coincident with a hard X-ray flare from a Galactic magnetar. Several repeating FRB sources exhibit time-variable rotation measures and compact, spatially coincident persistent radio emission, consistent with energetic-particle nebulae confined by young supernova ejecta. Secular changes in the dispersion measure (DM) of repeating FRBs have also been observed, offering a complementary probe of their local environments; for example, the DM of FRB 121102 rose until 2019 before declining in recent years. Although rising DM evolution has been attributed to shock ionization, shocked ejecta can cool efficiently through metal-line emission and recombine, especially if mixed with cooler gas. Here we argue that DM variations of the observed magnitude and timescale instead arise from changes in the ionization state of supernova ejecta irradiated by X-rays from a time-variable central engine. The dominant rapidly variable contribution comes from the dense, weakly ionized shell swept up by the expanding nebula, whose ionization and recombination times are shorter than those of the more extended ejecta. The model predicts that enhanced FRB activity should be accompanied by rising DM, with a response smoothed over the ionization/recombination time and superposed on a slower secular decline from ejecta expansion. In FRB 121102, the DM maximum occurred close to the burst-rich 2018–2019 activity episodes. If the recently reported renewed activity is sustained, its DM decline should flatten and may reverse into a fresh rise over the coming years.

GW190814 was a remarkable gravitational-wave (GW) event: a merger between a 23 M _⊙ black hole (BH) and a 2.6 M _⊙ compact object, with an extreme mass ratio that is difficult to reproduce through standard isolated-binary or dynamical formation channels. Recent work has shown that neutrino-cooled collapsar disks can become gravitationally unstable and fragment, producing neutron stars (NSs) or low-mass BHs in orbit around the newly formed central BH. These fragments may subsequently interact, scatter, merge with one another, or inspiral into the central remnant. We explore the possibility that GW190814 originated from such a collapsar-disk fragment merging with the central BH. A key prediction of this scenario is a temporal association with a stripped-envelope supernova preceding the GW event, and we identify the Type Ib supernova candidate SN 2019npv, which occurred inside the GW190814 credible volume approximately 60 days before coalescence, as a possible electromagnetic precursor. Although this delay is too long for a conventional kilonova counterpart, we show that three-body interactions among disk fragments can excite some compact objects to wide orbits and produce merger delays of weeks to months. While GW190814 itself was not expected to produce detectable tidal-disruption-powered emission, future delayed mergers in this channel could generate luminous transients through either reprocessed kilonova heating or shocks driven as merger ejecta collide with the preceding supernova ejecta. Finally, treating SN 2019npv as the electromagnetic counterpart makes GW190814 a bright standard siren and yields H 0 = 71 . 1 − 7.5 + 9.3 km s − 1 Mpc − 1 .

Quasi-periodic eruptions (QPEs) are recurring bursts of X-ray radiation originating from supermassive black holes (SMBHs). They are an unprecedented type of structured, high-amplitude SMBH variability, but the physical origins of their regularity, timescales, energetics, and emission are uncertain. We present new XMM-Newton observations of the QPEs in ZTF 19acnskyy/“Ansky,” constituting the deepest observations of individual bursts in any source thus far. The X-ray spectra reveal time-evolving P Cygni profiles comprising blueshifted absorption and redshifted emission from L-shell transitions of Fe xix – xxiv , with column densities N _H ∼ 10 ²²⁻²³ cm ⁻² and bulk velocities of ∣ v _w / c ∣ ∼ 0.2, indicating relativistic mass ejections during each eruption. We construct a time-dependent analytical model of a wind turning on to self-consistently compute its evolving luminosity and ionization properties, and find that the light curve and spectral lines can be simultaneously produced by a wide-angle outflow with M ̇ ∼ 1 0 − 9 – 1 0 − 8 M ⊙ s − 1 kinetically powering the X-rays with an efficiency of L X / E ̇ K ∼ 0.1 . For a covering fraction f _Ω ∼ 0.5, each eruption ejects ∼10 ⁻³ M _⊙ and ≳10 ⁴⁹ erg of kinetic energy, setting an upper bound on the QPE lifetime of ≲1000 bursts if the underlying mass reservoir is ∼1 M _⊙ , and implying that the bursts may result in detectable multiwavelength signatures of reverberation and feedback. These measurements provide new quantitative constraints on QPE energetics, emission mechanisms, and the mass/energy they recycle into their circumnuclear environments, as well as an observational probe for direct comparison with physical models and hydrodynamical simulations of QPEs.

Matter ejected from the neutron star crust during a magnetar giant flare will undergo r -process nucleosynthesis during decompression. Ultraheavy ions ( Z ≫ 26) may be accelerated to cosmic-ray energies by the reverse shock as the ejecta decelerates upon interacting with the ambient environment. We investigate the contribution of magnetars to the local ultraheavy cosmic-ray flux using semianalytic Galactic transport calculations, demonstrating that they may be significant contributors throughout Galactic history depending on the giant flare rate and ion acceleration efficiency. Although neutron star mergers inject orders of magnitude more energy into cosmic rays, they rarely occur within the spallation-limited propagation horizon for ultraheavy species, reducing their local contributions. As compared to lighter nuclei, which are dominantly accelerated by supernovae, the SuperTIGER experiment has presented tentative evidence for a distinct contribution to the cosmic-ray abundances near and above the first r -process peak ( Z ≈ 35–56). We argue that current abundance data may be consistent with either a magnetar giant flare or neutron star merger origin for these species. Measurements with single-element resolution through the third r -process peak, expected from the upcoming TIGERISS experiment, are required to discriminate between these sources for the heaviest cosmic rays.

