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When massive stars exhaust their fuel, they collapse and often produce the extraordinarily bright explosions known as core-collapse supernovae. On occasion, this stellar collapse also powers an even more brilliant relativistic explosion known as a long-duration γ-ray burst. One would then expect that these long γ-ray bursts and core-collapse supernovae should be found in similar galactic environments. Here we show that this expectation is wrong. We find that the γ-ray bursts are far more concentrated in the very brightest regions of their host galaxies than are the core-collapse supernovae. Furthermore, the host galaxies of the long γ-ray bursts are significantly fainter and more irregular than the hosts of the core-collapse supernovae. Together these results suggest that long-duration γ-ray bursts are associated with the most extremely massive stars and may be restricted to galaxies of limited chemical evolution. Our results directly imply that long γ-ray bursts are relatively rare in galaxies such as our own Milky Way. Not in our back yard In their death throes massive stars often produce supernovae, and occasionally a long-duration γ-ray burst (GRB). That suggests that GRBs and supernovae should be found in similar environments, but work based on more than a thousand hours of Hubble Space Telescope observation time shows that expectation to be wrong. Most long GRBs are found in small, faint, irregular galaxies. Supernovae appear equally divided between spiral and irregular galaxies. GRBs are concentrated in the brightest regions of their host galaxies whereas supernovae occur throughout their host galaxies. A happy conclusion of this finding is that GRBs, which would cause havoc here on Earth if exploding nearby, should be relatively rare in the Milky Way. γ-ray bursts are more concentrated in the very brightest regions of their host galaxies than are supernovae — in addition, the host galaxies of the γ-ray bursts are significantly fainter and more irregular than the hosts of the supernovae.

X-Rated Supernova A link between long γ-ray bursts (GRBs) and supernovae has been established, but whether there is a similar relationship between the weaker and softer X-ray flashes and supernovae is unclear. GRB/XRF 060218, spotted by the Swift satellite on 18 February this year, may supply that missing link. In the first of four papers on this novel burster, Campana et al . report the sighting of the X-ray signature of a shock break-out, possible evidence of a supernova in progress. Pian et al . report the optical discovery of a type Ic supernova 2006aj associated with GRB/XRF 060218. Soderberg et al . report radio and X-ray observations that show that XRF 060218 is 100 times less energetic than, but of a type that is ten times more common than cosmological GRBs. Mazzali et al . modelled the spectra and light curve of SN 2006aj to show that it had a much smaller explosion energy and ejected much less mass than other GRB-supernovae, suggesting that it was produced by a star with a mass was only about 20 times that of the Sun, leaving behind a neutron star, rather than a black hole. A report of the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. SN 2006aj was intrinsically less luminous than the gamma-ray burst (GRB)–supernovae connection, but more luminous than many supernovae not accompanied by a GRB. Long-duration γ-ray bursts (GRBs) are associated with type Ic supernovae 1 that are more luminous than average 2 , 3 , 4 , 5 and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB–supernovae were thought to be rare events 6 . Whether X-ray flashes—analogues of GRBs, but with lower luminosities and fewer γ-rays—can also be associated with supernovae, and whether they are intrinsically ‘weak’ events or typical GRBs viewed off the axis of the burst 7 , is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB–supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output 8 and the supernova radio flux 9 . Our data, combined with radio and X-ray observations 8 , 9 , 10 , suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB–supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB–supernovae.

