Cocoons of dying stars could explain fast blue optical transients


Since their discovery in 2018, fast blue optical transients (FBOTs) have completely surprised and utterly confused observational and theoretical astrophysicists.

So hot they glow blue, these mysterious objects are the brightest known optical phenomenon in the universe. But with only a few discoveries so far, the origins of FBOTs have remained elusive.

Now, an astrophysics team from Northwestern University presents a bold new explanation for the origin of these curious anomalies. Using a new model, astrophysicists believe that FBOTs could result from the active cooling cocoons that surround jets launched by dying stars. This is the first astrophysical model fully consistent with all FBOT-related observations.

the research has been published April 11 in the Monthly Notices of the Royal Astronomical Society.

When a massive star collapses, it can launch debris flows at speeds close to the speed of light. These outflows, or jets, collide with the collapsing layers of the dying star to form a “cocoon” around the jet. The new model shows that as the jet pushes the cocoon outward – away from the collapsing star’s core – it cools, releasing heat as the observed FBOT emission.

Gottlieb Ore

“A jet starts deep inside a star, then pushes its way out,” Northwestern said. Gottlieb Ore, who led the study. “As the jet travels through the star, it forms an extended structure, known as a cocoon. The cocoon envelops the jet, and it continues to do so even after the jet has escaped the star, which cocoon escapes with the jet. When we calculated the energy amount of the cocoon, it turned out to be as powerful as an FBOT.”

Gottlieb is a Rothschild Fellow of Northwestern’s Interdisciplinary Center for Exploration and Research in Astrophysics (CIERA). He co-wrote the article with a member of CIERA Sasha Chekovskoyassistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences.

The hydrogen problem

FBOTs (pronounced F-bot) are a type of cosmic explosion initially detected in the optical wavelength. As their name suggests, transients fade almost as quickly as they appear. FBOTs reach peak brightness within a few days and then quickly fade – much faster than the rise and decay of standard supernovae.

chekovskoy fast blue optical transients
Sasha Chekovskoy

After discovering FBOTs just eight years ago, astrophysicists wondered if the mysterious events were related to another transient class: gamma-ray bursts (GRBs). The strongest and brightest outbursts on all wavelengths, GRBs are also associated with dying stars. When a massive star runs out of fuel and collapses into a black hole, it launches jets to produce a powerful burst of gamma rays.

“The reason we think GRBs and FBOTs might be related is because both are very fast – moving at close to the speed of light – and both are asymmetrical in shape, breaking up the spherical shape of the star,” Gottlieb said. “But there was a problem. Stars that produce GRBs lack hydrogen. We don’t see any sign of hydrogen in GRBs, whereas in FBOTs we see hydrogen everywhere. So it couldn’t not be the same phenomenon.

By using their new model, Gottlieb and his co-authors believe they have found an answer to this problem. Hydrogen-rich stars tend to harbor hydrogen in their outermost layer – a layer too thick for a jet to penetrate.

“Basically, the star would be too massive for the jet to pass through,” Gottlieb said. “So the jet will never exit the star, and that’s why it doesn’t produce GRB. Now, in these stars, the dying jet transfers all its energy to the cocoon, which is the only component to escape from the star. The cocoon will emit FBOT emissions, which will include hydrogen. This is another area where our model is fully consistent with all FBOT observations.

Assemble the image

Although FBOTs shine in optical wavelengths, they also emit radio waves and X-rays. Gottlieb’s model also explains them.

When the cocoon interacts with the dense gas surrounding the star, this interaction heats the stellar material to release a radio emission. And when the cocoon extends far enough from the black hole (formed from the collapsed star), X-rays can escape from the black hole. X-rays join radio and optical light to form a complete picture of the FBOT event.

While Gottlieb is encouraged by his team’s findings, he says more observations and models are needed before we can definitively understand the mysterious origins of FBOTs.

When we calculated the amount of energy from the cocoon, it turned out to be as powerful as an FBOT.

Gottlieb Ore

“This is a new class of transients, and we know so little about them,” Gottlieb said. “We need to detect more of them earlier in their evolution before we can fully understand these explosions. But our model is able to draw a line between supernovae, GRBs and FBOTs, which I think is very elegant.”

“This study paves the way for more advanced simulations of FBOTs,” Chekovskoy said. “This next-generation model will allow us to directly connect the physics of the central black hole to observables, allowing us to reveal the otherwise hidden physics of the central FBOT engine.”

The study, “Shocked Jets in CCSNe May Power the Zoo of Fast Blue Optical Transients,” was supported by the National Science Foundation (Award Numbers AST-1815304 and AST-2107839). The authors developed the simulation using supercomputers at the Texas Advanced Computing Center at the University of Texas at Austin.


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