For better or worse, astronomers anthropomorphize pretty much everything in the universe. When we discuss how stars form, generate energy, and eventually flicker out, we assign these phases names like stellar birth, the star’s life cycle, and stellar death.

And, to be entirely frank, a whole lot of energy goes into understanding birth and death because these two parts of a star’s life are both the most beautiful and the most complicated, and who among us can really walk away from a captivating puzzle?

One of the newer puzzles to hit the scene is the Fast Blue Optical Transit or FBOT. First discovered in 2018, the most famous of these events was initially cataloged as SN 2018cow, or “The Cow”. Characterized by their abundance of blue and ultraviolet light and extremely brief appearance, these flashes are substantially brighter than a normal supernova and seem to require an explanation other than “A massive star collapsed and exploded its atmosphere in a tremendous flash of light that then cooled and expanded.” SN 2018cow was re-cataloged as AT (Astronomical Transient) 2018cow, and the search for an explanation began. Since its discovery, four more Fast Blue Optical Transients have been spotted.

And now, we may have a theory to explain what is going on. This work is published in Monthly Notices of the Royal Astronomical Society and led by Ore Gottlieb.

Massive stars were known to die in a few different ways. Beyond supernovae, there are also kilonovae that briefly flare out in gamma rays as compact stars, like neutron stars or when a neutron star and black hole merge. Then there are Long Gamma-Ray Bursts which are some weird mutant, extremely energetic explosion that involves a star with no hydrogen.

FBOTs have hydrogen but are brighter than normal supernovae, suggesting they could be massive stars, but ones that still have hydrogen. When the collapsing star forms jets, those jets of energy get absorbed by the hydrogen cocoon. Over time, that energy is re-released.

As Gottlieb explained: Basically, the star would be too massive for the jet to pierce through. 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.

While the theory matches observations over a variety of wavelengths of light, the number of examples it has to match is few, and there may be details we’re missing. I feel this work is the start of solving this puzzle, but as Gottlieb further explains: This is a new class of transients, and we know so little about them. We need to detect more of them earlier in their evolution before we can fully understand these explosions.

More Information

Northwestern University press release

“Shocked jets in CCSNe can power the zoo of fast blue optical transients,” Ore Gottlieb, Alexander Tchekhovskoy, and Raffaella Margutti, 2022 April 11, Monthly Notices of the Royal Astronomical Society