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Optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and a low-luminosity short-duration γ-ray burst are presented and modelled. When neutron stars collide Merging neutron stars are potential sources of gravitational waves and have long been predicted to produce jets of material as part of a low-luminosity transient known as a 'kilonova'. There is growing evidence that neutron-star mergers also give rise to short, hard gamma-ray bursts. A group of papers in this issue report observations of a transient associated with the gravitational-wave event GW170817—a signature of two neutron stars merging and a gamma-ray flash—that was detected in August 2017. The observed gamma-ray, X-ray, optical and infrared radiation signatures support the predictions of an outflow of matter from double neutron-star mergers and present a clear origin for gamma-ray bursts. Previous predictions differ over whether the jet material would combine to form light or heavy elements. These papers now show that the early part of the outflow was associated with lighter elements whereas the later observations can be explained by heavier elements, the origins of which have been uncertain. However, one paper (by Stephen Smartt and colleagues) argues that only light elements are needed for the entire event. Additionally, Eleonora Troja and colleagues report X-ray observations and radio emissions that suggest that the 'kilonova' jet was observed off-axis, which could explain why gamma-ray-burst detections are seen as dim. The merger of two neutron stars has been predicted to produce an optical–infrared transient (lasting a few days) known as a ‘kilonova’, powered by the radioactive decay of neutron-rich species synthesized in the merger 1 , 2 , 3 , 4 , 5 . Evidence that short γ-ray bursts also arise from neutron-star mergers has been accumulating 6 , 7 , 8 . In models 2 , 9 of such mergers, a small amount of mass (10 −4 –10 −2 solar masses) with a low electron fraction is ejected at high velocities (0.1–0.3 times light speed) or carried out by winds from an accretion disk formed around the newly merged object 10 , 11 . This mass is expected to undergo rapid neutron capture (r-process) nucleosynthesis, leading to the formation of radioactive elements that release energy as they decay, powering an electromagnetic transient 1 , 2 , 3 , 9 , 10 , 11 , 12 , 13 , 14 . A large uncertainty in the composition of the newly synthesized material leads to various expected colours, durations and luminosities for such transients 11 , 12 , 13 , 14 . Observational evidence for kilonovae has so far been inconclusive because it was based on cases 15 , 16 , 17 , 18 , 19 of moderate excess emission detected in the afterglows of γ-ray bursts. Here we report optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and with a low-luminosity short-duration γ-ray burst 20 . Our observations, taken roughly every eight hours over a few days following the gravitational-wave trigger, reveal an initial blue excess, with fast optical fading and reddening. Using numerical models 21 , we conclude that our data are broadly consistent with a light curve powered by a few hundredths of a solar mass of low-opacity material corresponding to lanthanide-poor (a fraction of 10 −4.5 by mass) ejecta.

In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster's gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.

Archival images of the progenitor system of supernova SN 2011fe are so sensitive that the presence of luminous red giants or most helium stars is directly ruled out. Identification of a supernova companion Supernova 2011fe in the Pinwheel galaxy, discovered by the Palomar Transient Factory on 24 August 2011, is the brightest type Ia supernova that's been seen from Earth for many years. Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. Two new reports of observations of SN 2011fe narrow down the range of possibilities for the mystery companion. Nugent et al . present some of the earliest data ever obtained from a type Ia supernova. They find that the exploding star was probably a carbon–oxygen white dwarf, and conclude from the lack of an early shock that the companion may have been a main sequence star. Li et al . analysed pre-discovery images in the Hubble Space Telescope archives and find that no object was visible before the explosion. That rules out luminous red giants and the vast majority of helium stars as the mass-donating companion to an exploding white dwarf. Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system 1 , 2 , but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models 1 , 3 : double-degenerate (involving two white dwarfs in a close binary system 2 , 4 ) and single-degenerate models 5 , 6 . In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy 7 . Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10–100 times fainter than previous limits on other type Ia supernova progenitor systems 8 , 9 , 10 , allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf.

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.

In this study, we analyzed the optical observations of a subluminous Type Ia supernova (SN Ia) 2017fzw, which exhibited high photospheric velocity (HV) at B-band maximum light. The absolute B-band peak magnitude was determined to be MmaxB=−18.65±0.13 mag, similar to 91bg-like SNe Ia. An estimation of the rate of decline for the B-band light curve was determined to be Δm15(B)=1.60±0.06 mag. The spectra of SN 2017fzw were similar to those of 91bg-like SNe Ia, with prominent Ti ii and Si ii λ5972 features at early phases, gradually transitioning to spectra resembling normal (mainly HV subclass) SNe Ia at later phases, with a stronger Ca ii NIR feature. Notably, throughout all phases of observation, SN 2017fzw displayed spectral evolution characteristics that were comparable to those of HV SNe Ia, and at peak brightness, the Si ii λ6355 velocity was determined to be 13,800 ± 415 km s−1 and a more pronounced Ca ii NIR feature was also detected. Based on these findings, we classify SN 2017fzw as a transitional object with properties of both normal and 91bg-like SNe Ia, providing support for the hypothesis of a continuous distribution of supernovae between these two groups.