Rapidly rotating, strongly magnetized neutron stars (“millisecond protomagnetars”) formed in stellar core collapse, neutron star mergers, and white dwarf accretion-induced collapse have long been proposed as central engines of gamma-ray bursts (GRBs) and accompanying supernovae/kilonovae. However, during the first few seconds after birth, neutrino heating drives baryon-rich winds from the neutron star surface, potentially limiting the magnetization and achievable Lorentz factors of the outflow and casting doubt on whether protomagnetars can launch ultrarelativistic jets at early times, as needed to power short-duration GRBs. We present three-dimensional general-relativistic magnetohydrodynamic simulations of neutrino-heated protomagnetar winds that incorporate M0 neutrino transport. While the global wind properties broadly agree with previous analytic estimates calibrated to one-dimensional models, our simulations reveal essential multidimensional effects. For rapidly rotating models with spin periods P ≈ 1 ms, centrifugal forces strongly enhance mass loss near the rotational equator, producing a dense, subrelativistic outflow ( v ∼ 0.1 c ). This equatorial wind naturally confines and collimates less baryon-loaded outflows emerging from higher latitudes, leading to the formation of a structured bipolar jet with a peak magnetization along the pole up to σ ∼ 30–100, sufficient to reach bulk Lorentz factors Γ _∞ ∼ 100 on larger scales. The resulting angular stratification of the outflow energy into ultrarelativistic polar and subrelativistic equatorial components is broadly consistent with the observed partition between beaming-corrected GRB energies and supernova/kilonova ejecta. Our results demonstrate that millisecond protomagnetars can launch relativistic jets within seconds of formation and highlight their potential role in powering the diverse electromagnetic counterparts of compact-object explosions.

We present photometric and spectroscopic observations of the 2017 Galactic nova V612 Sct, whose optical evolution was marked by multiple unusually large maxima. The eruption included two prominent flares lasting around a month each and reaching amplitudes of about 2.5 mag, followed by a series of smaller flares. Extensive spectroscopic monitoring reveals a striking pattern: with each flare, new absorption systems emerge at progressively higher velocities. This behaviour, also seen in other flaring novae, provides evidence for repeated episodes of mass ejection or outflow at increasing velocities. V612 Sct also alternated between Fe ii and He/N spectral phases during different stages of the eruption, establishing a clear connection between the photometric flares and major spectral transitions. We present two-dimensional dynamic spectra that directly trace the appearance of new absorption features contemporaneous with the light-curve flares. We also introduce a sonification of the spectroscopic sequence, offering an alternative representation of the temporal evolution of the eruption. These results support a picture in which repeated ejection episodes and shock formation play a central role in powering the multiple maxima observed in flaring novae.

We present the results from an extensive broadband (radio to X-rays) observing campaign of the engine-driven Type Ib SN 2012au in the first 13 yr of evolution. The early time ( δt ≤ 190 days) radio and X-ray evolution is well described by conventional models of a forward shock interacting with a wind-like circumstellar medium (CSM; ρ _CSM ∝ r ⁻² ). However, starting at δt ≈ 6.7 yr, we detect a significant radio rebrightening. This late-time emission is dominated by a luminous component characterized by a broad and rapidly evolving spectral peak and a shallow optically thin spectral slope, F _ν ∝ ν ^{−0.31 ± 0.02} . These properties imply a compact emitting region ( R ≲ 10 ¹⁶ cm) expanding at a remarkably slow velocity ( v _s ≲ 500 km s ⁻¹ ) into a high-density environment ( n _e ≥ 10 ⁴ cm ⁻³ ), accompanied by a hard electron power-law index p ≈ 1.6. No soft or hard X-ray emission is detected at any epoch, indicating that high-energy radiation is either strongly absorbed or intrinsically absent. In the context of aspherical shock–CSM interaction models, these observations imply extreme properties of the CSM (geometry, density, and total mass) that lack clear astrophysical motivation. Instead, we show that the emergence of radiation from a newborn pulsar wind nebula (PWN) naturally explains the radio spectral evolution and high-energy limits, where the emission is governed by the adiabatic expansion of a relic pair plasma. We conclude that SN 2012au represents the most compelling candidate for a young, newborn PWN discovered to date, a scenario that can be directly tested with pending very long baseline interferometry observations.

A growing fraction of tidal disruption events (TDEs) exhibit radio emission that rises only years after the optical or infrared flare, indicating delayed outflow activity. In some events, the outflow is inferred to be slow (∼0.02 c ) and massive (≳0.01–0.1 M _⊙ ), challenging models such as delayed jets and disk state transitions. We propose a new mechanism for such delayed outflows: repeated collisions between a TDE accretion disk and a preexisting stellar extreme-mass-ratio-inspiral (EMRI) orbiting the black hole. In this scenario, the delay reflects the viscous time required for the initially compact TDE disk to expand and intercept the EMRI orbit, rather than delayed jet launching or off-axis viewing effects. Once star–disk collisions commence, repeated impacts eject outflows with velocities comparable to the orbital speed, v _w ∼ 0.02–0.1 c . We develop a time-dependent model for the coupled evolution of the spreading disk and EMRI-induced mass loss, identifying regimes where the outflow is dominated by disk material or ablated stellar debris. Depending on disk viscosity, orbital period, and collision efficiency, masses ∼(10 ⁻³ –1) M _⊙ can be launched with energies up to 10 ⁵¹ erg, years after the TDE. These outflows produce radio emission through interaction with circumnuclear material or earlier TDE ejecta, consistent with observed late-time radio rebrightening. This model predicts a connection between delayed radio flares and EMRI-hosting systems, potentially including those exhibiting quasi-periodic eruptions (QPEs) powered by star–disk collisions, though the conditions for bright radio flares may not always match those necessary for detectable QPEs.