“Cow” and “Koala” lead explosive new class of space signals

The universe is full of powerful explosions from various sources, and now astronomers have described a brand new class of space signals. Named fast blue optical transients (FBOTs), these events are very bright and throw off incredible amounts of energy in a short amount of time.

FBOTs are so hot that they glow blue in optical wavelengths, but can be picked up in X-ray and radio waves as well. They come and go very fast, usually within a few days, and in that time, they seem to be emitting huge amounts of energy.

At a glance, FBOTs seem similar to other transient space events like gamma ray bursts, fast radio bursts (FRBs) and supernovae. But these cosmic explosions all have different signatures and sources. FBOTs, for instance, are much longer lasting than FRBs, which typically only last milliseconds, but shorter than supernovae, which can glow for months or years. FBOTs also contain hydrogen, while gamma ray bursts do not.

FBOTs have different signal features to supernovae and gamma ray bursts
FBOTs have different signal features to supernovae and gamma ray bursts

Bill Saxton, NRAO/AUI/NSF

So far, the newly created FBOT class only has three members. The first, named AT2018COW (nicknamed “The Cow”), was spotted in June 2018, when it shone around 100 times brighter than a supernova but faded away in just 16 days.

The second is called ZTF18abvkwla, nicknamed “The Koala.” It appeared just a few months after The Cow as a bright optical source in the sky, and disappeared even faster. When it was examined in radio wavelengths, astronomers realized it was 10 times brighter than The Cow.

And finally, the third event is known as CSS161010. It was actually spotted in 2016, but wasn’t able to be identified as an FBOT until now. And it’s the most powerful one of all – astronomers calculate that it launched up to 10 percent of the mass of the Sun at more than half the speed of light.

“We thought we knew what produced the fastest outflows in nature,” says Raffaella Margutti, senior author of the study. “We thought there were only two ways to produce them – by collapsing a massive star with a gamma ray burst or two neutron stars merging. We thought that was it. With this study, we are introducing a third way to launch these outflows. There is a new beast out there, and it’s able to produce the same energetic phenomenon.”

The anatomy of an FBOT
The anatomy of an FBOT

Bill Saxton, NRAO/AUI/NSF

But what causes these FBOTs? They’re tricky to study because they vanish so quickly, but astronomers have some clues. All three originated in tiny galaxies, which have stars with lower levels of metals. That allows the stars to retain more mass by the end of their lives, potentially leading to a more energetic supernova – which may be what we’re seeing.

Interestingly, they also all seem to host a black hole or neutron star at their centers. This could be further evidence of a supernova, or perhaps that the signals are created when a black hole tears apart a star that wanders too close.

“The observations prove the most luminous FBOTs have a ‘central engine’ – a source like a neutron star or black hole that powers the transient,” says Margutti. “It’s not yet clear if these bright FBOTs are rare supernovae, stars being shredded by black holes, or other energetic phenomena. Multi-wavelength observations of more FBOTs and their environment will answer this question.”

A paper describing The Koala was published in The Astrophysical Journal, while one describing CSS161010 was published in The Astrophysical Journal Letters.

Sources: Northwestern University, Keck Observatory

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