The detection of strong emission lines in an early-time spectrum of type IIb supernova SN 2013cu reveals Wolf–Rayet-like wind signatures, suggesting that the supernova’s progenitor may have been a Wolf–Rayet star with a wind dominated by helium and nitrogen, with traces of hydrogen. Progression from Wolf-Rayet star to type IIb supernova Wolf–Rayet stars, massive objects stripped of their outer hydrogen-rich envelope, are one of a number of candidates as supernova progenitors for type IIb, Ib and Ic explosions. This paper reports the detection of strong emission lines in early spectra — just 15 hours after the blast — from the type IIb supernova SN 2013cu that are consistent with a Wolf–Rayet star as progenitor. The extent of this dense supernova wind implies possible increased mass loss from the progenitor shortly before the explosion, consistent with recent theoretical predictions. An accompanying News & Views article suggests that the new findings are the most direct evidence yet that these massive stars do end their lives as supernovae. The explosive fate of massive Wolf–Rayet stars 1 (WRSs) is a key open question in stellar physics. An appealing option is that hydrogen-deficient WRSs are the progenitors of some hydrogen-poor supernova explosions of types IIb, Ib and Ic (ref. 2 ). A blue object, having luminosity and colours consistent with those of some WRSs, has recently been identified in pre-explosion images at the location of a supernova of type Ib (ref. 3 ), but has not yet been conclusively determined to have been the progenitor. Similar work has so far only resulted in non-detections 4 . Comparison of early photometric observations of type Ic supernovae with theoretical models suggests that the progenitor stars had radii of less than 10 12  centimetres, as expected for some WRSs 5 . The signature of WRSs, their emission line spectra, cannot be probed by such studies. Here we report the detection of strong emission lines in a spectrum of type IIb supernova 2013cu (iPTF13ast) obtained approximately 15.5 hours after explosion (by ‘flash spectroscopy’, which captures the effects of the supernova explosion shock breakout flash on material surrounding the progenitor star). We identify Wolf–Rayet-like wind signatures, suggesting a progenitor of the WN(h) subclass (those WRSs with winds dominated by helium and nitrogen, with traces of hydrogen). The extent of this dense wind may indicate increased mass loss from the progenitor shortly before its explosion, consistent with recent theoretical predictions 6 .

With the advent of new wide-field, high-cadence optical transient surveys, our understanding of the diversity of core-collapse supernovae has grown tremendously in the last decade. However, the pre-supernova evolution of massive stars, which sets the physical backdrop to these violent events, is theoretically not well understood and difficult to probe observationally. Here we report the discovery of the supernova iPTF 13dqy = SN 2013fs a mere ∼3 h after explosion. Our rapid follow-up observations, which include multiwavelength photometry and extremely early (beginning at ∼6 h post-explosion) spectra, map the distribution of material in the immediate environment (≲10 15  cm) of the exploding star and establish that it was surrounded by circumstellar material (CSM) that was ejected during the final ∼1 yr prior to explosion at a high rate, around 10 −3 solar masses per year. The complete disappearance of flash-ionized emission lines within the first several days requires that the dense CSM be confined to within ≲10 15  cm, consistent with radio non-detections at 70–100 days. The observations indicate that iPTF 13dqy was a regular type II supernova; thus, the finding that the probable red supergiant progenitor of this common explosion ejected material at a highly elevated rate just prior to its demise suggests that pre-supernova instabilities may be common among exploding massive stars. Type II supernova explosions are common, but our understanding of such events is not complete. Such an event was observed just three hours after the explosion started, providing important information about the early stages.

The discovery of afterglows associated with γ-ray bursts at X-ray 1 , optical 2 and radio 3 wavelengths and the measurement of the redshifts of some of these events 4 , 5 has established that γ-ray bursts lie at extreme distances, making them the most powerful photon-emitters known in the Universe. Here we report the discovery of transient optical emission in the error box of the γ-ray burst GRB980425, the light curve of which was very different from that of previous optical afterglows associated with γ-ray bursts. The optical transient is located in a spiral arm of the galaxy ESO184-G82, which has a redshift velocity of only 2,550 km s −1 ( ref. 6 ). Its optical spectrum and location indicate that it is a very luminous supernova 7 , which has been identified as SN1998bw. If this supernova and GRB980425 are indeed associated, the energy radiated in γ-rays is at least four orders of magnitude less than in other γ-ray bursts, although its appearance was otherwise unremarkable: this indicates that very different mechanisms can give rise to γ-ray bursts. But independent of this association, the supernova is itself unusual, exhibiting an unusual light curve at radio wavelengths that requires that the gas emitting the radio photons be expanding relativistically 8 , 9 .

