The death of a massive star is a spectacular event. When the star has consumed all of its nuclear fuel, its core collapses. This leads to a strong blast wave that ejects the stellar envelope at a velocity of about 1–3% of the speed of light, producing a supernova explosion1. In rare cases, the collapsing core also launches an energetic beam of matter and radiation, known as a jet, through the stellar envelope at almost the speed of light2. The jet emerges from the stellar surface and generates a bright burst of γ-rays, which can be seen only when the jet points directly towards Earth. Writing in Nature, Izzo et al.3 report spectroscopic observations of a supernova associated with a γ-ray burst, improving our understanding of the interplay between the jet and the exploding stellar envelope.
Theory predicts that, as a jet pushes its way through a stellar envelope, it heats the gas that surrounds its path, producing a highly pressurized, hot bubble known as a cocoon4–6 (Fig. 1a). As the jet breaks out of the star, so does the cocoon7 (Fig. 1b; see also the movies at go.nature.com/2sdoeao). The cocoon carries information about the interaction between the jet and the envelope and opens a window on the inner workings of the explosion.
The predicted signature of the cocoon initially includes a relatively faint flare of γ-rays that is emitted when the cocoon breaks out of the stellar envelope8–10. This is followed by a fading signal that is seen first in X-rays and later in ultraviolet and optical light11. In addition, the cocoon spreads sideways, engulfing the expanding envelope so that the supernova light must pass through the cocoon material on its way to Earth. During the first few days after the explosion, this passage leaves an imprint of broad absorption features on the spectrum of light from the supernova12. Later, as the cocoon expands, it becomes fully transparent, and the absorption features disappear.
For decades, astronomers have been observing the emissions from ejected envelopes and from jets — the supernovae and the γ-ray bursts, respectively. However, although cocoons are predicted to transport as much energy as the other two components11, identifying their observational signatures turned out to be extremely difficult. When a jet is pointing towards Earth, the γ-ray burst and the afterglow of the burst outshine the cocoon emission, as well as the supernova light, for several days. And when a jet is pointing away from Earth, the supernova is usually detected too late, after the absorption signal has disappeared. Izzo and colleagues present observations of an event in which a γ-ray burst is seen, but the afterglow of the burst is faint enough to allow the cocoon to be detected within a day of the stellar explosion.
According to theoretical models11, the cocoon moves faster than the envelope, but more slowly than the jet. Moreover, the jet is expected to drag material from the core of the exploding star and to deposit it in the cocoon13,14. Indeed, Izzo et al. find broad absorption features in the supernova spectra that correspond to an outflow that is moving up to one-third the speed of light. These features indicate that the fast-moving material is substantially enriched in ‘iron-peak’ elements (such as iron, cobalt and nickel), which must have been produced in the stellar core during the explosion. The authors’ observations provide direct evidence that material from the stellar core is dragged along by the jet and deposited in the outermost layers of the envelope.
Although the γ-ray burst itself was faint, suggesting a weak jet, the energy deposited in the cocoon revealed that the jet was highly energetic. This finding might imply that the observed faint γ-rays originate from a jet that is pointing slightly away from Earth. Alternatively, a more intriguing possibility, and one that I think is more probable, is that we are seeing the cocoon directly, and that the faint γ-rays are the predicted signal of the cocoon breaking out of the stellar envelope8–10. Evidently, Izzo and colleagues’ exquisite data set carries a lot of information, and considerable theoretical work will be needed to uncover its full ramifications.
Even after decades of extensive astrophysical study, the processes by which massive stars explode are not fully understood, and the role of jets, if they have one, is unknown. Moreover, although jets are rarely observed in these explosions, it is possible that they are much more common than the observations would suggest. Jets require specific conditions to successfully cross the entire stellar envelope and break out. Most probably, many jets are ‘choked’ — they dissipate all of their energy while still buried in the envelope15. Although such choked jets cannot be seen directly, their cocoons can still break out (Fig. 1c; see also the movies at go.nature.com/2sdoeao), and therefore hold the key to identifying these hidden jets.
Remarkably, choked jets in supernovae might have been identified already. Broad absorption features from fast-moving material were seen in early spectra of several supernovae, and have been interpreted as the signature of jet-driven cocoons12. Most intriguingly, these features have been found not only in supernovae that are associated with γ-ray bursts, but also in several that are not12. Izzo and colleagues’ findings support the interpretation of these observations as the marks of hidden jets.
The future looks promising. Next-generation wide-field optical surveys will detect many supernovae within less than a day of their explosion, enabling an effective search for the telltale cocoons. Izzo and colleagues’ results pave the way for these searches, which could resolve the long-standing enigma concerning the role of jets in the explosive deaths of massive stars.
Nature 565, 300-301 (2019)