A team of astronomers at the University of Southampton, UK, has found 73 rapidly evolving transients — very bright, but quick events — in a recent survey and are still struggling to explain their origin.
Images of one of the rapidly evolving transients, 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 / DES Collaboration.
Miika Pursiainen from the University of Southampton’s Department of Physics and Astronomy and co-authors found the rapidly evolving transients in data from the Dark Energy Survey Supernova Programme (DES-SN).
“The DES-SN survey looks for supernovae, the explosion of massive stars at the end of their lives,” they said.
“A supernova explosion can briefly be as bright as a whole galaxy, made up of hundreds of billions of stars.”
“We found the largest number of these quick events to date.”
Even for transient phenomena, these rapidly evolving transients are very peculiar: while they have a similar maximum brightness to different types of supernovae they are visible for less time, from a week to a month. In contrast supernovae last for several months or more.
The events appear to be both hot, with temperatures from 18,000 to 54,000 degrees Fahrenheit (10,000-30,000 degrees Celsius), and large, ranging in size from several up to a hundred times the distance from Earth to Sun. They also seem to be expanding and cooling as they evolve in time, as would be expected from an exploding event such as a supernova.
There is still debate on the origin of these transients.
“One possible scenario is that the star sheds a lot of material before a supernova explosion, and in extreme cases could be completely enveloped by a shroud of matter,” the astronomers said.
“The supernova itself may then heat the surrounding material to very high temperatures. In this case we see the hot cloud rather than the exploding star itself.”
To confirm any of this, the researchers will need a lot more data.
“We plan to continue our search for transients, and estimate how often they take place compared with more ‘routine’ supernovae,” Pursiainen said.
The astronomers presented their results yesterday at the European Week of Astronomy and Space Science (EWASS) in Liverpool, UK.
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Miika Pursiainen et al. Rapidly Evolving Transients in the Dark Energy Survey. EWASS 2018, abstract # 330
