Gamma-ray bursts (GRBs), among the most energetic transients in the Universe, are traditionally classified into long-duration (lasting more than 2 seconds) and short-duration (lasting less than 2 seconds) events. Long-duration GRBs are typically associated with the core collapse of massive stars (type II), whereas short-duration GRBs originate from the merger of compact binary systems (type I). These different progenitor channels are reflected in distinct observational signatures that provide key diagnostics for GRB classification. Here we report a peculiar event, GRB 160425A, comprising two sub-bursts separated by 4 minutes: a short-duration burst (G1) and a long-duration burst (G2). Nearly all standard prompt-emission diagnostics, including pulse morphology, duration, hardness ratio, minimum variability timescale, spectral properties and established empirical correlations, consistently categorize G1 as a short-like GRB (type I, merger origin) and G2 as a long-like (type II, collapsar origin) GRB. The coexistence of merger and collapsar signatures within a single event challenges existing progenitor frameworks and calls for a re-evaluation of GRB classification schemes and progenitor scenarios. GRB 160425A emitted two sub-bursts in short succession with differing properties, indicative of two distinct progenitor types: collapsar and stellar merger. This event raises questions about γ-ray burst progenitor scenarios.
The magnetic field is one of the fundamental observables in solar physics, whereas solar flares are violent, rapid magnetic energy-release events in the solar atmosphere. Despite decades of observations and theory regarding flare evolution and magnetic-field-related physical processes, the actual magnetic field variations during flares remain poorly understood. Here, using a mid-infrared solar telescope, we investigate the spectroscopic evolution of the Mg I 12.32-µm line during an X1.5-class flare. Owing to the high Zeeman sensitivity of this line, magnetic field strength can be directly determined from the measured line separation. Our analysis reveals apparent magnetic field weakening beneath flare ribbons in the impulsive phase. Radiative–hydrodynamic simulations and atmospheric inversions confirm that the formation height of the Mg I 12.32-µm line increases during the flare. We suggest this apparent field change probably originates either from elevated line-formation altitude or magnetic-field redistribution. This study demonstrates the capability of mid-infrared wavelengths to trace magnetic-field evolution during flares and advances our understanding of the flare energy build-up and release processes. AIMS mid-infrared spectroscopy of the Mg I 12.32 µm line during an X1.5 flare reveals magnetic field weakening beneath impulsive-phase ribbons via Zeeman splitting. These changes may arise from an elevated line-formation height or local magnetic redistribution.
Accretion onto black holes powers some of the most energetic sources in the Universe, yet its behaviour across the luminosity range remains poorly understood. Specifically, as the luminosity approaches the Eddington limit (LEdd), the inner accretion disk is expected to depart from the standard thin-disk solution, becoming vertically extended. The luminosity threshold and observational signatures of this transition remain, however, uncertain. We present multiwavelength spectroscopy obtained during the decay of a black hole X-ray binary outburst. At the peak (~0.2 LEdd), a cool optical and near-infrared spectrum dominated by strong recombination lines of hydrogen and He i is observed. However, as the luminosity decreases, it switches to a hot, highly irradiated, nearly featureless spectrum. This puzzling behaviour reveals the emergence of a vertically extended inner disk at the outburst peak, shielding the outer regions from high-energy radiation produced near the black hole—a scenario supported by contemporaneous X-ray spectra. Observations of similar systems suggest that this phenomenon may be common among black hole accretion disks. Self-shadowing structures at luminosities well below LEdd have major implications for black hole accretion flows and their observational properties, affecting inferred black hole spins, X-ray polarization measurements and the physical conditions of the outer disk and its associated outflows. A multiwavelength study reveals a vertically extended inner disk in a black hole X-ray binary at unexpectedly low luminosities, reshaping its emission and shielding the outer disk, with implications for its physical and observational properties.
The abundance of UV-bright, compact star-forming galaxies in the early Universe indicates that high-energy (Lyman continuum (LyC); λ < 912 Å) photons from massive stars alone could have re-ionized the intergalactic medium. However, the efficiency with which LyC photons escape these systems—and the relative roles of radiation, stellar winds and supernovae in regulating that escape—remain uncertain. Here, using medium-resolution spectra of six otherwise similar local star-forming galaxies spanning a range of starburst ages, we directly trace the launch of a multiphase galactic wind through individual spectral lines alongside measurements of the LyC escape fraction. We find that LyC escape peaks early (<3 Myr), during a period dominated by intense radiation and strong stellar winds but lacking a fast galactic wind. In more evolved systems (>3 Myr), we detect faster galactic winds of greater equivalent width and dust content. These observations suggest that LyC escape peaks early and is later suppressed by the launch and subsequent mass loading of a supernova-driven wind during the first ~10 Myr of star formation in compact star-forming galaxies. UV spectra of six similar galaxies show that stronger, faster, multiphase outflows accompany lower escape of ionizing radiation. Thus Lyman continuum escape may peak before strong galactic winds develop and then decline as the wind-column density grows.
Planets are inherently three-dimensional objects shaped by their thermal structure and dynamics. Observations of transiting exoplanets have begun to uncover this three-dimensional nature, with JWST measuring their dayside atmosphere structure with eclipse mapping and exploring spectroscopic asymmetries in the limbs of planets during transit. Here we apply the novel transmission strings mapping method to the hot Jupiter WASP-17 b. With this method, we are not only able to extract hemispheric limb asymmetries from east-to-west, but can also map equator-to-pole variations around the transmitting atmosphere as a function of angle, spanning 0.6–12 μm. We find a clear transition in the thermal scale height around the limb of the atmosphere, with an approximately 650 K temperature difference between the morning ( $$85{8}_{-233}^{+434}\,{\mathrm{K}}$$ ) and evening $$1{,}52{2}_{-114}^{+116}\,{\mathrm{K}}$$ ). These results are consistent with both nightside emission and dayside eclipse mapping measurements, and represent a fully comprehensive set of observations for a transiting exoplanet. We are also able to constrain the abundances of H2O and CO2 to constant values around the limb, providing observational constraints for results from global circulation models. The transmission string technique, combined with the outstanding observability of WASP-17 b, pushes the study of exoplanet atmospheres further into a three-dimensional understanding of hot Jupiter dynamics and chemistry. A new method of extracting atmospheric variations around the limb of a hot Jupiter using transmission mapping of JWST data finds a transition between the equator and pole of WASP-17 b. The authors measure a 650 K temperature difference between the morning and evening and find H2O and CO2 limb abundances consistent with circulation models.
