A superluminous supernova is one of the brightest events that astronomers can observe. One was recently spotted very soon after it exploded, giving scientists new clues about these exotic phenomena.
A superluminous supernova can flare 100 times brighter than a normal supernova – a spectacular explosion which can shine brighter than an entire galaxy. They were first discovered in 1998. Since then, around 50 more have been found in the past 10 years – and now one superluminous supernova is raising new questions.
SN 2017egm was first discovered on May 23rd by Arancha Delgado’s team at Cambridge University, using the European Space Agency’s star-surveying Gaia satellite. Three days later, a team led by Danfeng Xiang obtained an optical spectrum of the superluminous supernova; from analysing the spectrum, astronomers have determined that SN 2017egm was discovered about 10 days after exploding. This is one of the earliest spectra ever obtained for a superluminous supernova and will help us to understand how these events unfold.
A week after the initial discovery, Subo Dong’s team at the Kavli Institute for Astronomy and Astrophysics observed the superluminous supernova and determined that it is what is known as a Type I superluminous supernova, which means that it is poor in hydrogen.
Most superluminous supernovae are found in dwarf galaxies which don’t contain much besides hydrogen and helium – astronomers call these metal-poor galaxies. However, SN 2017egm was found in the massive spiral galaxy NGC 3191 – the first time we’ve found a superluminous supernova of this type in a galaxy like our own.
In another first, SN 2017egm was found to be around 420 million light-years away from Earth – around three times closer than any other superluminous supernova.
A team led by Matt Nicholl and based at the Harvard-Smithsonian Center for Astrophysics (CfA) investigated the properties of SN 2017egm and its host galaxy. Previously, the low metal content of a galaxy was thought to be essential in causing these explosions. However, NGC 3191 is comparatively rich in metals.
“Superluminous supernovas were already the rock stars of the supernova world,” said Nicholl. “We now know that some of them like heavy metal, so to speak, and explode in galaxies like our own Milky Way.”
Co-author Edo Berger, also from the CfA, added that if we could see a superluminous supernova in our own galaxy, “it would be much brighter than any supernova in recorded human history and would be as bright as the full Moon. However, they’re so rare that we probably have to wait several million years to see one.”
One of the aims of the CfA team was to investigate how SN 2017egm relates to other Type I superluminous supernovae. Surprisingly, despite its birth in a far more massive and metallic galaxy, it seems to be quite a typical superluminous supernova.
This suggests that the metallicity of a galaxy – the proportion of elements not hydrogen or helium – does not affect the properties of a superluminous supernova. The team further concludes in their paper, published today (July 31st) in the Astrophysical Journal Letters, that there is no hard upper limit on a host galaxy’s metallicity.
On July 4th, SN 2017egm entered Solar conjunction, where the Sun passes between the Earth and an object. This renders it unobservable until September 16th, when it will come out of Solar conjunction and astronomers can once again train their telescopes on it. Because it is so bright and because it was observed so soon after its explosion, it should be possible to study the supernova for at least a few more years.
“This should break all records for how long a superluminous supernova can be followed,” said co-author Raffaella Margutti from Northwestern University in Evanston, Illinois. “I’m excited to see what other surprises this object has in store for us.”
A version of the paper submitted to arXiv can be found here: https://arxiv.org/abs/1706.08517