These blazing blue explosions may be born when a compact dead star slams into a Wolf-Rayet star

Luminous fast blue optical transients (LFBOTs) are among the universe’s brightest and fastest explosions but their origin is not completely understood. A new study takes a closer look at the galaxies they occur in, offering two important clues about their nature. A paper outlining these results was uploaded to the preprint server arXiv on March 24.

LFBOTs are called cow-like events, nicknamed after the first member of this class—AT2018cow—discovered in 2018. They are extremely bright explosions whose brightness peaks within a week and fades to half its peak value in the following week. Their peak brightness is typically greater than 10⁴³ erg per second at optical wavelengths. This is comparable with that of superluminous supernovae, which take a few weeks to months to peak and are generally 10 to 100 times brighter than normal supernovae.

Moreover, LFBOTs’ light curve—a graph that shows changes in their brightness over time—cannot be explained by the decay of nickel-56, which is a common energy source for normal and core-collapse supernovae. There are several theories for their origins; however, there is a lack of consensus.

Peering into their galactic homes

In the new study, Anya Nugent of the Harvard and Smithsonian Center for Astrophysics and her team examined the galaxies in which 11 of these fast explosions took place. They closely looked at where the explosions occur within their galaxies, how close they lie to regions of active star formation, and what kinds of stellar populations surround them.

To do this, they modeled physical properties such as stellar mass, star formation rate, and chemical composition to match the galaxy’s light. This allowed them to reconstruct the underlying environments in which these mysterious LFBOTs occur.

The team then compared the simulated LFBOT environmental conditions with those of other well-studied explosions in astrophysics, including various types of supernovae, long gamma-ray bursts, and superluminous supernovae.

Here is what they found: these host galaxies are actively forming stars showing signs of recent activity. However, they are less extreme than galaxies hosting superluminous supernovae and more active than those hosting many supernovae. Chemically, they are less enriched in heavy elements than supernovae host galaxies, but are more metal-rich than hosts of long gamma-ray bursts or superluminous supernovae.

Interestingly, unlike many stellar explosions that occur in bright, star-forming regions, a significant fraction of LFBOTs were found to occur far from these regions, sometimes in the faint outskirts of their galaxies.

Pinning it down

Put together, these clues allowed researchers to rule out scenarios tied to galaxy centers, such as stars being torn apart after wandering too close to black holes in tidal disruption events. At the same time, they found less support for other ideas, such as magnetar-powered or failed supernovae.

Therefore, they propose an unusual scenario that checks all the boxes. It involves a compact object, like a neutron star or black hole, merging with a massive Wolf-Rayet star. The binary system, which likely started in a star-forming region, receives a “natal kick” when the compact object formed in a supernova. This kick can carry the system away from its birth site, far from the star-forming region where the two objects eventually merge, forming an LFBOT.

“We emphasize that the current sample of LFBOTs is quite small; as such, our findings should be viewed as a preliminary foundation for future population studies,” the team writes in the paper.

Vera C. Rubin Observatory’s upcoming survey is expected to detect hundreds of these events every year. This large population could help astronomers pin down the origin of these mysterious explosions.

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