Every hour survey telescopes on Earth, and in space, discover new supernovae in distant galaxies. A supernova, the destruction of a star in a powerful explosion, sometimes outshines its own host galaxy. Scientists can often follow such an event for months. In the past decades, keen observing and modeling of supernovae has given us some understanding of how these explosions work. Fortunately, our budding understanding is regularly challenged by new, strange supernovae. Supernova iPTF14hls is a strange one indeed. A new paper in the journal Nature, led by Iair Arcavi (Las Cumbres Observatory, California, USA) and with OKC co-authors Jesper Sollerman, Francesco Taddia, Ragnhild Lunnan and Anders Nyholm, attempts to explain this supernova. Former OKC member Christoffer Fremling also contributed.
Sitting in a dwarf galaxy about 500 million light years away, below the Big Dipper in the sky, supernova iPTF14hls initially seemed ordinary when discovered by the intermediate Palomar Transient Factory (iPTF) survey in September 2014. The iPTF, which finished in 2017, searched for and followed up transient events in the sky and involved several scientists from the OKC. When spectra were taken of iPTF14hls in early 2015 scientists observed strong signatures of hydrogen emission which usually indicate the explosion of a red supergiant star. However, when a red supergiant star explodes it typically shines at roughly constant brightness for about 100 days, before fading to obscurity. iPTF14hls did not. In fact, iPTF14hls increased in brightness and this prompted an intense follow-up campaign from telescopes around the world. During 2015, iPTF14hls faded and brightened in a way rarely seen from supernovae (see also supernova iPTF13z). New spectra were taken regularly by telescopes, including the Nordic Optical Telescope on La Palma. As the brightness of iPTF14hls swayed up and down, its spectrum added to the mystery by hardly changing at all. In 2016, iPTF14hls finally began to slowly fade while its spectrum evolved only slightly.
This new paper discusses many possible causes for the unique behavior of iPTF14hls. One mechanism, pulsational pair instability (PPI), was proposed in the 1960s and hypothesizes that sufficiently massive supergiant stars might create large numbers of electron-positron pairs in their cores by using some of the energy which would otherwise have gone into supporting the star against gravity. The lack of pressure support could give rise to pulsations which would eject the outer layers of the star. Shells of this material crashing into each other could be the cause of long-lasting and bright supernova-like events. The PPI mechanism might be occurring in iPTF14hls although that would make the strong Hydrogen signature in its spectrum hard to explain as PPI events are expected to remove the outer hydrogen-rich layers of a star.
A photographic plate containing observations of this galaxy taken in 1954 suggests that an outburst of the iPTF14hls progenitor took place around that time. It is thus likely that the system is surrounded by material from previous outbursts. Interaction between supernova ejecta and the surrounding material could make the supernova shine and change brightness for several years longer than expected. However, observations extending into both X-rays and radio waves do not show evidence for the shocks scientists expect in this scenario.
The paper also discusses whether a black hole, formed by the iPTF14hls supernova explosion, could be accreting material in an irregular way and give rise to the undulating brightness and durable character of this odd supernova. A simple model of accretion onto a black hole can match the declining brightness of iPTF14hls seen over the 450 days since its discovery, making black hole accretion another possible explanation.
More observations are being performed and the jury is still out on iPTF14hls. In the coming years, projects such as the Zwicky Transient Facility and the Large Synoptic Survey Telescope (both of which include OKC scientists) will start monitoring the sky. They may find more events like iPTF14hls, helping us to understand at which rate they occur and possibly what causes them.
