Astronomy

Supernova bursts onto the scene

The stellar explosions known as supernovae are spectacular but common cosmic events. A satellite telescope's chance observation of a burst of X-ray light might be the first record of a supernova's earliest minutes.

Once the processes of nuclear fusion that have bolstered it against its own gravity are exhausted, the core of a massive star collapses in on itself. The result is a cataclysmic explosion that sends a violent shock wave racing outwards. As this shock wave reaches the star's surface, it produces a short, sharp burst of X-ray or ultraviolet radiation, the prelude to the expulsion of most of the star's matter into the surrounding medium. Lasting days to months, we see this aftermath of the explosion as a supernova.

That is the theory, at any rate. But although supernovae themselves are common enough, the chain of events that lead up to them — in particular, the exact moment of 'shock break-out' — had never been seen. That all changes with a report from Soderberg et al. (page 469)1. They observed an intense, but short-lived, X-ray outburst from the same point in the sky where shortly afterwards a supernova flared up, and have thus provided valuable support for the prevalent theories of supernova progenitors.

The authors' discovery was serendipitous: they just happened to be examining the aftermath of a similar supernova, of 'type Ibc', in the same galaxy. The instrument they were using, NASA's Swift satellite, was primarily intended to pinpoint the mysterious flashes of intense, high-energy light known as γ-ray bursts. But, while pursuing this successful main career, the telescope has also developed a useful sideline in X-ray and optical follow-up observations of supernovae.

What Swift spotted1 was an X-ray outburst that lasted for some 10 minutes. Its energy content was around 1039 joules, about a hundred-thousandth of the energy expelled in the explosive motions of a supernova. Continued observation of the position of the outburst showed the emergence of a spectrum and an evolution of emission intensity over time typical of a type-Ibc supernova, albeit with a slightly fainter peak luminosity than normal.

The exploding object was also detected by NASA's Chandra X-ray observatory 10 days after the X-ray outburst, as well as in a series of radio measurements between 3 and 70 days after. Similar observations characterize type-Ibc supernovae, and are thought to relate to interaction of the expanding supernova with mass lost from its progenitor before the explosion, which encircles the star as a surrounding 'wind' (Fig. 1). The interaction generates shock waves that accelerate electrons to almost light speed. These electrons in turn emit radio-frequency synchrotron radiation as their paths curve in the ambient magnetic field, and scatter photons from the visible surface of the star, the photosphere, up to X-ray energies.

Figure 1: Shock break-out.
figure1

Soderberg and colleagues' observations1 of an X-ray outburst preceding a type-Ibc supernova provide sterling support for prevalent models of how these cosmic explosions occur. a, Once its nuclear fuel is exhausted, the core of a massive star collapses in on itself, generating a huge explosion that propagates outwards as a shock wave. The progenitor star is surrounded by a 'wind' of gas previously lost from the star. b, After a matter of minutes, the shock wave reaches the surface of the star (the photosphere) and radiation from the explosion that is trailing in its wake escapes, accelerating the surrounding gas outwards. The moment of this 'shock break-out' is what the authors succeeded in capturing. c, Days later, a layer of hot gas has developed where the rapidly expanding, but cool, supernova gas impacts on the surrounding wind. This shocked layer is the site of electrons moving close to the speed of light that are responsible for radio and X-ray emission, typical supernova signatures.

Taken together, these observations seem to add up to the identification of the X-ray outburst with the supernova — now designated SN 2008D — that followed. One caveat is that, although the energy of the outburst was close to predictions for the shock break-out of a type-Ibc supernova2, its duration was much longer than expected. The length of the burst should be determined by the time light needs to cross the supernova progenitor, which is 10 seconds or less. The implication, therefore, is that the photosphere of the progenitor star extends farther than expected, perhaps because it has shed a large amount of material before the supernova occurs.

Within the star, the energy behind the shock wave emanating from the core's collapse is dominated by radiation. Outside, it is dominated by gas energy. Shock break-out occurs at the transition between these two modes, when the radiation behind the internal shock wave spreads out into the circumstellar medium and accelerates its gas. As the inner, already accelerated layers of gas catch up with outer, slower-moving layers, an external gas shock wave develops. Soderberg et al.1 suggest that the observed spectrum of the X-ray burst is determined by the shock acceleration of photons from the supernova photosphere. Detailed hydrodynamic simulations that allow for the effects of a radiation field that is out of equilibrium will be needed before we can properly model the niceties of the transition.

Is there any other possible interpretation of the X-ray outburst, other than the emergence of a supernova shock? Might the outburst simply be a lower-energy cousin of a γ-ray burst? These bursts are thought to be produced by the same type of progenitor as type-Ibc supernovae, albeit less than 100 times as frequently. Their pathology is very different: rather than being the result of a spherical shock wave rippling through the star, they are assumed to be caused by directed, relativistic jets of particles and magnetic fields that are generated by a central black hole or neutron star and then burrow through their surrounds.

The X-ray outburst from SN 2008D was much weaker than a γ-ray burst, although that might simply represent a downwards extension of the permissible intensity range3,4. It has also recently been suggested4 that the outburst shares with γ-ray bursts certain relationships between the amount of energy radiated isotropically (that is, equally in all directions), its peak spectral energy and the peak luminosity of the ensuing supernova. But speaking against the γ-ray-burst interpretation is not only the weight of post-burst observations seeming to indicate a normal supernova, but also the lack of firm evidence for the relativistic motions that are a signature of γ-ray bursts.

The observations1 of SN 2008D would thus seem to be the earliest of light emanating from a supernova, just minutes after core collapse. NASA's Galaxy Evolution Explorer (GALEX) has recently seen the rising ultraviolet emission associated with shock break-out from type-II supernovae5,6. The progenitors of these supernovae are surrounded by an envelope of hydrogen gas, and the shock wave takes close to a day to traverse them.

As well as telling us more about the types of star that produce supernovae, such observations of shock break-out, by helping to tie down the time of core collapse, could provide useful auxiliary information in terrestrial hunts for exotic citizens of the cosmos thought to be produced in these cosmic cataclysms. The benefits could be felt by neutrino detectors, and by detectors searching for evidence of the elusive ripples in space-time known as gravitational waves.

References

  1. 1

    Soderberg, A. M. et al. Nature 453, 469–474 (2008).

  2. 2

    Matzner, C. D. & McKee, C. F. Astrophys. J. 510, 379–403 (1999).

  3. 3

    Xu, D., Zou, Y.-C. & Fan, Y.-Z. preprint at http://arxiv.org/abs/0801.4325 (2008).

  4. 4

    Li, L.-X. preprint at http://arxiv.org/abs/0803.0079 (2008).

  5. 5

    Schawinski, K. et al. preprint at http://arxiv.org/abs/0803.3596 (2008).

  6. 6

    Gezari, S. et al. preprint at http://arxiv.org/abs/arxiv:0804.1123 (2008).

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Chevalier, R. Supernova bursts onto the scene. Nature 453, 462–463 (2008). https://doi.org/10.1038/453462a

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