Astronomers struggle to explain mysterious short-lived explosions

Astronomers struggle to explain mysterious short-lived explosions

An international team of astronomers has discovered 72 new transients – bright explosions that fade away far more quickly than a supernova. Their research will help other scientists to understand stellar evolution. 

Images of one of the transient events, from 8 days before the maximum brightness to 18 days afterwards. This outburst took place at a distance of 4 billion light years. Image credit: M. Pursiainen / University of Southampton and DES collaboration

Over the past decade, astronomers have detected around 25 transients. These explosions can be as bright as a supernova, but while a supernova might last for several months, a transient might only last a week. So far nobody has been able to explain where they come from or why.

A group led by Miika Pursiainen of the University of Southampton set out to change that. Using data from the Dark Energy Survey Supernova Programme (DES-SN), they searched for events whose brightness faded quickly and checked that these did not overlap with any “normal” supernovae. They have now nearly quadrupled the number of transients we know about.

The events are hot, ranging from 10000 to 30000°C, and can be hundreds of billions of kilometres across. They also expand and cool down over time, indicating that they are some kind of explosion.

Astronomers have proposed various scenarios which might produce these transients, from neutron star collisions to stars being ripped apart by black holes (which researchers call a tidal disruption event), but Pursiainen’s team suggests that the transients are caused by something known as a shock breakout.

In this particular kind of shock breakout, when a star’s core collapses in on itself, it sends a shock wave through some of the material surrounding the star. Here, astronomers are not seeing a star explode – they are seeing a hot cloud cool down.

The Dark Energy Camera (DECam) mounted on the 4-metre Blanco Telescope. Image credit: Reidar Hahn, Fermilab

Shock breakout is an attractive explanation because the time it takes the hot material to cool depends most strongly on its mass. Hence shock breakout could produce the diverse range of events that Pursiainen’s group and others have seen.

However, some of the transients don’t brighten and fade in a way that would be consistent with shock breakout. It could be more than one process is at work, but the team need to search for more transients to see if this is the case. This is exactly what they plan to do in order to compare these transients with “normal” supernovae, along with seeing how far away these transients are from Earth.

“The DES-SN survey is there to help us understand dark energy, itself entirely unexplained,” said Pursiainen. “That survey then also reveals many more unexplained transients than seen before. If nothing else, our work confirms that astrophysics and cosmology are still sciences with a lot of unanswered questions!”

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