Astronomers discover the fastest optical flash of a Type Ia supernova

A team of astronomers at Kavli IPMU capture the fastest optical flash emitted from a new-born supernova to discover how Type Ia supernova explosions are ignited.

The end of a star’s life commonly results in a spectacular explosion. The majority of giant stars will explode as a supernova. However, this differs for white dwarf stars, which are the remnants of an intermediate mass star, such as our Sun, and it can explode if part of a close binary star system, where two stars are in orbit around each other. This type of supernovae is classified as Type Ia supernovae.

Due to the uniformity and extreme brightness of the Type Ia supernova, which is estimated as five billion times brighter than our Sun, they are widely used by researchers as a tool for distance measurements in astronomy. As a recent example, the Type Ia supernovae assisted researchers to discover the accelerating expansion of our universe.

However, despite the great success of the Type Ia supernova cosmology, researchers are still perplexed by questions such as what the progenitor systems of Type Ia supernovae are, and how Type Ia supernova explosions are ignited.

Examining early-phase supernova candidates

To decipher these long-standing issues, a team of astronomers attempted to catch footage of Type Ia supernovae within one day of their explosions, called early-phase Type Ia supernovae. This team, led by project researcher Ji-an Jaiang at the Kavli Institute from the Physics and Mathematics department (Kavli IPMU), used new-generation wide-field survey facilities, including the Tomo-e Gozen camera that is the first wide-field mosaic CMOS sensor imager in the world.

By regularly examining early-phase supernova candidates discovered by the Tomo-e transient survey, one transient, Tomo-e202004aaelb, caught Jiang’s attention.

“Tomo-e202004aaelb was discovered with high brightness on April 21 in 2020,” said Jaiang. “Surprisingly, its brightness showed significant variation in the next two days and then behaved like a normal early-phase Type Ia supernova. We have discovered several early-phase Type Ia supernovae that show interesting excess emission in the first few days of their explosions but have never seen such a fast and prominent early emission in optical wavelengths. Thanks to the high-cadence survey mode and the excellent performance of Tomo-e Gozen, we can perfectly catch this amazing feature for the first time. Such a prompt early flash should originate from a different origin compared to previously discovered early-excess Type Ia supernovae.”

The origin of the fast optical flash

Computational simulations by Kyoto University Associate Professor Keiichi Maeda exhibited that the origin of the mysterious fast optical flash can be explained by the energy released from an interaction between supernova ejecta and a dense and confined circumstellar material (CSM) soon after the supernova explosion.

“We have not seen such a short and bright flash from Type Ia supernovae before, even with a recently increasing number of very early discoveries soon after the supernova explosion in the last few years, including those discovered by our team, explained Maeda. “The nature of the CSM must reflect the nature of the progenitor star, and thus this is a key to understanding what kind of a star explodes and how they do so. A question is what makes this supernova so special.”

Through spectroscopic observations by the Seimei telescope of Kyoto University, the team discovered that the SN is a variant of brightest Type Ia supernovae.

“At the first look of the spectrum taken just after the initial flash, it stood out as something different from normal supernovae,” said Kyoto University Project Researcher Miho Kawabata. “We noticed that a brightest class of Type Ia supernovae might look like this one if they would be observed in such an early phase. Our classification was subsequently confirmed as the spectra evolve to look more and more similar to the previously found bright Type Ia supernovae.”

The team’s results prove that at least a fraction of Type Ia supernovae originates from a dense CSM environment, providing a stringent constraint on the progenitor system of these spectacular phenomena in our universe. Given that Tomo-e202004aaelb (SN 2020hvf) is much brighter than typical Type Ia supernovae used as the distance indicator, this discovery will enable Jiang and his collaborators to test various theories which have been proposed for these peculiar over-luminous Type Ia supernovae.

“Previously, we have constructed theoretical models of super-Chandrasekhar-mass rotating white dwarfs and their explosions” added Kavli IPMU Senior Scientist Ken’ichi Nomoto. “Such massive models can be consistent with the peak brightness of SN 2020hvf, but more theoretical work is necessary to explain the detailed observational properties. SN 2020hvf has provided a wonderful opportunity of collaboration between the theory and observations.”

The Projects future aspirations

Jiang’s team intend to continue looking for the answer of the long-standing origin issue of Type Ia supernovae by carrying out transient surveys with telescopes all over the world.

“We have used Type Ia supernovae to measure the expansion of the universe, although their origins are not well understood,” added Kavli IPMU Senior Scientist and University of Tokyo Professor Mamoru Doi. The early-phase photometry of Type Ia supernovae provides unique information to understand their origins, and hence, should contribute to more accurate measurements of the expansion of the universe in near future.”

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