A New Class of Cosmic X-ray Sources

An exciting breakthrough has emerged from the depths of the universe, as an international team of astronomers, spearheaded by researchers from the Astronomical Observatory of the University of Warsaw, has uncovered a new class of cosmic X-ray sources. Their remarkable findings have been published in The Astrophysical Journal Letters.

While many people are familiar with X-rays from medical procedures to examine bones or diagnose lung ailments, what is less known is that celestial phenomena also emit X-ray radiation. Dr. Przemek Mróz, the lead author of the study, explains, "Some cosmic events generate X-rays naturally, such as hot gas falling onto dense compact objects like white dwarfs, neutron stars, or black holes." Additionally, X-rays can emerge from oriented charged particles, like electrons, losing energy as they slow down.

The Optical Gravitational Lensing Experiment (OGLE)

The research team focused on analyzing data from the Optical Gravitational Lensing Experiment (OGLE), which spans over 20 years and is guided by the University of Warsaw’s astronomers. During their investigation, they identified an intriguing group of 29 objects within the Magellanic Clouds—the Milky Way’s two nearest satellite galaxies. These cosmic entities exhibited extraordinary behavior by experiencing long-duration outbursts, where their brightness spiked dramatically—often increasing by 10 to 20 times for several months. While some objects displayed recurring outbursts every few years, others flared up only once throughout the research period.

The Fascination of OGLE-mNOVA-11

One particularly fascinating object, dubbed OGLE-mNOVA-11, ignited an outburst in November 2023, granting researchers an exceptional opportunity for in-depth examination. "We observed this star using the Southern African Large Telescope (SALT), one of the world's largest and most advanced telescopes," Dr. Mróz stated. "Its observations revealed the optical spectrum carried telltale signs of ionized helium, carbon, and nitrogen—indicators of extremely high temperatures."

Observations from the Neil Gehrels Swift Observatory revealed that OGLE-mNOVA-11 reached astounding temperatures of about 600,000 degrees Celsius and boasted more than 100 times the luminosity of our Sun, despite being located over 160,000 light years away. The characteristics of this object presented strong similarities to a previously discovered system known as ASASSN-16oh, identified in 2016.

New Class: Millinovae

Dr. Mróz remarked, "We believe that OGLE-mNOVA-11, ASASSN-16oh, and the other 27 objects form an entirely new class of transient X-ray sources, which we've termed 'millinovae.' Their peak brightness is approximately a thousand times dimmer than classical novae, yet they reveal a complex stellar ballet."

Experts theorize that millinovae consist of binary star systems, featuring a compact white dwarf and a subgiant star that has shed hydrogen from its core. The gravitational dance between the two allows material to funnel from the subgiant onto the dancing white dwarf.

The Mystery of X-ray Emissions

The source of the intense X-ray emissions remains enigmatic. Scientists suggest two intriguing hypotheses: the X-rays might be generated as the subgiant material cascades onto the white dwarf’s surface, releasing immense energy. Alternatively, a lesser-known scenario posits that the X-rays arise from a thermonuclear runaway on the white dwarf's surface. As the material accumulates, hydrogen ignites, causing a thermonuclear explosion that doesn't eject debris, but could be significant enough to impact the white dwarf’s evolution.

Implications for Cosmology

Should this latter theory hold validity, millinovae could play a crucial role in cosmology and astrophysics. As a white dwarf accumulates mass, it might reach a critical point (approximately 1.4 solar masses), triggering a catastrophic event known as a Type Ia supernova.

Type Ia supernovae serve as cosmic benchmarks for measuring distances across the Universe. Observations of such supernovae have even led to the groundbreaking discovery that the universe is expanding at an accelerating rate—a discovery that earned the 2011 Nobel Prize in Physics. However, the true origin of these stellar explosions remains one of astronomy’s greatest mysteries.

Collaboration and Future Prospects

This collaborative effort unites bright minds from the Astronomical Observatory of the University of Warsaw alongside researchers from esteemed institutions, including the University of Southampton, the University of Leicester in the UK, the University of Cape Town, and the University of the Free State in South Africa. This groundbreaking discovery of millinovae not only enriches our understanding of the cosmos, but also paves the way for future astronomical advancements and explorations!