Fast blue optical transients (FBOTs) have entirely astonished and perplexed both observational and theoretical astrophysicists ever since they were discovered in 2018. These enigmatic objects are the brightest known optical phenomenon in the universe, being so hot that they glow blue.
However, with so few having been found thus far, the FBOTs’ origins have remained a mystery. The source of these intriguing abnormalities is now the subject of a daring new theory put forth by a Northwestern University astrophysics team.
The astrophysicists think that FBOTs might be caused by the actively cooling cocoons that surround jets launched by dead stars using a new model. It is the first astrophysics model that completely accounts for all FBOT-related observations.
The research was published April 11, 2022, in the Monthly Notices of the Royal Astronomical Society.
A big star’s collapse can cause the material to be ejected at speeds that are nearly as fast as light. These jets or outflows clash with the dying star’s collapsing layers, creating a “cocoon” around the jet. According to the new model, the cocoon cools as the jet pushes it away from the star’s core, emitting heat as an FBOT emission.
“A jet starts deep inside of a star and then drills its way out to escape,” said Northwestern’s Ore Gottlieb, who led the study. “As the jet moves through the star, it forms an extended structure, known as the cocoon. The cocoon envelopes the jet, and it continues to do so even after the jet escapes the star, this cocoon escapes with the jet. When we calculated how much energy the cocoon has, it turned out to be as powerful as an FBOT.”
In Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Gottlieb is a Rothschild Fellow. In the Weinberg College of Arts and Sciences at Northwestern, he coauthored the study alongside CIERA member Sasha Tchekovskoy, an assistant professor of physics and astronomy.
This study paves the way for more advanced simulations of FBOTs. This next-generation model will allow us to directly connect the physics of the central black hole to the observables, enabling us to reveal otherwise hidden physics of the FBOT central engine.
Sasha Tchekovskoy
The hydrogen problem
FBOTs, which are called “F-bots,” are a category of cosmic explosions that were first discovered in the optical spectrum. Transients disappear almost as rapidly as they arise, as their name suggests. Within a few days, FBOTs attain their peak brightness before rapidly fading away far faster than typical supernovae rise and decline.
Just eight years after FBOTs were discovered, astronomers questioned whether the puzzling occurrences were connected to another transient class: gamma-ray bursts (GRBs). GRBs, which are the strongest and brightest explosions at all wavelengths, are also connected to fading stars. A big star that has run out of fuel and collapsed into a black hole fires jets, which result in a strong gamma-ray emission.
“The reason why we think GRBs and FBOTs might be related is that both are very fast moving at close to the speed of light and both are asymmetrically shaped, breaking the spherical shape of the star,” Gottlieb said. “But there was a problem. Stars that produce GRBs lack hydrogen. We don’t see any signs of hydrogen in GRBs, whereas in FBOTs, we see hydrogen everywhere. So, it could not be the same phenomenon.”
Gottlieb and his coauthors believe they have solved this issue with their new approach. The outermost layer of hydrogen-rich stars is frequently too thick for a jet to pass through.
“Basically, the star would be too massive for the jet to pierce through,” Gottlieb said. “So the jet will never make it out of the star, and that’s why it fails to produce a GRB. However, in these stars, the dying jet transfers all its energy to the cocoon, which is the only component to escape 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.”
Putting the picture together
FBOTs emit X-rays and radio waves in addition to their brilliant optical brilliance. These also are explained by Gottlieb’s model. A radio emission is produced when the stellar material inside the cocoon interacts with the thick gas surrounding the star.
Additionally, the black hole (created by the collapsing star) can emit X-rays when the cocoon extends far enough away from it. The X-rays combine with optical and radio waves to create a complete picture of the FBOT event.
Gottlieb thinks additional observations and simulations are required before we can conclusively grasp FBOTs’ enigmatic beginnings, although he is pleased by his team’s discoveries.
“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 among supernovae, GRBs, and FBOTs, which I think is very elegant.”
“This study paves the way for more advanced simulations of FBOTs,” Tchekovskoy said. “This next-generation model will allow us to directly connect the physics of the central black hole to the observables, enabling us to reveal otherwise hidden physics of the FBOT central engine.”
The study, “Shocked jets in CCSNe can power the zoo of fast blue optical transients,” was supported by the National Science Foundation (award numbers AST-1815304 and AST-2107839). The Texas Advanced Computing Center at the University of Texas at Austin’s supercomputers were used by the writers to create the simulation.
