The Discovery of A Very Rare Supernova

Just over a year ago, we witnessed an unusually impressive supernova within a galaxy 200 million light-years away. However, astronomers have only just discovered how rare this supernova really was!

The Story of The Cow

TheCOWSDSS
A close up of Cow marked by two lines. Credit: SDSS

The Cow (or SN 2018cow) was a supernova detected on the 16th of June 2018. It’s luminosity peaked within a few days, whereas normal supernovae begin peaking after about three weeks. Not only that, but The Cow’s power output at one point was over 100 times greater than any normal supernova!

This sparked a lot of discussion, and astronomers concluded that the immense energy seen in The Cow’s supernova either stems from a high speed neutron star or a newborn black hole accreting matter. Both plus the supernova are possible endings for a star, specifically high mass stars.

A lot more observations confirmed the extreme distance and nature of The Cow, being roughly 200 million light years away in a different galaxy. Astronomer Anna Ho observed the Cow using radio-waves to research short-wavelength emissions from the stellar explosion. These radio wave emissions are from charged particles interacting with the star’s magnetic field as material is being thrown into space. Ho’s observations showed the material was being thrown at nearly 10% the speed of light!

SNCowps1imagesstsci
Credit: panSTARRS/STSci

Her and her team’s observations confirmed that either a black hole or neutron star was the source of this high-energy explosion. This was also confirmed by research done by  Daniel Perley and his team at Liverpool John Moores University, and Raffaella Margutti and her colleagues’ observations in the X-ray showing a reheating from the core of the event.

What was The Cow?

Whilst The Cow was a highly unusual supernova, it is not alone in it’s strangeness!

Supernovae that are exceedingly bright such as The Cow are known as superluminous supernova. SN 2003dh was the first case of this kind of supernova to be detected, which accompanied a gamma-ray-burst just under 600 Megaparsecs away.

However, SN 2003dh and The Cow aren’t terribly similar in their nature. In fact, there are many ways a superluminous supernova can form such as core-collapse, magnetars, gamma-ray-bursts, or perhaps even simply binary stars!

In the case of The Cow, we had an extremely massive star undergoing core-collapse into either a black hole or neutron star, sometimes called collapsars. Although our understanding of how these superluminous supernova come to be, another key factor in their creation may be metallicity in the star, as collapsars don’t leave a lot of iron behind.

There is some evidence to suggest that superluminous supernovae may also be produced when a normal supernova acquires energy from a large magnetic field. In this case, the strong magnetic field would be produced by a magnetar. Ig you would like to learn more, I found this publication by Daniel Kasen and Lars Bildsten on supernovae powered by magnetars. I will warn you, it is not for light reading!

And lastly, Binary Systems – The story of SN 2006gy

SN2006gyNASAPSUSwiftSNTeam
SN 2006gy in the top right corner. Credit: NASA/PSU/Swift SN Team

The more recent work on this type of supernova is on SN 2006gy. This supernova gets another fancy name: a hypernova, due to its extremely high kinetic energy. SN 2006gy was first spotted in 2006 in the galaxy NGC 1260, and it was bright enough to outshine its own galaxy and stay bright for a few years.

Initially , there were many theories as to what caused this bright event. One was that star that collapsed and exploded was a hypergiant (I’m talking over 100 solar masses here!) that shed lots of outer material, causing a nebulous appearance.

sn2006gy
SN 2006gy’s Iron spectrum compared to a theoretical iron I spectrum. Credit: MPA

Another idea was that because this supernova had a large amount of iron in its spectrum, it was probably a very extreme type 1a supernova. These types are caused by a white dwarf accretes mass from a companion star.

 

The star could instead have exploded from pair-instability, where radiation produced in the star’s core decreases the outward pressure on a star. The star retaliates by increasing fusion, but in the process explodes and leaves no star remnant.

20170519-thorne-zytkow-object
This is an artists’s rendering of a Thorne-Zytkow object, which is a similar model to SN 2006gy.

However, none of these were quite the case.

This year a new theory has been published, which proposes that instead of the white dwarf accreting mass from its giant companion, the companion star completely engulfs the white dwarf. This explains the iron spectral lines, the large shell, and seems to fit very well.

 

These were just two bizarre cases of superluminous supernovae that I wanted to share with you. I really enjoyed learning about superluminous supernovae, and never heard of them until I read about SN 2006gy’s explanation. Some of the information I got was from this publication about stellar deaths, specifically section 4 on supernovae. You can also get some data from SN 2006gy by going to Swift Supernovae. SN 2006gy is the 27th objects, so scroll all the way down to find it!

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