An event that initially resembled an ordinary supernova explosion continued to erupt brightly for more than 600 days. Standard theoretical models cannot explain the event's properties. See Letter p.210
When a star that has more than about eight times the mass of the Sun dies, it either collapses to a black hole or explodes as a supernova1. In the latter case, a blast at the centre of the star — usually caused by the formation of a stellar remnant called a neutron star — ejects the surrounding material at high speed. The expansion of this material releases trapped energy, providing a nearly constant luminosity (equivalent to that of about 100 million Suns) for roughly 100 days, before fading. Supernovae lasting more than 130 days are extremely rare2. On page 210, Arcavi et al.3 report that a supernova known as iPTF14hls glowed brilliantly for more than 600 days, making it the longest-lived bright supernova ever observed.
The supernova iPTF14hls was discovered in September 2014 by the Intermediate Palomar Transient Factory telescope near San Diego, California, and seemed to be an ordinary supernova already well under way. It was infrequently observed over the next few months, and was eventually classified4 as a common type of supernova called type II-P in January 2015. Its subsequent failure to decline led Arcavi and colleagues to flag it for special attention.
Not only did iPTF14hls not fade, but as months stretched into years, the authors found that its brightness varied by as much as 50% on an irregular timescale, as if it were exploding again and again. Although not among the most luminous supernovae ever seen, iPTF14hls was brighter than an ordinary type II-P explosion and, by lasting so long, it radiated much more energy.
Just as the light from iPTF14hls refused to fade, the spectrum of the light refused to evolve. Usually, as a supernova expands, it reveals deeper, more-slowly moving material, and the lines in its spectrum get narrower. But in the case of iPTF14hls, Arcavi et al. found that the light-emitting region maintained the same speed throughout the supernova's lifetime.
This result might make sense if the authors had observed the emission from a single shell of ejected matter. However, the radius of such a shell would have increased over time. Radiation theory tells us that if something gets bigger but maintains its luminosity, it should also get cooler. Confounding expectations again, iPTF14hls remained at the same temperature (about 6,000 kelvin), suggesting that the radius of the light-emitting region was roughly constant. What was going on?
Supernovae shine for one of four reasons5: radioactive decay; radiation released by the shock-heated envelope of a massive star as it expands and cools (ordinary type II-P supernovae); colliding shells that convert kinetic energy into light (type IIn supernovae); or radiation from a central, compact stellar object such as a magnetar. In the case of iPTF14hls, radioactivity can be ruled out because the isotopes that have the correct lifetime to explain the emission are not sufficiently abundant in the explosion. Likewise, radiation from a shock-heated envelope would require an envelope mass and an explosion energy that are incompatible with our understanding of stellar evolution.
It therefore falls to a magnetar or colliding shells to explain iPTF14hls. Arcavi and colleagues explore both possibilities and rule out the simplest models. Using standard formulae, which might not be adequate for this event, they conclude that the initial luminosity of a magnetar would be too high to explain their observations. Similarly, the colliding shells that are typical of type IIn supernovae would produce X-ray and radio emission that was not seen for iPTF14hls, and narrower spectral lines than were observed.
Having ruled out all of the standard theoretical models, Arcavi et al. tie their hopes to an alternative scenario: a pulsational pair-instability supernova6. In this model, during the death of an extremely massive star, violent thermonuclear instabilities in the final stages of nuclear fusion lead to repeated supernova-like outbursts. Each outburst ejects a few solar masses of material in pulses, and can do so for a long time without destroying the star. For stars born with masses near 105 solar masses, the outbursts can continue for about 2 years — in agreement with the overall timescale for iPTF14hls.
The model also predicts that, before the violent pulses, the star loses about half of its mass to stellar winds, and perhaps to earlier pulses — the authors note that a possible eruption occurred 60 years before iPTF14hls, in the same location. About 10 solar masses are then ejected in the explosion itself. The remaining 40 solar masses or so collapse to a black hole, which might provide additional luminosity by accreting matter that failed to escape.
Pulsational pair-instability supernovae have not been definitively observed, but, in theory, the emitted radiation results from colliding shells (Fig. 1). These shells move more quickly than in type IIn supernovae, with the fastest material being ejected in the first pulse. Consequently, the shells collide far from their origin, and the emitted radiation can readily escape, rather than being trapped and cooled as the supernova expands. The combined kinetic energy of many such shells (almost 1051 erg) can account for the total radiated energy from iPTF14hls.
Nevertheless, there are problems with this scenario. For example, the model does not explain the supernova's constant temperature. Additionally, Arcavi et al. estimate that the high-speed ejection of tens of solar masses is required to explain certain lines in the observed spectrum. Such an ejection would need about 20 times more energy than could be supplied by a pulsational pair-instability supernova6. However, the authors' estimate is uncertain, and relies on the assumption that the supernova emits like a black body (releasing light that has a specific spectrum) at all times. In fact, the colliding shells might have provided extra ionizing radiation that would reduce the energy requirement, although no such radiation was detected.
As of now, no detailed model has been published that can explain the observed emission and constant temperature of iPTF14hls, let alone the possible eruption 60 years before the supernova. A better understanding of iPTF14hls could provide insight into the evolution of the most massive stars, the production of the brightest supernovae and possibly the birth of black holes that have masses near 40 solar masses — such as those associated with the first direct detection of gravitational waves7. For now, the supernova offers astronomers their greatest thrill: something they do not understand.