When a star passes within the tidal radius of a supermassive black hole, it will be torn apart 1 . For a star with the mass of the Sun ( M ⊙ ) and a non-spinning black hole with a mass <10 8 M ⊙ , the tidal radius lies outside the black hole event horizon 2 and the disruption results in a luminous flare 3–6 . Here we report observations over a period of ten months of a transient, hitherto interpreted 7 as a superluminous supernova 8 . Our data show that the transient rebrightened substantially in the ultraviolet and that the spectrum went through three different spectroscopic phases without ever becoming nebular. Our observations are more consistent with a tidal disruption event than a superluminous supernova because of the temperature evolution 6 , the presence of highly ionized CNO gas in the line of sight 9 and our improved localization of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely 10,11 . While the supermassive black hole has a mass >10 8 M ⊙ 12,13 , a star with the same mass as the Sun could be disrupted outside the event horizon if the black hole were spinning rapidly 14 . The rapid spin and high black hole mass can explain the high luminosity of this event. Transient object ASASSN-15lh was previously cast as the most luminous supernova ever discovered. Now, however, there is convincing evidence that its flare was a tidal disruption event: a rapidly-spinning black hole tearing apart a neighbouring star.

Hydrogen-poor superluminous supernovae (SLSN-I) are a class of rare and energetic explosions that have been discovered in untargeted transient surveys in the past decade 1 , 2 . The progenitor stars and the physical mechanism behind their large radiated energies (about 10 51  erg or 10 44  J) are both debated, with one class of models primarily requiring a large rotational energy 3 , 4 and the other requiring very massive progenitors that either convert kinetic energy into radiation through interaction with circumstellar material (CSM) 5 – 8 or engender an explosion caused by pair-instability (loss of photon pressure due to particle–antiparticle production) 9 , 10 . Observing the structure of the CSM around SLSN-I offers a powerful test of some scenarios, although direct observations are scarce 11 , 12 . Here, we present a series of spectroscopic observations of the SLSN-I iPTF16eh, which reveal both absorption and time- and frequency-variable emission in the Mg ii resonance doublet. We show that these observations are naturally explained as a resonance scattering light echo from a circumstellar shell. Modelling the evolution of the emission, we infer a shell radius of 0.1 pc and velocity of 3,300 km s −1 , implying that the shell was ejected three decades before the supernova explosion. These properties match theoretical predictions of shell ejections occurring because of pulsational pair-instability and imply that the progenitor had a helium core mass of about 50–55 M ⊙ , corresponding to an initial mass of about 115 M ⊙ . Probing the pre-explosion environments of hydrogen-poor superluminous supernovae is important for understanding how they exploded. Here, Lunnan et al. infer the presence of a fast-moving circumstellar shell around iPTF16eh through the detection of a resonance-line light echo, which indicates the massive progenitor experienced pulsational pair instability shell ejections.

Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of γ-ray, optical, infrared, submillimetre, millimetre and radio observations of the burst GRB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.

The discovery of the unusual supernova SN1998bw, and its possible association with the γ-ray burst GRB 980425 1 , 2 , 3 , provide new insights into the explosion mechanism of very massive stars and the origin of some classes of γ-ray bursts. Optical spectra indicate that SN1998bw is a type Ic supernova 3 , 4 , but its peak luminosity is unusually high compared with typical type Ic supernovae 3 . Here we report our findings that the optical spectra and the light curve of SN1998bw can be well reproduced by an extremely energetic explosion of a massive star composed mainly of carbon and oxygen (having lost its hydrogen and helium envelopes). The kinetic energy of the ejecta is as large as +(2–5)× 10 52  erg, more than ten times that of previously observed supernovae. This type of supernova could therefore be termed ‘hypernova’. The extremely large energy suggests the existence of a new mechanism of massive star explosion that can also produce the relativistic shocks necessary to generate the observed γ-rays.