This is an artist's concept of one of brightest explosions ever seen in space. Called a Luminous Fast Blue Optical Transient (LFBOT), it shines intensely in blue light and evolves rapidly, reaching peak brightness and fading again in a matter of days, unlike supernovae which take weeks or months to dim. Only a handful of previous LFBOTs have been discovered since 2018. And they all happen inside galaxies where stars are being born. But this illustration shows that Hubble discovered the LFBOT flash seen in 2023 happened between galaxies. This only compounds the mystery of what these transient events are. Because astronomers don't know the underling process behind LFBOTs, the explosion shown here is purely conjecture based on some known transient phenomenon.
NASA, ESA, NSF's NOIRLab, Mark
Garlick , Mahdi Zamani
A very rare, strange burst of
extraordinarily bright light in the universe just got even stranger – thanks to
the eagle-eye of NASA's
Hubble Space Telescope.
The phenomenon, called a Luminous
Fast Blue Optical Transient (LFBOT), flashed onto the scene where it wasn't
expected to be found, far away from any host galaxy. Only Hubble could pinpoint
its location. And, the results are leaving astronomers even more confounded. To
start with, they don’t know what LFBOTs are. The Hubble results suggest they know even less by ruling out some
possible theories.
LFBOTs are among the brightest
known visible-light events in the universe – going off unexpectedly like camera
flashbulbs. Only a handful have been found since the first discovery in 2018 – an event located about 200 million light-years
away that was nicknamed "the Cow." Presently, LFBOTs are detected
once per year.
After its initial detection, the
latest LFBOT was observed by multiple telescopes across the electromagnetic
spectrum, from X-rays to radio waves. Designated AT2023fhn and nicknamed
"the Finch," the transitory event showed all the tell-tale
characteristics of an LFBOT. It shined intensely in blue light and evolved
rapidly, reaching peak brightness and fading again in a matter of days, unlike
supernovae, which take weeks or months to dim.
But unlike any other LFBOT seen
before, Hubble found that the Finch is located between two neighboring galaxies
– about 50,000 light-years from a nearby spiral galaxy and about 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," said Ashley Chrimes, lead author of the Hubble paper reporting the
discovery in an upcoming issue of the Monthly Notices of the Royal Astronomical Society
(MNRAS). He is also
a European Space Agency Research Fellow, formerly of Radboud University,
Nijmegen, Netherlands.
A NASA Hubble Space Telescope image of a Luminous Fast
Blue Optical Transient (LFBOT) designated AT 2023fhn, indicated by pointers. It
shines intensely in blue light and evolves rapidly, reaching peak brightness
and fading again in a matter of days, unlike supernovae which take weeks or
months to dim. Only a handful of previous LFBOTs have been discovered since
2018. The surprise is that this latest transient, seen in 2023, lies at a large
offset from both the barred spiral galaxy at right and the dwarf galaxy to the
upper left. Only Hubble could pinpoint its location. And, the results are
leaving astronomers even more confounded because all prevous LFBOTs have been
found in star-formig regions in the spiral arms of galaxy. It’s not clear what
astronomical event would trigger such a blast far outside of a galaxy.
NASA, ESA, STScI, Ashley Chrimes (ESA-ESTEC/Radboud
University)
While these awesome explosions have
been assumed to be a rare type of supernova called core-collapse supernovae,
the gargantuan stars that turn into supernovae are short-lived by stellar
standards. Therefore, the massive progenitor stars don't have time to travel
very far from their birthing place – a cluster of newborn stars – before
exploding. All previous LFBOTs have been found in the spiral arms of galaxies
where star birth is ongoing, but the Finch is not in any galaxy.
"The more we learn about
LFBOTs, the more they surprise us," said Chrimes. "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 Zwicky
Transient Facility – an extremely wide-angle ground-based camera that scans the entire
northern sky every two days – first alerted astronomers to the Finch on April
10, 2023. Once it was spotted, the researchers triggered a pre-planned program
of observations that had been on standby, ready to quickly turn their attention
to any potential LFBOT candidates that arose.
Spectroscopic measurements made
with the Gemini South telescope in Chile found that the Finch is a
scorching 36,000 degrees Fahrenheit. Gemini also helped determine its distance
from Earth so its luminosity could be calculated. Together with data from other
observatories including NASA's Chandra X-ray Observatory and the National Science Foundation's
ground-based Very Large Array radio telescopes, these findings confirmed the explosion was indeed
an LFBOT.
The LFBOTs could be the result of
stars being torn apart by an intermediate-mass black hole (between 100 to 1,000
solar masses). NASA's James
Webb Space Telescope's high resolution and infrared sensitivity might eventually be used to
find that the Finch exploded inside a globular star cluster in the outer halo
of one of the two neighboring galaxies. A globular star cluster is the most
likely place an intermediate-mass black hole could be found.
To explain the unusual location of
the Finch, the researchers are considering the possibility that it is the
result of a collision of two neutron stars, travelling far outside their host
galaxy, that have been spiraling toward each other for billions of years. Such
collisions produce a kilonova – an explosion 1,000 times more powerful than a
standard nova. However, one very speculative theory is that if one of the
neutron stars is highly magnetized – a magnetar – it could greatly amplify the
power of the explosion even further to 100 times the brightness of a normal
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."
Because astronomical transients can
pop up anywhere and at any time, and are relatively fleeting in astronomical
terms, researchers rely on wide-field surveys that can continuously monitor
large areas of the sky to detect them and alert other observatories like Hubble
to do follow-up observations.
A larger sample is needed to
converge on a better understanding of the phenomenon, say researchers. Upcoming
all-sky survey telescopes, such as the ground-based Vera C. Rubin Observatory,
may be able to detect more, depending on the underlying astrophysics.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
Source: NASA's Hubble Finds Bizarre Explosion in Unexpected Place - NASA Science
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