For almost a decade, scientists have been baffled by a series of mysterious blue flashes emerging from deep space.
Since the first lit up astronomers’ telescopes in 2018, only 14 of these strange pulses have been detected, making them some of the rarest astronomical events on record.
Known as Luminous Fast Blue Optical Transients (LFBOTs), these lights burn faster and up to 100 times brighter than anything else scientists have seen.
Now, a group of scientists think their origin lies in the unlikely collision between a black hole and an ultra–hot sun.
LFBOTs come and go astonishingly fast, reaching their dazzling peak and fading into oblivion over days compared to the weeks or months of most stellar explosions.
Strangely, they also maintain their unique blue glow over the entire course of their brief display, which suggests they must be extremely hot the whole time.
Lead author of a new paper investigating these flashes, Dr Anya Nugent, from Harvard & Smithsonian, says 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.
Scientists have been baffled by mysterious blue flashes emerging from distant galaxies for almost a decade. Now, researchers say they could be caused by black holes hitting bright stars. Pictured: A blue flash coming from AT 2024wpp, a galaxy 1.1 billion light–years from Earth
In their pre–print paper, Dr Nugent and her co–authors examine the kinds of galaxies that the few confirmed blue flashes have been seen in.
By measuring the rates of star formation, mass, and levels of metallic elements in these galaxies, the researchers paint a picture of how LFBOTs might form.
This data suggests that 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 one part of a binary star system, in which two stellar giants orbit around a central point.
As these stars creep closer together, the larger of the two starts to feast on 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.
At the same time, the cannibal grows so fat on hydrogen stolen from the donor that it collapses under its own enormous bulk.
The star collapses inwards and detonates in a supernova explosion that leaves behind a stellar remnant, in the form of a black hole or neutron star.
Known as Luminous Fast Blue Optical Transients (LFBOTs), these lights burn faster and up to 100 times brighter than anything else scientists have seen (artist’s impression)
Scientists now say that LFBOTs may emerge from Wolf–Rayet stars, a massive class of stars that form when a black hole strips away the sun’s outer hydrogen layer. Pictured: A Wolf–Rayet star 15,000 light–years from Earth that is 30 times the mass of the sun
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.
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.’
Wolf–Rayet stars are ideal candidates for producing LFBOTs for a few critical reasons.
First, the light from LFBOT’s doesn’t show the telltale signature of elemental hydrogen, which suggests they should come from stars that have lost their hydrogen layer.
Wolf–Rayet stars are also massive and dense, which allows the black hole to feed as fast as possible and produce a colossal burst of light.
Finally, Professor Metzger adds: ‘They can also have dense material around them from earlier episodes of mass loss, which gives the explosion something to crash into and helps power the observed emission.’
When a black hole falls into its neighbouring Wolf–Rayet star, the destruction releases a massive burst of energy which briefly appears as LFBOT, such as this one spotted by NASA in 2023
Previously, scientists had thought that the bright surge of light could either come from an unusual type of supernova explosion or from a large star being torn apart by gravitational forces.
However, the LBOTs that scientists have seen don’t come from galaxies where either of these would be likely.
These galaxies either tend to have a rate of new star formation that is either too fast or too slow to fit with known types of supernovae.
Additionally, the black hole collision theory also helps to solve the biggest mystery surrounding these blue flashes.
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 on its own more than 50,000 light–years from the nearest spiral galaxy.
This is extremely unusual because, if they are being triggered by stars, you would expect to see them more often in regions where stars are the most tightly packed.
Scientists had thought that the bright surge of light could come from an unusual type of supernova explosion, but the galaxies where LFBOTs are found don’t match those where these explosions are likely
Dr Nugent says: ‘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.’
The researchers admit that, since the number of LBOTs 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.
