Astronomers offer explanation for mysterious fast flares
(ORDO NEWS) — Some large stars, when dying, create bright short-term flashes in the optical and other ranges. Scientists have associated their occurrence with the peculiarities of the movement of jets – narrow streams of relativistic particles that carry away pieces of a dying star with them.
Many of the events that telescopes detect in deep space remain a mystery. So, from time to time, instruments record the appearance of short-term blue optical transients ( Fast Blue Optical Transients, FBOTs) – very powerful flashes of optical radiation, reaching peak brightness in just a few days and starting to emit in radio waves and X-rays, and then just as quickly falling off.
The most famous of the FBOTs was the AT2018cow transient, which was registered in 2018 and received the unofficial nickname “Cow” (Cow).
The nature of FBOTs remains a mystery, although it is clear that they are associated with the death of massive stars and, possibly, the formation of black holes or neutron stars from their remains.
A new hypothesis to explain such events was put forward by Ore Gottlieb and his colleagues from the American Northwestern University, whose article was published in the journal Monthly Notices of the Royal Astronomical Society.
When a sufficiently massive star dies and its outer layers collapse, jets are ejected from the interior – narrow streams of radiation and particles accelerated to near-light speeds. Flying away, they can create powerful gamma-ray bursts that cover almost the entire spectrum.
However, FBOTs only occur at certain wavelengths. According to the model proposed by Gottlieb and his co-authors, they occur if the star is too large, and the energy of the jet is not enough to break through the uppermost, hydrogen-rich layers.
A narrow stream draws out a part of this substance, taking away the red-hot “cocoon” of matter, which, quickly cooling down, begins to radiate brightly in the optical range.
Moving on, this “cocoon” collides with the surrounding matter and radiates in radio waves. And when it expands enough, the black hole formed in the place of the star opens and its X-ray emission becomes visible, completing the typical FBOT picture.
“This is a completely new class of transients, and we don’t know enough about them yet,” says Gottlieb. “We need new observations, especially in the early stages of their evolution, to better understand these outbreaks.
However, our model finds parallels between supernova explosions [type II — the collapse of old massive stars to form a black hole or neutron star], gamma-ray bursts and FBOTs — and, in my opinion, quite elegantly.”
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