An international team of astronomers, led by scientists from the University of Warsaw, has announced the discovery of a new class of cosmic X-ray sources. The discovery concerns 29 objects in the Magellanic Clouds that emit X-ray radiation in an unprecedented way. This finding could advance our ability to measure distances in space and deepen our understanding of the history and beginnings of the Universe.
The scientists described their research in a scientific article published in The Astrophysical Journal Letters.
X-ray radiation in space
X-ray radiation, which is a type of electromagnetic radiation, is most commonly known for its use in medicine, where it is used for taking X-ray images (for example in diagnosing bone fractures).
“Far fewer people know that some celestial bodies can also be sources of X-ray radiation. X-rays with the lowest energies are usually emitted by very hot objects, such as gas falling onto a compact object – a white dwarf, neutron star, or black hole. X-rays can also be generated by the accelerated motion of charged particles, such as electrons,” said Dr Przemysław Mróz from the Astronomical Observatory of the University of Warsaw, the lead author of the publication.
The mysteries of the Magellanic Clouds
The Magellanic Clouds are two dwarf galaxies, which are the closest galactic neighbours to the Milky Way. They are most visible from the Southern Hemisphere, and their study continues to uncover many mysteries. Thanks to over 20 years of data from the OGLE sky survey, Polish scientists have discovered unusual sources of X-ray radiation within these galaxies.
The OGLE programme, which has been operational since 1992, is one of the most advanced astronomical projects in the world. Using telescopes at the Las Campanas Observatory in Chile, astronomers can systematically observe billions of stars. It is thanks to this unique database that 29 objects have been observed emitting X-ray radiation in a way that differs from previously known sources of such radiation.
Instead of the short, violent flares typical of many known X-ray sources in space, these objects emitted radiation for extended periods – even for several months. Additionally, their brightness temporarily increased by as much as 10–20 times, making these objects a true puzzle for astronomers. These studies may lead to new discoveries about the physics of the cosmos and the nature of X-ray radiation.
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Observation of Millinovae
In some cases, the flares repeated every few years, while in others, over 20 years of observations, only one brightening event was recorded. One of the observed objects, named OGLE-mNOVA-11, brightened in November 2023, allowing scientists to study it more closely. Key observations were made using the South African Large Telescope (SALT), one of the largest optical telescopes in the world, and the Swift satellite X-ray telescope.
Located at an altitude of 1,800 meters above sea level at the Sutherland Observatory in South Africa, SALT helped scientists study the physical composition of the object, detecting signals from partially ionized helium, carbon, and nitrogen atoms, indicating extremely high temperatures.
Additional observations using the X-ray telescope revealed that the object had a temperature of 600,000 degrees Celsius, and its X-ray emission was over 100 times stronger than the radiation from the Sun. The new class of stars discovered by scientists has been named Millinovae because, at their peak brightness, they are approximately 1,000 times weaker than classical nova stars.
Supernovae and the history of the Universe
It is unclear which mechanism is responsible for the production of X-ray radiation during the explosion of million-star objects. The authors of the study outlined two hypotheses: the first suggests that the X-ray radiation comes from matter being dumped onto the surface of a white dwarf.
The second hypothesis proposes that it originates from a thermonuclear explosion on the surface, where the accumulated hydrogen-rich gas reaches a critical temperature.
If the second hypothesis is true, million-star objects could be important in astrophysical research, as – as claim the researchers – a white dwarf, when it reaches 1.4 times the mass of the Sun, could explode as a Type Ia supernova. Such supernovae are crucial for measuring distances in the Universe and studying its beginnings and history.
Observations of these supernovae contributed to the discovery of the accelerating expansion of the Universe, which was honored with the Nobel Prize in Physics in 2011.
More:
- Mróz, P., Król, K., Szegedi, H., Charles, et al., (2024). Millinovae: A New Class of Transient Supersoft X-Ray Sources without a Classical Nova Eruption. The Astrophysical Journal Letters, 977(2), L37. https://doi.org/10.3847/2041-8213/ad969b.
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