Royal Observatory of Belgium

We review the key observations and theories relevant to the internal structure and dynamics of the Galilean satellites. Key observations include: the bulk densities and degree-two gravity coefficients of the moons; the presence of conductive subsurface layers, as inferred from magnetic induction; and the surface compositions. All the moons, with the possible exception of Callisto, appear to be differentiated (denser components have separated from lighter components). Ganymede and Io have iron cores; Europa may have one. The outer three moons all likely possess subsurface oceans; for Europa the ocean overlies rock, while for the other two it overlies higher-pressure ice phases. Io is partially molten but does not possess a shallow magma ocean. Tidal heating is the dominant energy source at Europa and Io, and may have affected Ganymede’s long-term evolution. The dynamics of the subsurface oceans are of considerable theoretical interest but are only weakly tied to current or likely future observations. We identify seven outstanding questions regarding internal structures, some of which will be answered by the forthcoming JUICE, Europa Clipper and Tianwen-4 missions.

Solar flares are the most powerful, magnetically driven, explosions in the heliosphere. The nature of magnetic energy release in the solar corona that heats the plasma and accelerates particles in a flare, however, remains poorly understood. Here, we report high-resolution coronal observations of a flare by the Solar Orbiter mission that reveal initially weaker but rapid reconnection events, on timescales of a few seconds at most, leading to a more prominent activity of a similar nature that explosively causes a flare. Signatures of this process are further imprinted on the widespread raining plasma blobs with short lifetimes, giving rise to the characteristic ribbon-like emission pattern associated with the flare. Our observations unveil the central engine of a flare and emphasize the crucial role of an avalanche-like magnetic energy release mechanism at work.

We describe the scientific objectives and instrument design of the ASPIICS coronagraph launched aboard the Proba-3 mission of the European Space Agency (ESA) on 5 December 2024. Proba-3 consists of two spacecraft in a highly elliptical orbit around the Earth. One spacecraft carries the telescope, and the external occulter is mounted on the second spacecraft. The two spacecraft fly in a precise formation during 6 hours out of 19.63 hour orbit, together forming a giant solar coronagraph called ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun). Very long distance between the external occulter and the telescope (around 144 m) represents an increase of two orders of magnitude compared to classical externally occulted solar coronagraphs. This allows us to observe the inner corona in eclipse-like conditions, i.e. close to the solar limb (down to 1.099 Rs) and with very low straylight. ASPIICS will provide a new perspective on the inner solar corona that will help solve several outstanding problems in solar physics, such as the origin of the slow solar wind and physical mechanism of coronal mass ejections.

The recently discovered gravitational wave event GW231123 was interpreted as the merger of two black holes with a total mass of 190-265 M circle dot, making it the heaviest such merger detected to date. While much of the postdiscovery literature has focused on its astrophysical origins, primary analyses have exhibited considerable discrepancies in the measurement of source properties between waveform models, which cannot reliably be reproduced by simulations. Such discrepancies may arise when an unaccounted overlapping signal is present in the data, or from phenomena that produce similar effects, such as gravitational lensing or overlapping noise artifacts. In this work, we analyze GW231123 using a flexible model that allows for two overlapping signals, and find that it is favored over the isolated signal model with Bayes factors of similar to 102-104, depending on the waveform model. These values lie within the top few percent of the background distribution. Similar effects are not observed in GW190521, another high-mass event. Under the overlapping signals model, discrepancies in the measurement of source properties between waveform models are largely mitigated. We also find that neglecting an additional signal in overlapping-signal data can lead to discrepancies in the estimated source properties resembling those reported in GW231123. Although the overlapping signal model provides a higher Bayesian evidence, the astrophysical prior probability of two short signals overlapping is low. However, we find that the two recovered sources show similar properties. This, taken with the higher evidence of the two signal model, suggests that gravitational lensing may provide an alternative explanation.

Understanding the transfer of mass and angular momentum in binary interactions is crucial for modeling the evolution of any interacting binary after the first mass-transfer phase. Mass-transfer physics assumptions shape the predictions for later stages of binary evolution, such as the immediate progenitors of stripped-envelope supernovae and gravitational-wave mergers. We constrain the efficiency and stability of thermal-timescale mass transfer in massive binary evolution using the observed population of 62 massive interacting binaries on the main sequence ("Algols") in the Milky Way and the Large and Small Magellanic Clouds. We find that purely conservative or nonconservative mass transfer cannot explain the current mass ratio and orbital period of all massive Algols. Angular momentum conservation rules out conservative mass transfer in similar to 28% of massive Algols in the SMC. About three-quarters of all massive Algols are consistent with having undergone inefficient mass transfer (less than or similar to 50%), while the remaining systems, mostly residing in the LMC and the Milky Way, require mass transfer to have been more efficient than 25%. For our fiducial assumption on the extent of envelope stripping, the current sample of massive Algols does not require mass transfer to be efficient at the shortest orbital periods (similar to 2 days) at any metallicity. We find evidence that mass transfer on the main sequence needs to be stable for initial accretor-to-donor mass ratios as unequal as similar to 0.6. Unless biased by observational selection effects, the massive Algols in the SMC seem to have undergone less efficient mass transfer than those in the LMC and the Milky Way.