An extraordinarily bright explosion in an unexpected place has been detected by the Hubble Space Telescope, a joint project of NASA and the European Space Agency.
In a study published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society journal, a team of researchers describes a rare phenomenon known as a Luminous Fast Blue Optical Transient (LFBOT).
These cosmic explosions were first discovered in 2018, and only a handful have been found since then. Their origins are poorly understood, but a number of potential explanations have been proposed.
Researchers have identified a particular set of LFBOT characteristics that have been detected so far. These phenomena shine intensely in blue light and evolve rapidly, reaching peak brightness before fading away in a few days.
The latest LFBOT to be discovered is unusual in that it appeared in a location where no one expected it to be, far between two galaxies, which puzzled scientists.
The LFBOT, known as AT2023fh and nicknamed “the Finch,” was observed with multiple telescopes after it was first spotted in April by the Zwicky Transient Facility, a wide-angle, ground-based camera that scans the entire northern sky every two days.
But only Hubble was able to accurately pinpoint its location—about 50,000 light-years from a nearby spiral galaxy and roughly 15,000 light-years from a smaller galaxy.
“The Hubble observations were really the crucial thing. They made us realize that this was unusual compared to the other ones like that, because without the Hubble data we would not have known,” Ashley Chrimes, a European Space Agency research fellow and lead author of the study, said in a press release.
One explanation for LFBOTs is that they represent a rare type of supernova known as core-collapse supernovae. Supernovae are cataclysmic cosmic explosions that occur when massive stars reach the end of their lives.
These kinds of stars are found in the spiral arms of galaxies where star birth is occurring. All previous LFBOTs have been found in these spiral arms. But the Finch was spotted in intergalactic space, challenging this theory of the origin of LFBOTs.
“The more we learn about LFBOTs, the more they surprise us,” Chrimes said. “We’ve now shown that LFBOTs can occur a long way from the center of the nearest galaxy, and the location of the Finch is not what we expect for any kind of supernova.”
To explain the unusual location of the Finch—which reached temperatures of 36,000 degrees Fahrenheit, according to measurements from the Gemini South telescope in Chile—researchers have hypothesized that it may have resulted from the collision of two neutron stars traveling far outside their host galaxy.
Neutron stars are the extremely dense remnants of supermassive stars that have exploded as supernovae. The collision of two neutron stars can produce huge explosions known as kilonovas. It is also possible that one of the neutron stars is highly magnetized—an object known as a magnetar—and that could greatly amplify the explosion’s power.
Another potential explanation is that LFBOTs could result from stars being torn apart by an intermediate-mass black hole, which ranges between 100 and 1,000 solar masses.
“The discovery poses many more questions than it answers,” Chrimes said. “More work is needed to figure out which of the many possible explanations is the right one.”