On December 23, 2024, an international team of astronomers announced the discovery of a new type of X-ray sources in the Magellanic Clouds, two galaxies orbiting the Milky Way. The findings, published in the Astrophysical Journal Letters, reveal a cluster of 29 unusual celestial objects exhibiting dramatic brightness fluctuations, increasing their luminosity by 10 to 20 times over several months.
Among these, an object designated OGLE-mNOVA-11 became active in November 2023, providing researchers with a unique opportunity for in-depth study. Observations from the South African Large Telescope indicated the presence of ionized helium, carbon, and nitrogen atoms, suggesting extremely high temperatures. X-ray emissions recorded by the Neil Gehrels Swift Observatory corresponded to temperatures around 600,000 degrees Celsius.
OGLE-mNOVA-11 shares characteristics with ASASSN-16oh, identified in 2016, leading scientists to propose that these objects represent a new class of transient X-ray sources termed 'millinovas.' These are thought to be binary systems consisting of a white dwarf and a subgiant star that has exhausted hydrogen in its core and expanded in size.
The origin of X-ray emissions in millinovas remains under investigation, with two primary hypotheses put forth. The first suggests that the X-rays result from material being pulled from the subgiant to the white dwarf. The second posits that nuclear reactions occurring on the surface of the white dwarf, triggered by accumulated matter, are responsible.
If the latter hypothesis is confirmed, millinovas could significantly impact astrophysics, as a white dwarf may reach critical mass and trigger a Type Ia supernova explosion. Such events are crucial for measuring cosmic distances and understanding the accelerating expansion of the universe.