A very rare and peculiar burst of ultra-bright light in the universe became even more peculiar thanks to NASA's Hubble Space Telescope's Eagle Eye. The phenomenon, known as a Luminous Fast Blue Transient (LFBOT), unexpectedly flashes far from any galaxy. Only Hubble can pinpoint its location. The results only left astronomers even more confused. First, they don't know what LFBOTs are. Hubble's results show that they rule out some possible theories and know even less.
LFBOT is one of the brightest known visible light events in the universe - a sudden explosion like a camera flash. Since its first discovery in 2018, only a handful have been discovered - an event located about 200 million light-years away and nicknamed "The Cow." Currently, LFBOT is detected once a year.
After LFBOT was first detected, multiple telescopes observed it across the electromagnetic spectrum from X-rays to radio waves. Named AT2023fhn and nicknamed "Finch," this brief event displayed all the telltale characteristics of LFBOT. It shines with an intense blue light, evolves rapidly, and peaks in brightness before fading within days, unlike supernovae, which take weeks or months to dim.
But unlike any other LFBOT seen before, Hubble found Finch located between two neighboring galaxies - a nearby spiral galaxy about 50,000 light-years away and a smaller galaxy about 15,000 light-years away.
The image titled "AT2023fhnHSTWFC3/UVIS," with color key, scale bar and compass arrows, shows three galaxies against a velvety black space background. The largest is the white and blue spiral galaxy at the center of the image. The two smaller galaxies are the white patches on the left. A strange white blob with a red pointer near the top of the image is the dazzling light of an unknown object's explosion, but is not associated with any galaxy. Image credit: NASA, ESA, STScI, AshleyChrimes (ESA-ESTEC/Radbroud University)
"The Hubble observations were really critical. They made us realize that this object was unusual compared to other similar objects, because without Hubble data, we wouldn't know it at all," said Ashley Christmas, lead author of a Hubble paper to be published in the upcoming Monthly Notices of the Royal Astronomical Society (MNRAS). He is also a researcher at the European Space Agency and worked at Radboud University in Nijmegen, the Netherlands.
While these horrific explosions have long been thought to be a rare type of supernova known as a core-collapse supernova, massive stars that go supernova are short-lived by stellar standards. Therefore, huge primordial stars don't have time to travel too far from their birthplace - the population of newborn stars - before exploding. All previous LFBOTs have been found in the spiral arms of galaxies where star birth is ongoing, but Finch is not in any galaxy.
"The more we learn about LFBOTs, the more they surprise us," Klems said. "We have now shown that LFBOTs can occur far away from the centers of the nearest galaxies, and Finch's location does not match what we would expect from any kind of supernova."
The Zwicky Transient Facility, an ultra-wide-angle ground-based camera that scans the entire northern sky every two days, first alerted astronomers to "Finch" on April 10, 2023. After its discovery, researchers initiated pre-planned observing programs that remain on standby, ready to quickly turn their attention to any potential LFBOT candidate.
Spectroscopic measurements using the Gemini South telescope in Chile found that Finch was as hot as 36,000 degrees Fahrenheit. The Gemini telescope also helped determine its distance from Earth, allowing its luminosity to be calculated. These findings, along with data from other observatories, including NASA's Chandra X-ray Observatory and the National Science Foundation's ground-based Very Large Array radio telescope, confirm that the explosion was indeed a low-frequency celestial explosion.
One theory is that LFBOTs could be the result of stars being swallowed by intermediate-mass black holes (between 100 and 1,000 solar masses). The high resolution and infrared sensitivity of NASA's James Webb Space Telescope may eventually be used to discover that Fincher exploded within a globular star cluster in the outer halo of one of two neighboring galaxies. Globular star clusters are the most likely places to find intermediate-mass black holes.
To explain Finch's unusual position, researchers are considering the possibility that it is the result of a collision between two neutron stars that, far from their host galaxies, have been spiraling into each other for billions of years. This collision creates a kilonova - an explosion 1,000 times more powerful than an ordinary supernova. However, one very speculative theory is that if one of the neutron stars was highly magnetized - a magnetar - it would greatly further amplify the power of the explosion, making it up to 100 times as bright as a normal supernova.
"This discovery raises many more questions than it answers," Krismus said. "We need to do more work to find the correct explanation among the many possible explanations."
Because astronomical transients can occur at any time and anywhere and are relatively short-lived in astronomical terms, researchers need to rely on wide-field sky surveys that continuously monitor large areas of the sky to detect them and alert other observatories such as Hubble to make follow-up observations.
The researchers say that to better understand the phenomenon, more samples are needed. Upcoming all-sky survey telescopes, such as the ground-based Vera C. Rubin Observatory, may be able to detect even more phenomena, depending on the astrophysics behind them.