A stellar explosion caught a birth

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The first moment of a supernova captured by an amateur astronomer

An international team of researchers, including Keiichi Maeda from Kyoto University's Department of Astronomy, reported evidence for the detection of the first light from a supernova, the so-called shock breakout. The result was reported in the journal Nature .

This moment of a supernova becoming visible in the sky was captured by an amateur astronomer, and has helped the team validate theoretical predictions about the initial evolution of such stellar explosions.

How the structure of a star affects its supernova explosion properties has remained an open question, and understanding it would be a significant step forward in astrophysics research. Current theory suggests an explosive shockwave travels through the star’s interior before reaching the surface, producing a sharp peak of electromagnetic emissions.

The strength and duration of this signal, known as shock breakout is believed to largely depend on the outer structure of the star and on the presence or absence of matter around it. However, testing this theory requires the observation of the exact moment a star becomes a supernova.

Melina Bersten, a researcher at the National Scientific and Technical Research Council-Argentina, had said the chances of capturing such an event are slim.

“If we think that on average each galaxy roughly produces one supernova per century, and that a century contains nearly 900 thousand hours, then the chance of observing the right galaxy at the right moment is not much greater than one in a million. However, the actual chances are smaller, at least down to one in ten million. One needs to take into account the facts that we can only see the galaxy during the night time and that the sky must be clear.”

Luckily, on September 20, 2016, amateur astronomer Víctor Buso from Rosario, Argentina, was testing his new camera on his rooftop observatory in hopes of photographing his first supernova. After an hour of taking images Buso noticed a new object had appeared, and it became more obvious with time. He had captured the moment a supernova had exploded!

Named SN 2016gkg, a team of researchers led by Bersten, analyzed the images. The rapid brightening rate combined with a very low luminosity had no analogue among known supernovae, and the team concluded Buso had discovered SN 2016gkg during a shock breakout.

“When Buso told us how he had observed and what he had witnessed, we realized this was a unique finding,” said Bersten.

By comparing the photometry of the images with their computer simulations, the team found an initial sharp rise in supernova light that could only be explained by shock emergence

“To our surprise, images had a great quality considering they were obtained from the middle of a large city in the midst of the pampas”, notes Dr. Gastón Folatelli from IALP, who led the data analysis. He adds “sky conditions seem to have been nearly ideal on that night!”

Their conclusion was supported by the fact that the models required no modification in order to consistently reproduce the initial rise and the rest of the supernova evolution. Moreover, SN 2016gkg happened to be a rather ordinary event, which would imply that the observed phase is common to all supernovae.

Illustration of a shock breakout

Dr. Keiichi Maeda

Sequence of combined images obtained by Víctor Buso as SN 2016gkg arises in the outskirts of galaxy NGC 613. Labels indicate the time each image was taken. The supernova location is indicated by the red circles. Notably the supernova appears and stabrightens and steadily brightens within one hour, as shown in the lower-right panel.

Observed light curve of SN 2016gkg (dots) and explosion model (red line). The model reproduces three distinct phases in the supernova evolution that occur on different time scales. First, the shock breakout (with a time scale of hours), then the post-shock cooling emission (days), and finally the emission due to radioactive heating (weeks). Buso's discovery observations, shown in blue, indicate such a rapid rise that can only be attributed to the shock breakout.

Paper information

【DOI】 https://doi.org/10.1038/nature25151

M. C. Bersten, G. Folatelli, F. García, S. D. Van Dyk, O. G. Benvenuto, M. Orellana, V. Buso, J. L. Sánchez, M. Tanaka, K. Maeda, A. V. Filippenko, W. Zheng, T. G. Brink, S. B. Cenko, T. de Jaeger, S. Kumar, T. J. Moriya, K. Nomoto, D. A. Perley, I. Shivvers & N. Smith (2018). A surge of light at the birth of a supernova. Nature, 554, 497–499.