Mysterious explosion from empty region of space puzzles astronomers
It feels like every time astronomers get a handle on a cosmic phenomenon, a new one pops up that sends them back to the drawing board. Case in point – Hubble has spotted a strange burst of light in a region of space where there didn’t seem to be a trigger.
A few years ago, astronomers officially described a new type of explosion in space – Luminous Fast Blue Optical Transients (LFBOTs), or just FBOTs. That name is pretty descriptive of what they are – very bright flashes of light in the optical part of the spectrum, that tend to shine brightly at blue wavelengths before fading in a matter of days.
Only a few examples of LFBOTs have been identified in data going back to 2016, but from their common characteristics astronomers had started to pull together a list of hypotheses about their origin. The leading candidate was a rare event called a core-collapse supernova, which occurs when a giant star exhausts its fuel supply and explodes under its own gravity.
But then along comes a new LFBOT that upends the whole thing. Designated AT2023fhn and nicknamed the Finch, this event was discovered by the Zwicky Transient Facility (ZTF) telescope on April 10, 2023, and at first glance it had all the usual features. It was bright, blue, and faded over a few days. An analysis of its spectrum by the Gemini South telescope estimated its temperature to be a toasty 20,000 °C (36,000 °F).
It was only when Hubble took a look that the whole thing unraveled. When the iconic telescope pinpointed its location in space, it became clear the usual hypotheses wouldn’t apply. Previous LFBOTs have all been found inside galaxies where active star formation is in progress – but the Finch seems to be floating around on its own in intergalactic space, at least 15,000 light-years from the nearest galaxy. The giant stars that die in core-collapse supernovae are only short-lived, they shouldn’t have had enough time to float out to such an isolated pocket of space.
“The more we learn about LFBOTs, the more they surprise us,” said Ashley Chrimes, lead author of the study. “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.”
The astronomers are considering other possible explanations, including a star that’s being torn apart by an intermediate-mass black hole, or a collision between two neutron stars. If one of those neutron stars is a highly magnetized version, known as a magnetar, it could boost the brightness 100 times higher than a regular supernova.
“The discovery poses many more questions than it answers,” said Chrimes. “More work is needed to figure out which of the many possible explanations is the right one.”
The team says that follow-up observations by the James Webb Space Telescope could reveal new clues. For example, the explosion may have come from a globular star cluster located in the outer halo of a nearby galaxy, which would lend weight to the black hole hypothesis.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.