Extremely fast cosmic explosions that puzzle scientists could be explosions arising from dying stars, new research suggests.
Nicknamed fast blue optical transients (FBOTs), because of their “blue” heat and very rapid evolution, only a few of these bursts have been recorded. (Astronomers often give them evocative nicknames, such as “the camel” or “the koala”.)
A new model suggests that the explosions happen amidst the chaotic environment of a huge dying star. These stars shoot powerful jets, which crash into layers of gas that break away from the star as its fusion slows. The model suggests that as the cocoon cools, it rapidly releases heat, which we see as FBOT.
“When we calculated how much energy the cocoon has, it turned out to be as powerful as an FBOT,” said Ore Gottlieb, a high-energy theoretical astrophysicist at Northwestern University who led the research, in a statement.
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FBOTs are most visible at optical wavelengths and change in a matter of days. They get bright quickly, then fade away, much faster than a supernova star burst normally. They are also new to science, having been first observed just four years ago.
Scientists have offered other origin stories to explain these strange explosions. Some astronomers have said that FBOTs may be related to gamma-ray bursts, another set of powerful bursts associated with dying stars. These explosions, known as GRBs, occur when huge stars collapse into black holes, generating gamma rays.
Gottlieb said he’s not sure. Although GRBs and FBOTs move at nearly the speed of light and have asymmetric shapes, there is a fundamental difference between the two screens. “Stars that produce GRBs lack hydrogen,” he explained. “In FBOTs, we see hydrogen everywhere. So it can’t be the same phenomenon.”
The model created by Gottlieb and his collaborators explains this discrepancy between these two types of explosions (the GRB and the FBOT). Because the hydrogen-rich stars that power the FBOTs have the gas in the outermost layer of the cocoon, the jet cannot penetrate that far as the layer is too thick, the scientists argue.
“That’s why you can’t produce a GRB,” Gottlieb said. In these situations, the pod emits FBOT emissions, which occur when the jet sends all of its energy into the gas pod, which in turn glows.
FBOTs are also visible in radio waves and X-rays, which also seem to have an explanation in the model. As the cocoon crashes into dense gas near the star, the motion creates radio emissions as the stellar material heats up.
X-rays come later in the process, when a black hole is created from the collapsed star; the black hole’s X-rays are believed to leak from the thin, fainter edges of the cocoon as it expands.
The researchers added that more observational data would help confirm the model and create more advanced simulations of FBOT and GRB behavior. A study based on the current research was published April 11 in the Monthly Notices of the Royal Astronomical Society.