The most powerful supernova?

An artist’s impression of the superluminous supernova ASASSN-15lh as it would appear from an exoplanet located about 10,000 light years away.

RIGHT now, astronomers are viewing a ball of hot gas 3.8 billion light years away that is radiating the energy of hundreds of billions of suns. At its heart is an object more than 15 kilometres across. Astronomers are not entirely sure what it is but suspect that the gas ball is the result of a supernova. If true, it is the most powerful supernova ever seen.

An international team of astronomers spotted the possible supernova, now called ASASSN-15lh, when it first flared to life in June 2015. This observation has been reported in a recent issue of Science.

The scientists say the object at the centre could be a very rare type of star called a magnetar, but one so powerful that it pushes the energy limits allowed by physics. “If it really is a magnetar, it’s as if nature took everything we know about magnetars and turned it up to 11 [on a scale of 1 to 10],” said the team’s co-principal investigator, Krzysztof Stanek of the Ohio State University.

The gas ball was spotted by the All Sky Automated Survey for Supernovae (ASAS-SN, pronounced “assassin”) collaboration. Led by Ohio State, the project uses a cadre of small telescopes around the world to detect bright objects in our universe.

Although ASAS-SN has discovered some 250 supernovae since the collaboration began in 2014, the explosion that powered ASASSN-15lh stands out for its sheer magnitude. It is 200 times more powerful than the average supernova, 570 billion times brighter than our sun, and 20 times brighter than all the stars in our Milky Way galaxy combined.

“The honest answer is at this point that we do not know what could be the power source for ASASSN-15lh,” said Subo Dong, lead author of the Science paper and a Youth Qianren Research Professor of astronomy at the Kavli Institute for Astronomy and Astrophysics at Peking University. He added that the discovery “may lead to new thinking and new observations of the whole class of superluminous supernova”.

Todd Thompson, professor of astronomy at Ohio State, offered one possible explanation. The supernova could have spawned an extremely rare type of star called a millisecond magnetar, a rapidly spinning and very dense star with a very strong magnetic field.

To shine so bright, this magnetar would also have to spin at least 1,000 times a second, and convert all that rotational energy to light with nearly 100 per cent efficiency, Thompson explained. It would be the most extreme example of a magnetar that scientists believe to be possible.

The Hubble Space Telescope will help settle the question later this year, in part because it will allow astronomers to see the host galaxy surrounding the object. If the team finds that the object lies in the centre of a large galaxy, then perhaps it ’is not a magnetar at all, and the gas around it is not evidence of a supernova, but instead some unusual nuclear activity around a supermassive black hole.

R. Ramachandran