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Gamma-ray bursts (GRBs) are divided into two populations 1 , 2 ; long GRBs that derive from the core collapse of massive stars (for example, ref.  3 ) and short GRBs that form in the merger of two compact objects 4 , 5 . Although it is common to divide the two populations at a gamma-ray duration of 2 s, classification based on duration does not always map to the progenitor. Notably, GRBs with short (≲2 s) spikes of prompt gamma-ray emission followed by prolonged, spectrally softer extended emission (EE-SGRBs) have been suggested to arise from compact object mergers 6 – 8 . Compact object mergers are of great astrophysical importance as the only confirmed site of rapid neutron capture ( r -process) nucleosynthesis, observed in the form of so-called kilonovae 9 – 14 . Here we report the discovery of a possible kilonova associated with the nearby (350 Mpc), minute-duration GRB 211211A. The kilonova implies that the progenitor is a compact object merger, suggesting that GRBs with long, complex light curves can be spawned from merger events. The kilonova of GRB 211211A has a similar luminosity, duration and colour to that which accompanied the gravitational wave (GW)-detected binary neutron star (BNS) merger GW170817 (ref.  4 ). Further searches for GW signals coincident with long GRBs are a promising route for future multi-messenger astronomy. A possible kilonova associated with a nearby, long-duration gamma-ray burst suggests that gamma-ray bursts with long and complex light curves can be spawned from the merger of two compact objects, contrary to the established gamma-ray burst paradigm.

Dust associated with various stellar sources in galaxies at all cosmic epochs remains a controversial topic, particularly whether supernovae play an important role in dust production. We report evidence of dust formation in the cold, dense shell behind the ejecta–circumstellar medium (CSM) interaction in the Type Ia-CSM supernova (SN) 2018evt three years after the explosion, characterized by a rise in mid-infrared emission accompanied by an accelerated decline in the optical radiation of the SN. Such a dust-formation picture is also corroborated by the concurrent evolution of the profiles of the Hα emission line. Our model suggests enhanced CSM dust concentration at increasing distances from the SN as compared to what can be expected from the density profile of the mass loss from a steady stellar wind. By the time of the last mid-infrared observations at day +1,041, a total amount of 1.2 ± 0.2 × 10 −2   M ⊙ of new dust has been formed by SN 2018evt, making SN 2018evt one of the most prolific dust factories among supernovae with evidence of dust formation. The unprecedented witness of the intense production procedure of dust may shed light on the perceptions of dust formation in cosmic history. By day 1,041 after explosion, SN Ia-CSM 2018evt had produced an estimated 0.01 solar masses of dust in the cold, dense shell behind the supernova ejecta–circumstellar medium interaction, ranking it as one of the most prolific dust-producing supernovae ever recorded.

In the transitional mass range (~8–10 solar masses) between white dwarf formation and iron core-collapse supernovae, stars are expected to produce an electron-capture supernova. Theoretically, these progenitors are thought to be super-asymptotic giant branch stars with a degenerate O + Ne + Mg core, and electron capture onto Ne and Mg nuclei should initiate core collapse 1 – 4 . However, no supernovae have unequivocally been identified from an electron-capture origin, partly because of uncertainty in theoretical predictions. Here we present six indicators of electron-capture supernovae and show that supernova 2018zd is the only known supernova with strong evidence for or consistent with all six: progenitor identification, circumstellar material, chemical composition 5 – 7 , explosion energy, light curve and nucleosynthesis 8 – 12 . For supernova 2018zd, we infer a super-asymptotic giant branch progenitor based on the faint candidate in the pre-explosion images and the chemically enriched circumstellar material revealed by the early ultraviolet colours and flash spectroscopy. The light-curve morphology and nebular emission lines can be explained by the low explosion energy and neutron-rich nucleosynthesis produced in an electron-capture supernova. This identification provides insights into the complex stellar evolution, supernova physics, cosmic nucleosynthesis and remnant populations in the transitional mass range. Electron-capture supernovae are thought to come from progenitors with a narrow range of masses, and thus they are rare. Here the authors present six indicators of an electron-capture supernova origin, and find that supernova 2018zd fulfils all six criteria.

