Supernova ASASSN-18bt caught in the act

What happens exactly at the moment when a Type Ia supernova exoplodes? Astronomers from IFA are co-authors on a new paper in The Astrophysical Journal with a new discovery, partly based on data from the Kepler satellite.

2018.11.26 | Ole J. Knudsen

The Kepler satellite ends it's career with at bang! New knowledge on Type Ia supernovae. Logo: NASA Keplerscience.

The Kepler satellite ends it's career with at bang! New knowledge on Type Ia supernovae. Logo: NASA Keplerscience.

Newly discovered supernova provides new insight on the origin of exploding stars


A supernova discovered by an international group of astronomers provides an unprecedented look at the first moments of a stellar explosion. Max Stritzinger and Simon Holmbo (Aarhus), working in collaboration with lead author Ben Shappee (University of Hawaii), found a mysterious signature in the light from the exploding star’s first hours of evolution.

The discovery is published on 27 November 2018 in The Astrophysical Journal.

Title of the paper is: Seeing Double: ASASSN-18bt Exhibits a double-power-law Rise in the Early-Time K2 Light Curve

Type Ia supernovae are fundamental to our understanding of the cosmos. Their nuclear cauldron is crucial for generating many of the elements of the universe  and their high intrinsic luminosity enables them to serve as cosmic lightbulbs to measure distances across the expanses of the universe. Despite their importance in understanding the expansion history of universe the actual mechanism triggering a Type Ia supernova explosion has remained elusive for decades.

That’s why catching them in the act is crucial.

Astronomers have long tried to get detailed data at the initial moments of the explosions with the intention to determine how their progenitor stars actually explode. This finally happened in February 2018 with the discovery of the Type Ia supernova called ASASSN-18bt (also known as SN 2018oh).

ASASSN-18bt was discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN), an international network of telescopes led by Ohio State University, in collaboration with Aarhus University, University of Hawaii and Peking University, that images the entire sky for supernovae on a daily basis. NASA’s planet hunter Kepler Space Telescope was able to take complementary data of this object in the days prior to discovery as it was simultaneously imaging the same location of the sky where the supernova was located over the course of one of its last K2 science campaigns. Kepler was designed to be incredibly sensitive to small changes in light for its main mission of detecting extrasolar planets and was therefore able to obtain especially detailed information on the infant emission of light from ASASSN-18bt extending from the moment of explosion to weeks later.

“ASASSN-18bt is the nearest and brightest supernova yet observed by Kepler, so it offered an excellent opportunity to test the predominant theories of supernova formation,” said Shappee.

The supernova was observed in the galaxy UGC04780, situated some 160 million lightyears away in the constellation of Cancer. Left image shows the galaxy before the explosion. To the right an exposure two days after with the bright supernova in the red ring, close to the galactic centre. Images: PANSTARRS-1


Combining data from ASAS-SN, Kepler, and telescopes around the world, the team realized ASASSN-18bt looked unusual during its first couple of days.

“When the supernova exploded and increased in brightness it evolved quite differently than expected, showing an early linear-rising phase followed by an exponential-rising phase. The early light curve evolution is difficult to explain using conventional wisdom, indicating the possibility of a triggering mechanism radically different than previously presumed”, said Stritzinger. Diagram from the paper in APJ.

Comparing ASASSN-18bt with other Type Ia supernovae indicates there could be a subpopulation of objects that exhibit early excess of emission. This supports a hypothesis put forth in a recent paper led by Stritzinger and published in September 2018 entitled: Red vs. Blue early observations of type Ia supernova reveal two distinct populations? (ApJ Letters 2018, 864, 35). Here they first entertained the idea of two different populations of Type Ia supernovae; those that show an early excess of blue emission and those that do not.

An early excess in emission suggests that the explosion may have been triggered on the surface of the white dwarf. The possibility of Type Ia supernovae being triggered from nuclear burning of helium-rich material accreted on the surface of the white dwarf and donated by a companion star was previously considered highly unlikely. ASASSN-18bt suggest otherwise and it may just be that nature follows a variety of paths leading to the terminal disruptions of white dwarfs. Image from the NASA/JPL video linked below.

Thanks to ASAS-SN and the current generation of surveys now monitoring the sky every night, astronomers are in the process of discovering a variety of supernovae near their ephemeral moment of explosion. Although the Kepler space telescope is no longer in service, the data it obtained of ASASSN-18bt ensured its mission finished by going out with a bang. As new discoveries come in they will hopefully home in on the solution to the longstanding mystery on the origins of these cosmic explosions.

This NASA press release discuss the various theories for the origin of Ia supernovae.



This research was supported in part by a research grant (13261) from the VILLUM FONDEN, the U.S. National Science Foundation (NSF), and a sabbatical grant from Aarhus University’s Faculty of Science & Technology. ASAS-SN is supported in part by the Gordon & Betty Moore Foundation, the Villum Foundation and the NSF. This research made use of the NASA/IPAC Extragalactic Database, which is operated by the Jet Propulsion Laboratory, Caltech, under contract with NASA. This research has made use of NASA’s Astrophysics Data System Bibliographic Services

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