Artist’s impression of a classical nova eruption. (Credit: Krzysztof Ulaczyk / Astronomical Observatory, University of Warsaw.)
WARSAW — While most of us associate X-rays with medical visits and bone scans, these high-energy emissions also tell fascinating stories about cosmic phenomena. In an exciting astronomical discovery, researchers have identified a new category of stellar fireworks that bridge the gap between two well-known types of exploding stars. These cosmic newcomers, dubbed “millinovae,” shine about a thousand times fainter than classical novae but still pack enough punch to command attention from astronomers worldwide.
Astronomers from the University of Warsaw, leading an international team, have found 29 of these stellar systems in our galactic backyard — specifically in the Large and Small Magellanic Clouds, two satellite galaxies orbiting our Milky Way. Their findings, published in The Astrophysical Journal Letters, shed light on a previously unknown class of binary star systems that undergo periodic outbursts.
“Some cosmic phenomena produce X-rays naturally,” explains Dr. Przemek Mróz, the lead author of the study, in a statement. “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.”
At the heart of each millanova lies a fascinating cosmic dance between two stars: a white dwarf (the dense remnant of a Sun-like star) and its companion. The white dwarf’s powerful gravity pulls material from its partner, creating a disk of hot gas around itself. This process occasionally triggers bright outbursts that can last for months.
The story began with a peculiar star system called ASASSN-16oh, discovered in 2016. Unlike typical nova explosions, which brighten dramatically over days and fade quickly, this system showed an unusually long, symmetric brightening that lasted months. Even more intriguingly, it emitted supersoft X-rays – a high-energy signature typically associated with much more powerful explosions.
To determine whether ASASSN-16oh was a cosmic oddball or representative of a larger population, the research team analyzed two decades of observational data from the Optical Gravitational Lensing Experiment (OGLE). They scoured through light curves of 76 million stars, ultimately identifying 29 systems showing similar behavior.
The research team’s breakthrough came when one of these objects, designated OGLE-mNOVA-11, began an outburst in November 2023. “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 observations revealed that OGLE-mNOVA-11 was generating X-rays corresponding to temperatures of 600,000 degrees Celsius. Given its immense distance of over 160,000 light years, the system was pumping out more than 100 times the luminosity of our Sun.
The source of the X-rays remains mysterious, with scientists proposing two possible explanations. In one scenario, the X-rays might be produced as the subgiant’s material falls onto the white dwarf’s surface, releasing energy. Alternatively, they could result from a thermonuclear runaway on the white dwarf surface, where accumulated hydrogen ignites but doesn’t create an explosion powerful enough to eject material.
The discovery of millinovae may have far-reaching implications for our understanding of cosmic evolution. These systems could help solve one of astronomy’s persistent mysteries: the origins of Type Ia supernovae. These stellar explosions serve as “standard candles” for measuring cosmic distances, and their study led to the discovery of the universe’s accelerating expansion — a finding so significant it earned the 2011 Nobel Prize in Physics. However, astronomers still debate exactly what kinds of stellar systems lead to these crucial cosmic events.
If millinovae represent a pathway for white dwarfs to gradually gain mass without violent eruptions, they might be the missing link in our understanding of how these stars reach the critical mass needed to trigger a Type Ia supernova explosion – approximately 1.4 times the mass of our Sun.
Like cosmic mood rings changing from cool to hot, these binary systems demonstrate yet another way that stars can surprise us, reminding us that the universe still holds many secrets waiting to be discovered.
Paper Summary
Methodology
The research combined systematic data mining of long-term OGLE survey observations with targeted follow-up using multiple telescopes. Researchers first identified candidates by searching for specific light curve patterns among 76 million stars. They then used additional data, including proper motion measurements and multi-wavelength observations, to confirm the nature of these objects.
Results
The team identified 29 millanova systems, characterized by outbursts lasting 10-600 days with amplitudes of 1-3.7 magnitudes in brightness. During these outbursts, the objects brightened by 10 to 20 times their usual brightness. Some systems showed recurring outbursts every few years, while others flared up only once during the observation period. Follow-up observations of OGLE-mNOVA-11 confirmed X-ray emission and spectroscopic properties consistent with the prototype system ASASSN-16oh.
Limitations
The study was limited to objects in the Magellanic Clouds, potentially missing similar systems in other galaxies. Only two systems (ASASSN-16oh and OGLE-mNOVA-11) have been observed with comprehensive follow-up, including X-ray observations. The exact mechanism producing the X-ray emissions remains uncertain between two competing theories.
Discussion and Takeaways
The discovery suggests a new class of binary star systems that might help explain how white dwarfs grow to become Type Ia supernovae. The consistent properties of these systems, including their location in color-magnitude space and characteristic outburst behavior, suggest they represent a distinct population with specific physical properties. Their study could contribute to our understanding of cosmic distance measurements and the universe’s expansion.
Funding and Disclosures
The research represents a broad international collaboration between scientists from the Astronomical Observatory of the University of Warsaw and researchers from various institutions, including the University of Southampton, the University of Leicester (UK), the University of Cape Town, and the University of the Free State (South Africa). The research was funded in part by the National Science Centre, Poland, and utilized multiple international facilities including OGLE, SALT, and the Neil Gehrels Swift Observatory. Polish participation in SALT was funded by grant No. MEiN 2021/WK/01.