Accreting supermassive black holes (SMBHs) can exhibit variable emission across the electromagnetic spectrum and over a broad range of timescales. The variability of active galactic nuclei (AGNs) in the ultraviolet and optical is usually at the few tens of per cent level over timescales of hours to weeks1. Recently, rare, more dramatic changes to the emission from accreting SMBHs have been observed, including tidal disruption events2–5, ‘changing look’ AGNs6–9 and other extreme variability objects10,11. The physics behind the ‘re-ignition’, enhancement and ‘shut-down’ of accretion onto SMBHs is not entirely understood. Here we present a rapid increase in ultraviolet–optical emission in the centre of a nearby galaxy, marking the onset of sudden increased accretion onto a SMBH. The optical spectrum of this flare, dubbed AT 2017bgt, exhibits a mix of emission features. Some are typical of luminous, unobscured AGNs, but others are likely driven by Bowen fluorescence—robustly linked here with high-velocity gas in the vicinity of the accreting SMBH. The spectral features and increased ultraviolet flux show little evolution over a period of at least 14 months. This disfavours the tidal disruption of a star as their origin, and instead suggests a longer-term event of intensified accretion. Together with two other recently reported events with similar properties, we define a new class of SMBH-related flares. This has important implications for the classification of different types of enhanced accretion onto SMBHs.Increased UV–optical nuclear emission in a nearby galaxy together with a spectrum showing emission lines typical of unobscured AGNs and Bowen fluorescence features suggests a longer-term event of intensified accretion onto the central supermassive black hole.

Type Ia supernovae are thermonuclear explosions of white dwarf stars. They play a central role in the chemical evolution of the Universe and are an important measure of cosmological distances. However, outstanding questions remain about their origins. Despite extensive efforts to obtain natal information from their earliest signals, observations have thus far failed to identify how the majority of them explode. Here, we present infant-phase detections of SN 2018aoz from a very low brightness of −10.5 AB absolute magnitude, revealing a hitherto unseen plateau in the B band that results in a rapid redward colour evolution between 1.0 and 12.4 hours after the estimated epoch of first light. The missing B -band flux is best explained by line-blanket absorption from Fe-peak elements in the outer 1% of the ejected mass. The observed B  −  V colour evolution of the supernova also matches the prediction from an over-density of Fe-peak elements in the same outer 1% of the ejected mass, whereas bluer colours are expected from a purely monotonic distribution of Fe-peak elements. The presence of excess nucleosynthetic material in the extreme outer layers of the ejecta points to enhanced surface nuclear burning or extended subsonic mixing processes in some normal type Ia SN explosions. Very early observations of a type Ia supernova—from within one hour of explosion—show a red colour that develops and rapidly disappears. These data provide information on the initial explosion mechanism: surface nuclear burning on the white dwarf or extreme mixing of the nuclear burning process.

A rare class of supernovae (SNe) is characterized by strong interaction between the ejecta and several solar masses of circumstellar matter (CSM) as evidenced by strong Balmer-line emission. Within the first few weeks after the explosion, they may display spectral features similar to overluminous Type Ia SNe, while at later phase their observation properties exhibit remarkable similarities with some extreme case of Type IIn SNe that show strong Balmer lines years after the explosion. We present polarimetric observations of SN2018evt obtained by the ESO Very Large Telescope from 172 to 219 days after the estimated time of peak luminosity to study the geometry of the CSM. The nonzero continuum polarization decreases over time, suggesting that the mass loss of the progenitor star is aspherical. The prominent H\\(\\alpha\\) emission can be decomposed into a broad, time-evolving component and an intermediate-width, static component. The former shows polarized signals, and it is likely to arise from a cold dense shell (CDS) within the region between the forward and reverse shocks. The latter is significantly unpolarized, and it is likely to arise from shocked, fragmented gas clouds in the H-rich CSM. We infer that SN2018evt exploded inside a massive and aspherical circumstellar cloud. The symmetry axes of the CSM and the SN appear to be similar. SN\\,2018evt shows observational properties common to events that display strong interaction between the ejecta and CSM, implying that they share similar circumstellar configurations. Our preliminary estimate also suggests that the circumstellar environment of SN2018evt has been significantly enriched at a rate of \\(\\sim0.1\\) M\\(_\\odot\\) yr\\(^{-1}\\) over a period of \\(>100\\) yr.

