Cocoons of dying stars could explain fast blue optical transients

(ORDO NEWS) — Since they were discovered in 2018, fast blue optical transients (FOTSs) have completely surprised and baffled observers and theoretical astrophysicists alike.

So hot that they glow blue, these mysterious objects are the brightest known optical phenomenon in the universe. But so far only a few such objects have been discovered, the origin of the BGOP remains elusive.

A group of astrophysicists at Northwestern University have come up with a bold new explanation for these curious anomalies.

Using the new model, astrophysicists believe that BHOPs may result from actively cooling cocoons that surround jets launched by dying stars. This is the first astrophysical model that is in full agreement with all the observations related to the BGOP.

When a massive star collapses, it can eject streams of debris at close to the speed of light. These streams, or jets, collide with the collapsing layers of the dying star, forming a “cocoon” around the jet.

The new model shows that as the jet pushes the cocoon outward – away from the core of the collapsing star – it cools, releasing heat in the form of the observed BHOP radiation.

“The jet originates deep inside the star and then works its way out to get out,” says Ore Gottlieb of Northwestern University, who led the study. “As the jet moves 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 leaves the star, this cocoon leaves it with the jet. When we calculated, how much energy the cocoon has, it turned out that it is as powerful as BGOP.”

Gottlieb is a Rothschild Fellow at the Northwestern Center for Interdisciplinary Research in Astrophysics (CIERA). He co-wrote the paper with CIERA member Sasha Shchekovskaya, assistant professor of physics and astronomy at the Weinberg College of Arts and Sciences, Northwestern University.

The hydrogen problem

of BHOP is a type of cosmic explosions originally detected in the optical wavelength range. As their name suggests, transients disappear almost as quickly as they appear. GBOPs reach their peak brightness within a few days and then fade rapidly – much faster than standard supernovae grow and decay.

Since discovering the BGOP just four years ago, astrophysicists have wondered if these mysterious events are related to another class of transient phenomena, gamma ray bursts (GRBs).

The strongest and brightest explosions in 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 that produce a massive burst of gamma rays.

“The reason we think that GRB and BGOP could be related is that they are both very fast – moving at close to the speed of light, and both have an asymmetric shape, breaking the star’s spherical shape,” Gottlieb said. . “But there’s a problem.

The stars that make GRBs don’t have enough hydrogen. We don’t see any signs of hydrogen in GRBs, while in GBOP we see hydrogen everywhere. So it can’t be the same thing.”

Using their new model, Gottlieb and his coauthors believe they may have found the answer to this problem. Hydrogen-rich stars tend to contain hydrogen in their outer layer – too thick for the jet to penetrate.

“In principle, the star would be too massive for the jet to pierce through it,” Gottlieb said. Therefore, the jet will never exit the star, and that is why it will not be able to produce GRB.

However, in such stars, the dying jet transfers all its energy to the cocoon, which is the only component capable of leaving the star. The cocoon will emit BHOP emissions, which will include hydrogen. This is another area where our model is in complete agreement with all of the BGOP observations.”

Putting the picture together

Although GOFs glow brightly in the optical wavelength range, they also emit radio waves and X-rays. Gottlieb’s model explains this as well.

When the cocoon interacts with the dense gas surrounding the star, this interaction heats up the stellar material, resulting in radio emission.

And when the cocoon expands far enough from the black hole (formed from a collapsed star), X-rays can seep out of the black hole. X-rays combine with radio and optical light to form a complete picture of the BGOP event.

While Gottlieb is encouraged by his team’s results, he says more observations and more models need to be made before we can finally understand the mysterious origins of BGOP.

“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 evolution before we can fully understand these explosions. But our model is able to draw a line between supernovae, GRBs and GBOP, which I think is very elegant.”

“This study paves the way for more advanced BHOP simulations. This next-generation model will allow us to directly relate the physics of the central black hole to observable objects, allowing us to uncover the hidden physics of the BHOP central engine.”

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