Luminous Fast Blue Optical Transients (LFBOTs) are a rare class of events whose origins are poorly understood. The prototypical example of LFBOTs is AT2018cow, a powerful astronomical explosion spotted in 2018 in a galaxy 200 million light-years away in the constellation of Hercules.
An artist’s impression of the LFBOT event AT2023fhn. Image credit: NOIRLab / NSF / AURA / M. Garlick / M. Zamani.
In 2018, astronomers witnessed an intense astronomical explosion far more powerful than the average supernova — 10 to 100 times brighter.
It was designated AT2018cow (nicknamed the Cow) and became the first example of LFBOTs.
Since AT 2018cow, several other LFBOTs have been discovered (both in real time and archival searches). These include ZTF18abvkwla (the Koala), CSS161010, ZTF20acigmel (the Camel), AT2020mrf and AT 2022tsd (the Tasmanian Devil).
LFBOTs shine intensely blue and they evolve rapidly, reaching peak brightness and fading again in a matter of days, unlike supernovae which take weeks or months.
The physics of how these objects form remains a mystery, and now the latest addition to the LFBOT collection raises more questions than it answers.
Named AT2023fhn (the Finch), the newly-detected event shows all the tell-tale characteristics of an LFBOT with an additional, unexpected characteristic.
Unlike any other LFBOTs seen before, it is an unusually long way from either of its two nearest galaxies — about 50,000 light-years from a nearby spiral galaxy and about 15,000 light-years from a smaller satellite galaxy.
“The more we learn about LFBOTs, the more they surprise us,” said Dr. Ashley Chrimes, an astronomer at ESA and Radboud University.
“We’ve now shown that LFBOTs can occur a long way from their nearest galaxy, and the location of the Finch is not what we expect for a supernova.”
AT2023fhn was discovered in data from several telescopes, including the Gemini Multi-Object Spectrograph (GMOS) instrument on the Gemini South telescope, one half of the International Gemini Observatory operated by NSF’s NOIRLab.
GMOS measured the temperature of the object to be a scorching 20,000 degrees Celsius (approximately 36,000 Fahrenheit) and helped determine its distance from Earth so its luminosity could be calculated.
Together with other observations, these findings confirmed the explosion was indeed an LFBOT.
However, the mystery of the AT2023fhn’s origin, as well as how it ended up so far away from its host galaxy, remains.
“Besides the speculation that LFBOTs are a particularly rare class of supernova, we have several other ideas about how they could form,” Dr. Chrimes said.
“For example, they could be the result of stars being torn apart by black holes, but finding a black hole so far from a galaxy is also not expected.”
“To explain the unusual location of the Finch, we are considering the possibility that it was the explosion of an extremely fast-moving star.”
“Alternatively, it could be the culmination of a much longer process, such as a collision of two neutron stars that have been spiraling toward each other for billions of years, though these are mere speculations.”
“The Finch may have caused problems for current theories of the nature of LFBOTs, but in science it is often the surprising results that reveal the gaps in our knowledge and lead to advances in our understanding of the Universe.”
“The discovery poses many more questions than it answers. More work is needed to figure out which of the many possible explanations is the right one.”
The paper will be published in the Monthly Notices of the Royal Astronomical Society: Letters.
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A.A. Chrimes et al. 2023. AT2023fhn (the Finch): a Luminous Fast Blue Optical Transient at a large offset from its host galaxy. MNRASL, in press; arXiv: 2307.01771
