A groundbreaking discovery by an international team of astronomers has revealed a completely new class of cosmic X-ray sources.
Led by researchers from the Astronomical Observatory of the University of Warsaw, this finding, published in Astrophysical Journal Letters, is shedding light on mysterious celestial phenomena.
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.
Theoretical Explanations and Astrophysical Implications
The source of the X-rays remains a mystery, but scientists have proposed two possible explanations. According to one scenario, the X-rays might be produced as the subgiant’s material falls onto the white dwarf’s surface, releasing energy.
Alternatively, the X-rays could result from a thermonuclear runaway on the white dwarf surface. As material accumulates on the white dwarf, hydrogen ignites, causing a thermonuclear explosion—but not a violent enough one to eject material.
If the latter hypothesis is correct, then millinovae may play a crucial role in astrophysics. As a white dwarf grows in mass, it can eventually reach a critical threshold (about 1.4 solar masses), at which point it may explode as a Type Ia supernova.
Astronomers use Type Ia supernovae as standard candles for measuring cosmic distances. In particular, observations of Type Ia supernovae led to the discovery of accelerating expansion of the Universe, a finding that won the 2011 Nobel Prize in Physics. However, the exact progenitors of Type Ia supernovae remain unknown.
Reference: “Millinovae: A New Class of Transient Supersoft X-Ray Sources without a Classical Nova Eruption” by Przemek Mróz, Krzysztof Król, Hélène Szegedi, Philip Charles, Kim L. Page, Andrzej Udalski, David A. H. Buckley, Gulab Dewangan, Pieter Meintjes, Michał K. Szymański, Igor Soszyński, Paweł Pietrukowicz, Szymon Kozłowski, Radosław Poleski, Jan Skowron, Krzysztof Ulaczyk, Mariusz Gromadzki, Krzysztof Rybicki, Patryk Iwanek, Marcin Wrona and Mateusz J. Mróz, 12 December 2024, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ad969b
The article is a collaboration between scientists from the Astronomical Observatory of the University of Warsaw and researchers from, among others, the University of Southampton, the University of Leicester (UK), the University of Cape Town and the University of the Free State (South Africa).
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