A mysterious cosmic explosion has created a brilliant flash of light in the space between two distant galaxies over 3 billion light-years away. The optical flash, one of the brightest blue light explosions in the universe, lasted only a few days. It is the latest example of a rare species of brief astronomical event called Luminous Fast Blue Optical Transients (LFBOT).

Mystery of LFBOT

LFBOTs remain an absolute mystery to this day. The first event was observed only in 2018. Designated AT2018COW, it was located in the spiral arm of its galaxy, 200 million light-years away. Nicknamed the Cow, it was up to 100 times brighter than a normal supernova and was also bright in radio waves, ultraviolet, and X-rays.

If it had been a supernova, it would have behaved strangely. Usually, they remain bright for weeks or even months and have a recognizable spectrum. In contrast, the Cow faded after a few days. Similar bursts of light are discovered at a rate of about one per year and are nicknamed after animals based on the last three letters of their designation.

Other LFBOTs have been nicknamed the Camel, the Koala, and the Tasmanian Devil. The latest discovery, detected by the Zwicky Transient Facility at the Palomar Observatory in California on April 10, is designated AT2023FHN and has therefore been nicknamed the Finch.

Unusual Location of the Explosion

After the initial detection by LFBOT, a planned sequence of observations was carried out by ground-based and space telescopes. Gemini South telescope in Chile measured the spectrum and found it to be 20,000°C. Not as hot as some massive stars, but certainly not as hot as a supernova. Redshift measurements place it at about 3 billion light-years away, an immense distance at which only the Hubble Space Telescope could observe the host galaxy.

Strangely, the Finch is not located in a galaxy at all. All previous LFBOTs were observed in the spiral arms of galaxies. Hubble observed that the cosmic explosion was instead in intergalactic space, about 50,000 light-years from a spiral galaxy and 15,000 light-years from a small galaxy.

Hubble Space Telescope image of the cosmic explosion AT2023fhn, known as the Finch, and its position near two galaxies. Credit: NASA/ESA
Hubble Space Telescope image of the cosmic explosion AT2023fhn, known as the Finch, and its position near two galaxies. Credit: NASA/ESA

Its location rules out the possibility of it being the supernova of a massive star in explosion. Instead, there are rogue stars that are ejected from a galaxy into the cosmos after encountering a supermassive black hole.

Some Possible Solutions

“The more we learn about LFBOTs, the more they surprise us,” said Ashley Chrimes (ref.), a researcher at the European Space Agency (ESA) and the lead author of a new article describing the observed LFBOT (ref.). “We have shown that LFBOTs can occur far from the center of the nearest galaxy. The Finch’s position is not what we expect for any type of supernova”.

Chrimes and her team are focusing on two possible explanations. One is that the cosmic explosion was a flash of light caused by a star being ripped apart by an intermediate-mass black hole. These black holes reside in the cores of some globular clusters, hiding at the outskirts of galaxies. Chrimes plans to eventually use the powerful optics of the James Webb Space Telescope to search for any faint globular clusters in the same location as the Finch.

Alternatively, it could have been a Kilonova, the explosion resulting from the collision of two neutron stars. The Laser Interferometer Gravitational-Wave Observatory was not operational at that time to detect any gravitational waves resulting from a neutron star merger. “The discovery raises more questions than answers” Chrimes said. “Further work is needed to understand which of the many possible explanations is the correct one”.

Stefano Gallotta
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