A team of astronomers has discovered a new class of cosmic X-ray sources, called “millinovae,” in nearby galaxies.
This breakthrough was led by researchers from the Astronomical Observatory of the University of Warsaw and has been published in The Astrophysical Journal Letters.
X-rays are commonly associated with medical imaging, but they also occur naturally in space.
Certain cosmic events, such as hot gas falling onto dense objects like white dwarfs or neutron stars, or the slowing down of charged particles, can emit X-rays.
The researchers found 29 unusual objects in the Magellanic Clouds, two small galaxies orbiting the Milky Way. These objects showed long-lasting outbursts, brightening 10 to 20 times more than usual for several months. Some outbursts occurred repeatedly over the years, while others were observed only once.
The discoveries came from analyzing more than 20 years of data from the Optical Gravitational Lensing Experiment (OGLE), a survey run by the University of Warsaw.
One object, named OGLE-mNOVA-11, began an outburst in November 2023, allowing for detailed observation.
OGLE-mNOVA-11 was studied using the Southern African Large Telescope (SALT) and the Neil Gehrels Swift Observatory.
SALT detected extremely high temperatures through signatures of ionized helium, carbon, and nitrogen, while the Swift Observatory measured X-ray temperatures of about 600,000°C. At a distance of 160,000 light years, the object emitted over 100 times the brightness of the sun.
Similar properties were seen in another system called ASASSN-16oh, discovered in 2016. The researchers believe these two objects, along with the other 27, belong to a new class of X-ray sources.
The name “millinovae” reflects their brightness, which is about 1,000 times less than classical novae.
Millinovae are thought to be binary star systems where two stars orbit each other closely over a few days. One is a white dwarf—a dense, dead star—and the other is a subgiant star that has expanded after using up its core hydrogen.
The proximity between them allows material from the subgiant to flow onto the white dwarf.
The exact source of the X-rays remains unclear. One possibility is that the X-rays are produced as the subgiant’s material crashes onto the white dwarf. Another theory suggests a thermonuclear reaction on the white dwarf’s surface, where hydrogen builds up and ignites without causing a major explosion.
If the thermonuclear reaction theory is correct, millinovae could help scientists understand how white dwarfs grow in mass. When a white dwarf reaches about 1.4 times the mass of the sun, it may explode as a Type Ia supernova—a critical tool for measuring cosmic distances.
This discovery brings astronomers closer to understanding these powerful cosmic events, which have played a vital role in uncovering the accelerating expansion of the universe.
Source: University of Warsaw.
