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Using the Gemini South telescope, part of the International Gemini Observatory funded by the U.S. National Science Foundation and operated by NSF NOIRLab, alongside the Magellan Baade Telescope, astronomers have made a groundbreaking discovery. They have observed a recurrent nova beyond our Milky Way in near-infrared light for the first time, unveiling distinctive chemical emissions and reporting one of the highest recorded temperatures during such an event, indicating an exceptionally intense eruption.
Novae arise in binary star systems, where a white dwarf—the remnants of a deceased star—draws material from a neighboring companion star. When the gathered material on the white dwarf’s surface becomes sufficiently hot, it triggers a cataclysmic explosion.
Historically, most novae have been recorded to erupt only once. However, a select few, deemed recurrent novae, have shown the ability to erupt multiple times, with intervals between eruptions ranging from just one year to several decades.
Within the Milky Way, fewer than twelve recurrent novae have been documented, while a larger number exist outside our galaxy. Investigating these extragalactic novae is crucial for astronomers to understand how various cosmic environments influence nova phenomena.
The first discovered recurrent nova beyond our galaxy was LMC 1968-12a (LMC68), located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. It manifests a recurrence period of roughly four years—the third shortest known for any nova—associated with a white dwarf and a significantly larger companion, the red subgiant. Since its discovery in 1968, LMC68 has been under regular observation, particularly since 1990.
The latest eruption of LMC68 occurred in August 2024, initially detected by the Neil Gehrels Swift Observatory, which has monitored the nova monthly since its previous eruption in 2020. Given its documented eruption frequency, astronomers had anticipated the event, which unfolded precisely as predicted.
Follow-up observations were conducted using the Magellan Baade Telescope nine days after the outburst and 22 days later with the Gemini South telescope.
The team employed spectroscopy to study LMC68’s near-infrared light, allowing an examination of the nova during its ultra-hot phase, where numerous elements are energized. Observing this phase is essential for astronomers to comprehend the extreme processes at work during the eruption. This study marks the inaugural near-infrared spectroscopic observation of an extragalactic recurrent nova.
After its eruption, LMC68’s brightness diminished quickly, yet the FLAMINGOS-2 instrument on Gemini South detected a significant signal from ionized silicon atoms, specifically silicon stripped of nine of its fourteen electrons—a process requiring immense energy levels through violent collisions or radiation.
Notably, in the earlier spectrum captured by Magellan, the near-infrared light from ionized silicon was found to be 95 times more luminous than the total light emitted from the Sun across all wavelengths. While the signal from Gemini showed a decrease over several days, ionized silicon remained prominent in the spectrum.
“The ionized silicon shining at almost 100 times brighter than the Sun is unprecedented,” remarked Tom Geballe, an emeritus astronomer at NOIRLab and co-author of the related study published in the Monthly Notices of the Royal Astronomical Society. “It’s surprising, not only the intensity of this signal but also the absence of expected features.”
Typically, novae in the Milky Way release various near-infrared signatures from highly excited elements. In contrast, the spectrum of LMC68 predominantly exhibited the ionized silicon feature. “We expected to also identify signals from energized sulfur, phosphorus, calcium, and aluminum,” noted Geballe.
“The unexpected absence of these elements, combined with the robust presence of the silicon signal, suggested a significantly high gas temperature, a finding supported by our models,” added Sumner Starrfield, a Regents Professor of Astrophysics at Arizona State University.
The research team estimates that the expelled gas’s temperature in the early aftermath of the nova eruption reached approximately 3 million degrees Celsius (5.4 million degrees Fahrenheit), marking it as one of the hottest novae ever documented. This extreme temperature hints at a highly explosive event, which the researchers attribute to the environmental conditions influencing the nova.
The Large Magellanic Cloud possesses lower metallicity than the Milky Way, meaning it has a reduced abundance of heavier elements beyond hydrogen and helium. In high-metallicity environments, these heavier elements can trap heat at the white dwarf’s surface, leading to earlier explosions in the accretion cycle. Conversely, the lack of heavier elements in lower metallicity settings allows more material to accumulate on the surface before reaching ignition temperatures, resulting in more violent eruptions. Additionally, the collision of expelled gas with the companion star’s atmosphere can generate significant shock waves, further elevating temperatures.
Prior to data collection, Starrfield had hypothesized that low-metallicity material accreted by a white dwarf would yield a more violent explosion, and the current findings align with that prediction.
“Given the limited number of recurrent novae detected in our galaxy, our understanding of these phenomena has been piecemeal,” stated Martin Still, NSF program director for the International Gemini Observatory. “By extending our observations to other galaxies with the aid of advanced telescopes like Gemini South, astronomers can enhance their understanding and measure how these objects behave in various chemical landscapes.”
Notes
[1] The recurrent nova with the briefest eruption interval is M31N 2008-12a, at just one year, whereas V2487 Ophiuchi has the longest, at 98 years.
[2] Spectroscopy is the analytical technique that captures an object’s light and separates it into a spectrum, enabling scientists to discern the chemical elements present from the specific wavelengths of light emitted.
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