Gemini South Observes Ultra-Hot Nova Erupting With Surprising Chemical Signature

March 5, 2025

Astronomers uncover extremely hot and violent eruption from first ever near-infrared analysis of a recurrent nova outside of the Milky Way Galaxy

Using the Gemini South telescope, one half of the International Gemini Observatory, partly funded by the U.S. National Science Foundation and operated by NSF NOIRLab, and the Magellan Baade Telescope, astronomers have for the first time observed a recurring nova outside of the Milky Way in near-infrared light. The data revealed highly unusual chemical emissions as well as one of the hottest temperatures ever reported for a nova, both indicative of an extremely violent eruption.

Nova explosions occur in binary star systems in which a white dwarf — the dense remnant of a dead star — continually siphons stellar material from a nearby companion star. As the outer atmosphere of the companion gathers onto the surface of the white dwarf it reaches temperatures hot enough to spark an eruption.

Almost all novae discovered to-date have been observed to erupt only once. But a few have been observed to erupt more than once, and are classified as recurrent novae. The span between eruptions for these novae can vary from as little as one year to many decades [1].

Less than a dozen recurrent novae have been observed within our Milky Way Galaxy, while far more are extragalactic, meaning located outside of the Milky Way. Studying extragalactic novae helps build astronomers’ understanding of how different environments affect nova eruptions.

The first recurrent extragalactic nova to be observed was LMC 1968-12a (LMC68), located in the Large Magellanic Cloud — a satellite galaxy of the Milky Way. This nova has a recurrent timescale of about four years — the third-shortest of any nova — and consists of a white dwarf and a companion red subgiant (a star much larger than the Sun). It was discovered in 1968 and its eruptions have been observed fairly regularly since 1990.

Its most recent eruption, in August 2024, was first captured by the Neil Gehrels Swift Observatory, which has been closely monitoring the nova every month since its 2020 eruption. Given its known recurrent timescale, astronomers were anticipating this eruption, and LMC68 delivered right on cue.

Follow-up observations were conducted nine days after the initial outburst with the Carnegie Institution’s Magellan Baade Telescope, and 22 days after the initial outburst with the Gemini South telescope, one half of the International Gemini Observatory, funded in part by the U.S. National Science Foundation and operated by NSF NOIRLab.

Using the technique of spectroscopy [2], the team observed LMC68’s near-infrared light, which allowed them to study the nova’s ultra-hot phase during which many elements have been highly energized. By studying this phase astronomers can learn about the most extreme processes at play in the eruption. This study is the first ever near-infrared spectroscopic observation of an extragalactic recurrent nova.

After its initial eruption LMC68’s light faded rapidly, but Gemini South’s FLAMINGOS-2 instrument still captured a strong signal from ionized silicon atoms, specifically silicon atoms that have been stripped of nine of their 14 electrons, which requires incredible amounts of energy in the form of radiation or violent collisions.

In the earlier spectrum from Magellan, the near-infrared light from just the ionized silicon alone shined 95 times brighter than the light emitted by the Sun added up across all its wavelengths (X-ray, ultraviolet, visible, infrared, and radio). When Gemini observed the line several days later the signal had faded, but the silicon emission still dominated the spectrum.

“The ionized silicon shining at almost 100 times brighter than the Sun is unprecedented,” says Tom Geballe, NOIRLab emeritus astronomer and co-author of the paper appearing in the Monthly Notices of the Royal Astronomical Society. “And while this signal is shocking, it’s also shocking what’s not there.”

Novae found in the Milky Way typically emit numerous near-infrared signatures from highly-excited elements, but LMC68’s spectra contained only the ionized silicon feature. “We would’ve expected to also see signatures of highly energized sulfur, phosphorus, calcium and aluminum,” says Geballe.

“This surprising absence, combined with the presence and great strength of the silicon signature, implied an unusually high gas temperature, which our modeling confirmed,” adds co-author Sumner Starrfield, Regents Professor of Astrophysics at Arizona State University.

The team estimates that, during the nova’s early post-explosion phase, the temperature of the expelled gas reached 3 million degrees Celsius (5.4 million degrees Fahrenheit), making it one of the hottest novae ever recorded. This extreme temperature suggests a highly violent eruption, which the team theorizes is due to the conditions of the nova’s environment.

The Large Magellanic Cloud and its stars have a lower metallicity than the Milky Way, meaning it contains a lower abundance of elements heavier than hydrogen and helium, referred to as metals by astronomers. In high-metallicity systems, heavy elements trap heat on the white dwarf’s surface such that eruptions occur early in the accretion process. But without these heavy elements, more matter builds up on the white dwarf’s surface before it gets hot enough to ignite, causing the explosion to erupt with far greater violence. Additionally, the expelled gas collides with the atmosphere of the companion red subgiant, causing a huge shock that elevates the temperatures in the collision.

Prior to collecting their data, Starrfield predicted that the accretion of low-metallicity material onto a white dwarf would result in a more violent nova explosion. The observations and analysis presented here are broadly in agreement with that prediction.

“With only a small number of recurrent novae detected within our own galaxy, understanding of these objects has progressed episodically,” says Martin Still, NSF program director for the International Gemini Observatory. “By broadening our range to other galaxies using the largest astronomical telescopes available, like Gemini South, astronomers will increase the rate of progress and critically measure the behavior of these objects in different chemical environments.”

More Information

This research was presented in a paper titled “Near-infrared spectroscopy of the LMC recurrent nova LMCN 1968-12a” appearing in the Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stae2711

The team is composed of A. Evans (Keele University), D. P. K. Banerjee (Physical Research Laboratory, Ahmedabad), T. R. Geballe (International Gemini Observatory/NSF NOIRLab), A. Polin (Purdue University), E. Y. Hsiao (Florida State University), K. L. Page (University of Leicester), C. E. Woodward (University of Minnesota), S. Starrfield (Arizona State University).

NSF NOIRLab, the U.S. National Science Foundation center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), NSF Kitt Peak National Observatory (KPNO), NSF Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and NSF–DOE Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. 

The scientific community is honored to have the opportunity to conduct astronomical research on I’oligam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence of I’oligam Du’ag to the Tohono O’odham Nation, and Maunakea to the Kanaka Maoli (Native Hawaiians) community.

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