For almost a decade, scientists have been perplexed by a series of enigmatic blue flashes emanating from deep space. Since the first such event illuminated astronomers' telescopes in 2018, only 14 of these strange pulses have been detected, making them among the rarest astronomical phenomena ever recorded.
Known as Luminous Fast Blue Optical Transients (LFBOTs), these lights burn faster and up to 100 times brighter than any other celestial events observed. Now, a team of scientists believes their origin lies in the improbable collision between a black hole and an ultra-hot star.
What Are LFBOTs?
LFBOTs appear and vanish astonishingly quickly, reaching their dazzling peak and fading into obscurity within days, compared to the weeks or months typical of most stellar explosions. Strangely, they maintain their distinctive blue glow throughout their brief display, suggesting they remain extremely hot the entire time.
Lead author of a new paper investigating these flashes, Dr. Anya Nugent from Harvard & Smithsonian, states this makes them 'unlike anything we have observed before.' However, Dr. Nugent now believes that LFBOTs' origins are just as violent, unusual, and unlikely as their spectacular effects.
The Collision Hypothesis
In their pre-print paper, Dr. Nugent and her co-authors examine the types of galaxies where the few confirmed blue flashes have been observed. By measuring star formation rates, mass, and levels of metallic elements in these galaxies, the researchers paint a picture of how LFBOTs might form.
This data suggests they could be caused by ultra-dense objects like black holes or neutron stars colliding with an exceptionally bright sun called a Wolf-Rayet star. Wolf-Rayet stars begin their lives as part of a binary star system, where two stellar giants orbit a central point. As these stars creep closer, the larger one starts to consume its neighbour's outer layers.
If they are just the right size, the 'donor' star's outer hydrogen layer is stripped away without destroying it completely, leaving the bright helium core known as a Wolf-Rayet star. Simultaneously, the cannibal grows so fat on stolen hydrogen that it collapses under its own enormous bulk. The star collapses inward and detonates in a supernova explosion, leaving behind a stellar remnant in the form of a black hole or neutron star.
That black hole continues to feed on its neighbour until, over hundreds of thousands of years, it falls into the stellar core and destroys it, triggering an LFBOT.
Why Wolf-Rayet Stars?
Wolf-Rayet stars are ideal candidates for producing LFBOTs for several critical reasons. First, the light from LFBOTs does not show the telltale signature of elemental hydrogen, suggesting they should come from stars that have lost their hydrogen layer. Wolf-Rayet stars are also massive and dense, allowing the black hole to feed as quickly as possible and produce a colossal burst of light.
Co-author Professor Brian Metzger of Columbia University told the Daily Mail: 'When the compact object plunges into the Wolf-Rayet star, it can rapidly accrete the stellar material and release a huge amount of gravitational energy. Some of that energy drives powerful outflows or jets, which then collide with material around the star. That interaction can produce a very hot, bright flash of light on a short timescale.'
Solving the Mystery of Location
Previously, scientists had thought that the bright surge of light could come from an unusual type of supernova explosion or from a large star being torn apart by gravitational forces. However, the LFBOTs observed do not come from galaxies where either of these would be likely. These galaxies either have a rate of new star formation that is too fast or too slow to fit with known types of supernovae.
Additionally, the black hole collision theory helps solve the biggest mystery surrounding these blue flashes: their location. LFBOTs are often found in the very outer reaches of their host galaxies, far away from the densely packed galactic centre. For example, one flash was spotted breaking out from a region some 55,000 light-years away from its galaxy's core. Another known as 'The Finch', found by NASA in 2023, was spotted more than 50,000 light-years from the nearest spiral galaxy.
This is extremely unusual because, if they were triggered by stars, you would expect to see them more often in regions where stars are most tightly packed. Dr. Nugent explains: 'We think it's because their progenitors must have received a 'kick' to push them out of their birthsite and away from these regions in their host galaxies. Stars can get strong kicks from supernova explosions, and if LFBOTs do indeed come from compact object-Wolf-Rayet star mergers, it is more than likely that the star that created the compact object underwent a supernova that gave the binary system the kick.'
Future Observations
The researchers admit that, since the number of LFBOTs is still so small, this is far from a 'closed case.' Many more observations will be needed before scientists can be absolutely certain what is triggering these bizarre explosions. However, they expect that the Vera C. Rubin Observatory and its newly started decade-long Legacy Survey of Space and Time should help provide those answers.
What Are Black Holes?
Black holes are so dense and their gravitational pull is so strong that no form of radiation can escape them, not even light. They act as intense sources of gravity that hoover up dust and gas around them. Their intense gravitational pull is thought to be what stars in galaxies orbit around. How they are formed is still poorly understood. Astronomers believe they may form when a large cloud of gas up to 100,000 times bigger than the sun collapses into a black hole. Many of these black hole seeds then merge to form much larger supermassive black holes, which are found at the centre of every known massive galaxy. Alternatively, a supermassive black hole seed could come from a giant star, about 100 times the sun's mass, that ultimately forms into a black hole after it runs out of fuel and collapses. When these giant stars die, they also go 'supernova', a huge explosion that expels the matter from the outer layers of the star into deep space.
