Millinovae: A New Class of Cosmic Explosions
Millinovae are a recently discovered class of cosmic explosions that are shedding light on some of the universe’s most mysterious phenomena. These powerful events, which are about a thousand times less luminous than classical novae, are thought to be caused by the interaction between a white dwarf star and a nearby companion star.
How Millinovae Illuminate the Universe:

• Unveiling Hidden Stellar Systems: Millinovae occur in binary star systems where a white dwarf star is accreting matter from its companion. By studying these explosions, astronomers can learn more about the properties of these systems and the evolution of stars.
• Probing Extreme Environments: The intense temperatures and pressures generated during a millinova can create conditions that are difficult to replicate on Earth. This makes them ideal laboratories for studying fundamental physics and nuclear processes.
• Shedding Light on Dark Matter: Some scientists believe that millinovae could be a source of dark matter, the mysterious substance that makes up most of the matter in the universe. By studying these explosions, we may be able to learn more about the nature of dark matter.
  A Recent Discovery:
  In 2023, astronomers discovered a new millinova, named OGLE-mNOVA-11, which provided a unique opportunity for detailed study. Observations revealed that the star emitted more than 100 times the luminosity of the Sun, with temperatures reaching 600,000 degrees Celsius. This discovery further solidified the existence of millinovae and their potential to illuminate the dark corners of the universe.
  

Cosmic X-ray Phenomena

Most people are familiar with X-rays from medical settings, where they help produce images of bones or diagnose conditions like lung disease. In these cases, the X-rays are created using artificial sources.

What many don’t realize is that celestial objects can also emit X-ray radiation. “Some cosmic phenomena produce X-rays naturally,” says Dr. Przemek Mróz, the study’s lead author. “For example, X-rays may be produced by a hot gas falling onto compact objects like white dwarfs, neutron stars, or black holes. X-rays can also be generated by decelerating charged particles, such as electrons.”

Discovery of Unusual Celestial Objects

A team of researchers discovered 29 unusual objects in the Magellanic Clouds, two satellite galaxies near the Milky Way. These objects exhibited surprising behavior: during long-lasting outbursts, typically spanning a few months, their brightness increased 10 to 20 times. While some of these objects showed recurring outbursts every few years, others flared up only once during the observation period.

The team discovered these objects by analyzing over 20 years of data collected by the Optical Gravitational Lensing Experiment (OGLE) survey, led by astronomers from the University of Warsaw.

Characteristics and Observations of OGLE-mNOVA-11

One detected object, named OGLE-mNOVA-11, began an outburst in November 2023, providing a unique opportunity for a detailed study.

“We observed this star with the Southern African Large Telescope (SALT), one of the largest telescopes in the world,” says Dr. Mróz. “Its optical spectrum revealed signatures of ionized atoms of helium, carbon, and nitrogen, indicating extremely high temperatures

The star was also observed by the Neil Gehrels Swift Observatory, which detected X-rays corresponding to a temperature of 600,000 degrees Celsius. Given its distance of over 160,000 light years, OGLE-mNOVA-11 emitted more than 100 times the luminosity of the Sun.

The Nature and Impact of Millinovae

The object’s unusual properties closely resembled another system, called ASASSN-16oh, discovered in 2016 by the All Sky Automated Survey for SuperNovae.

“We believe OGLE-mNOVA-11, ASASSN-16oh, and the other 27 objects form a new class of transient X-ray sources,” says Dr. Mróz. “We’ve named them millinovae, as their peak brightness is roughly a thousand times lower than that of classical novae.”

Millinovae are thought to be binary star systems consisting of two objects orbiting each other with a period of a few days. A white dwarf—a dense remnant of a once-massive star—closely orbits a subgiant star that has exhausted the hydrogen in its core and expanded. The proximity between the two stars allows material to flow from the subgiant to the white dwarf.

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