The universe is full of powerful explosions from different sources, and now astronomers have described a whole new class of space signals. These events are called Fast Blue Optical Transients (FBOTs), which are very bright and throw out amazing energy in a short period of time. FBOTs are so hot that they glow blue at optical wavelengths, but can also be obtained in X-rays and radio waves. They come very quickly, and they go fast, usually within a few days, during which time they seem to be releasing a tremendous amount of energy.
At first glance, FBOTs appear to resemble other transient space events, such as gamma-ray bursts, rapid radio waves (FRBs), and supernovae. But these cosmic explosions have different characteristics and sources. For example, FBOT is much longer than FRBs, which typically have only a few milliseconds, but much shorter than supernovae, which can glow for months or years. FBOT also contains hydrogen, while gamma-ray storms do not.
So far, the newly created FBOT class has only three members. The first, at2018COW (nicknamed “Cow”), was discovered in June 2018, when it was about 100 times brighter than a supernova, but disappeared in just 16 days. The second is named ZTF18 abvkwla, nicknamed “Koala”. It appears in the sky as a bright source of optics a few months after the emergence of “cows” and disappears faster. When it was examined at radio wavelengths, astronomers found that it was 10 times brighter than a “cow”.
Finally, the third event is called CSS161010. It was actually discovered in 2016, but has not been identified as FBOT until now. And it’s one of the most powerful — astronomers calculate, it emits up to 10 percent of the mass of the sun at more than half the speed of light.
“We thought we knew what was the fastest outflow in nature,” said Raffaella Margutti, senior author of the study. “We think there are only two ways to produce them— a large mass star or two neutron stars merged with gamma-ray bursts. Through this study, we introduced a third way to emit these outflows. A new ‘beast’ has emerged that can produce the same energy phenomenon. “
But what causes these FBOTs? Because they disappear so fast, it’s tricky to study, but astronomers have some clues. All three originate in tiny galaxies, where stars have lower metal content. This allows these stars to retain more mass at the end of their lives, potentially leading to higher-energy supernovae — which may be what we see now. Interestingly, they all seem to have a black hole or neutron star at their center. This could be further evidence of supernovae, or signals from black holes that tear apart stars that roam too close.
“Observations show that the most glowing FBOT has a ‘central engine’ — a source like a neutron star or a black hole that powers transients,” Marguetti said. “It is not clear whether these bright FBOTs are rare supernovae, stars crushed by black holes, or other energy phenomena. Multi-wavelength observations of more FBOT and its environment will answer this question. “
The paper describing the koala was published in the Astrophysical Journal, while the paper describing CSS 161010 was published in the Astrophysical Journal Letters.