The detection of the luminous, blue progenitor system of the type Iax supernova 2012Z suggests that this supernova was the explosion of a white dwarf accreting material from a helium-star companion. A type Iax supernova progenitor SN 2012Z, discovered in the Lick Observatory Supernova Search on 29 January 2012, is a type Iax supernova. Sometimes referred to as 'mini supernovae', these are initially spectroscopically similar to some type-Ia supernovae but diverge with time and are much less energetic and fainter. It is not clear what triggers a type Iax explosion. This paper reports the detection of a progenitor in deep observations of NGC 1309, the host galaxy of SN 2012Z, obtained with the Hubble Space Telescope and including the location of the supernova before its explosion. Its optical properties and similarity to the progenitor of the helium nova V445 Puppis suggest that SN 2012Z was probably an explosion of a white dwarf accreting from a helium-star companion. Type Iax supernovae are stellar explosions that are spectroscopically similar to some type Ia supernovae at the time of maximum light emission, except with lower ejecta velocities 1 , 2 . They are also distinguished by lower luminosities. At late times, their spectroscopic properties diverge from those of other supernovae 3 , 4 , 5 , 6 , but their composition (dominated by iron-group and intermediate-mass elements 1 , 7 ) suggests a physical connection to normal type Ia supernovae. Supernovae of type Iax are not rare; they occur at a rate between 5 and 30 per cent of the normal type Ia rate 1 . The leading models for type Iax supernovae are thermonuclear explosions of accreting carbon–oxygen white dwarfs that do not completely unbind the star 8 , 9 , 10 , implying that they are ‘less successful’ versions of normal type Ia supernovae, where complete stellar disruption is observed. Here we report the detection of the luminous, blue progenitor system of the type Iax SN 2012Z in deep pre-explosion imaging. The progenitor system's luminosity, colours, environment and similarity to the progenitor of the Galactic helium nova V445 Puppis 11 , 12 , 13 suggest that SN 2012Z was the explosion of a white dwarf accreting material from a helium-star companion. Observations over the next few years, after SN 2012Z has faded, will either confirm this hypothesis or perhaps show that this supernova was actually the explosive death of a massive star 14 , 15 .

Asteroseismology probes the internal structures of stars by using their natural pulsation frequencies 1 . It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars 2 , red giants 3 , high-mass stars 4 and white dwarfs 5 . However, a large group of pulsating stars of intermediate mass—the so-called δ Scuti stars—have rich pulsation spectra for which systematic mode identification has not hitherto been possible 6 , 7 . This arises because only a seemingly random subset of possible modes are excited and because rapid rotation tends to spoil regular patterns 8 – 10 . Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, which enables definitive mode identification. The space motions of some of these stars indicate that they are members of known associations of young stars, as confirmed by modelling of their pulsation spectra. The pulsation spectra of intermediate-mass stars (so-called δ Scuti stars) have been challenging to analyse, but new observations of 60 such stars reveal remarkably regular sequences of high-frequency pulsation modes.

Observations of an event (several energetic eruptions leading to a terminal explosion that is surprisingly hydrogen-rich) with the spectrum of a supernova do not match with other observations of supernovae. A very unusual supernova Thousands of 'core-collapse' supernovae have been observed over the past 15 years, with common observational elements such as hydrogen absorption lines that slow over time and a single light-curve peak or luminosity that plateaus for around 100 days before declining. Iair Arcavi and colleagues report observations of the supernova iPTF14hls, which does not display the usual elements. Its light curve has multiple peaks and extends over 600 days. They conclude that the properties could be explained by ejection of several tens of solar masses of gas a few hundred days before the explosion, but there is no viable explanation for how this occurred. Although multiple pre-supernova eruptions are predicted by the pulsational pair instability, that model is inconsistent with the energetics involved here and the continued presence of hydrogen absorption lines with no decrease in velocity. Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining 1 . Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability 2 , 3 , 4 , 5 . That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.

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.

Galaxy-cluster gravitational lenses can magnify background galaxies by a total factor of up to ~50. Here we report an image of an individual star at redshift z  = 1.49 (dubbed MACS J1149 Lensed Star 1) magnified by more than ×2,000. A separate image, detected briefly 0.26″ from Lensed Star 1, is probably a counterimage of the first star demagnified for multiple years by an object of ≳3 solar masses in the cluster. For reasonable assumptions about the lensing system, microlensing fluctuations in the stars’ light curves can yield evidence about the mass function of intracluster stars and compact objects, including binary fractions and specific stellar evolution and supernova models. Dark-matter subhaloes or massive compact objects may help to account for the two images’ long-term brightness ratio. An individual star at z  = 1.49 is gravitationally lensed and highly magnified by a foreground galaxy cluster. Fluctuations in the star’s emission provide insight on the mass function of intracluster stars, compact objects and the presence of dark-matter subhaloes.