The non-detection of companion stars in Type Ia supernova (SN) progenitor systems lends support to the notion of double-degenerate (DD) systems and explosions triggered by the merging of two white dwarfs. This very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN\\,2018gv with the ESO Very Large Telescope at \\(-\\)13.6 days relative to the \\(B-\\)band maximum light, or \\(\\sim\\)5 days after the estimated explosion --- the earliest spectropolarimetric observations to date of any Type Ia SN. These early observations still show a low continuum polarization (\\(\\lesssim\\)0.2\\%) and moderate line polarization (0.30\\(\\pm\\)0.04\\% for the prominent \\ion{Si}{2} \\(\\lambda\\)6355 feature and 0.85\\(\\pm\\)0.04\\% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first \\(\\sim\\)100 days after the maximum light indicate that SN\\,2018gv is a normal Type Ia SN with similar spectrophotometric behavior to SN\\,2011fe.

Stars that contain only trace amounts of elements heavier than helium, referred to as having low \"metallicity\", preserve the chemical fingerprints of the first generation of stars and supernovae. In the Milky Way, the lowest metallicity stars show an extreme over-abundance of carbon relative to other elements, which has been hypothesized to be a unique result of the first low-energy supernovae. However, the origin of this signature has remained a mystery, since no such stars have been discovered in the ancient dwarf galaxies where they are thought to have formed. Here, we present observations of a star in the >10 billion year old ultra-faint dwarf galaxy Pictor II, that shows the lowest iron and calcium abundances outside the Milky Way (<1/43,000th solar and ~1/160,000th solar), with a factor of >3000x relative carbon enhancement. As the first unambiguous second-generation star in a relic dwarf galaxy, this object demonstrates that carbon-enhanced second-generation stars can originate in primordial small-scale systems. This star supports the hypothesis that carbon-enhancement is produced by low-energy-supernovae, since the yields of energetic supernovae are harder to retain in small-scale environments. This key local signature of chemical enrichment by the first stars traces a regime inaccessible to current high-redshift observations, which cannot detect the early enrichment of the smallest galaxies.

We present a catalog of 643 diffuse galaxies identified through a citizen science search of the Fornax cluster, of which we estimate 21.8% are nucleated (139/637; 6 inconclusive). This marks the first crowd-sourced effort to construct a cluster-scale census of diffuse galaxies. These objects were visually identified using a combination of the Fornax Deep Survey and Dark Energy Camera Legacy Survey imaging across 26 deg\\(^2\\). Over 1,400 volunteers cataloged the candidates within this sky area at a rate of 1.15 days/deg\\(^2\\). Our catalog is highly complete relative to existing dwarf catalogs of Fornax (\\(> 80\\%\\) of objects recovered) down to an effective radius \\(r_{\\mathrm{eff}} = 5^{\\prime \\prime}\\), the minimum size we suggested volunteers classify, and to an effective r-band surface brightness as faint as \\(\\langle \\mu_r \\rangle \\simeq26\\) mag arcsec\\(^{-2}\\). We detect 97 candidates that existing automated searches of Fornax did not find and three candidates not found by any prior search, automated or visual. The stellar mass distribution of our sample is consistent with similar dwarf studies of Fornax, with the nucleated fraction peaking at 80% for a host galaxy mass of \\(\\sim\\)10\\(^{8.5}M_{\\odot}\\). The efficiency and completeness of our catalog thus establishes citizen science as a valuable tool for mapping diffuse galaxy populations in future sky surveys, such as the Legacy Survey of Space and Time.

We investigate the ultra-diffuse galaxy (UDG) UGC 9050-Dw1, which was selected because of its disturbed morphology as part of a larger sample of UDGs that display evidence for significant interactions. We use the Hubble Space Telescope's Advanced Camera for Surveys to identify globular clusters (GCs) associated with UGC 9050-Dw1, and the Jansky Very Large Array to measure its \\(HI\\) content. UGC 9050-Dw1, a neighbor to the low surface brightness spiral UGC 9050, exhibits a unique UV bright central ``clump'' with clearly associated \\(HI\\) gas and an extended stellar tidal plume to the north. We identify \\(52^+4_-6\\) GCs, implying a specific frequency \\(S_N = 122_-24^+30\\), one of the highest reported for a UDG of this luminosity. Additionally, \\( 20\\%\\) of the total light of the galaxy is contributed by GCs. Nearly uniform GC colors suggest they were formed during a single intense episode of star formation. We propose that UGC 9050-Dw1 formed via a rare dwarf merger event where induced, clumpy star formation led to its current observed properties.