Rare blue cosmic explosions, called LFBOTs, could originate from the collision between a black hole or neutron star and a Wolf-Rayet star. Since 2018, these events have only been detected fourteen times, and their persistent blue color as well as their evolution over a few days resemble nothing else known.
Unlike classical supernovae, which fade over several weeks, LFBOTs reach their peak brightness in a few days and remain blue, a sign of extremely high temperature. Their rarity makes their origin difficult to pinpoint.
A new study proposes an unprecedented scenario: a black hole or neutron star collides with a Wolf-Rayet star, the hottest star in the universe. These massive stars have lost their hydrogen envelope. In this model, a binary system evolves until one of the stars collapses into a compact object that then strikes its companion hundreds of years later.
Competing models, such as supernovae or tidal disruption events by a black hole, struggle to explain all the properties of LFBOTs, especially their dense circumstellar environment. Researchers therefore believe these explosions come from a unique process, and the collision with a Wolf-Rayet star matches all observations well.
To confirm this hypothesis, more LFBOTs will need to be discovered. The Vera C. Rubin Telescope, with its LSST survey, should allow detection of hundreds, including the faintest and most distant ones. This will reveal how these phenomena have evolved over cosmic time.
Wolf-Rayet stars: ultra-hot giants
Wolf-Rayet stars are among the most massive and hottest in the universe. Their surface temperature can reach 30,000 to 200,000 degrees Celsius. Their peculiarity is that they have lost their outer hydrogen envelope, leaving their helium core exposed. This loss is due to very powerful stellar winds that blow matter away at high speed.
These stars are rare and have short lifespans, on the order of a few million years. They often evolve in binary systems, where a companion star can accelerate their stripping. Their fate is generally to end as a supernova; some can give birth to black holes or neutron stars.
In the proposed model for LFBOTs, a Wolf-Rayet star is impacted by a compact object. The collision releases colossal energy, producing the observed blue explosion. Understanding these stars therefore helps to better grasp the origins of these luminous phenomena.