Researchers from the University of Warsaw have identified a previously unknown group of 29 celestial objects in the Magellanic Clouds, characterized by prolonged outbursts lasting several months. These outbursts resulted in brightness increases of 10 to 20 times, distinguishing them from known classes of astronomical objects.
The findings, published in the Astrophysical Journal Letters, were led by Dr. Przemysław Mróz. The objects were discovered through over 20 years of observations from the OGLE sky survey.
One notable object, designated OGLE-mNOVA-11, brightened in November 2023, prompting detailed observations using the Southern African Large Telescope (SALT). Spectra revealed signals from partially ionized helium, carbon, and nitrogen, indicating extremely high temperatures.
Additional observations with the Swift X-ray satellite detected X-ray emissions corresponding to temperatures around 600,000 degrees Celsius. Given its distance of over 160,000 light-years, the X-ray luminosity of this object was over a hundred times greater than that of the Sun.
The unusual properties of OGLE-mNOVA-11 bear similarities to ASASSN-16oh, discovered in 2016. Both objects and others in the study form a new class of X-ray sources termed 'millinovae,' which are approximately a thousand times less luminous than classical novae at peak brightness.
The researchers propose that these sources are binary star systems, involving a white dwarf and a subgiant. Material from the subgiant transfers to the white dwarf, leading to X-ray emissions during outbursts.
Two hypotheses explain the X-ray production: one suggests X-rays result from material falling onto the white dwarf's surface, while the other involves nuclear fusion igniting on the white dwarf due to accumulated hydrogen gas. If the latter is accurate, millinovae could be pivotal for astrophysical and cosmological studies, especially as white dwarfs near the Chandrasekhar limit may eventually trigger Type Ia supernovae, crucial for measuring cosmic distances.
This collaborative study involved researchers from the University of Warsaw, the University of Southampton, the University of Leicester, the University of Cape Town, and the University of the Free State.