Astronomers have observed an extremely bright nova explosion from the recurrent nova LMC68, revealing an unprecedented dominance of ionized silicon and record-high temperatures.
The recent observations challenge existing models of nova composition and shed new light on explosive stellar processes in low-metallicity environments.
Although rare, some white dwarf nova explosions—known as recurrent novae—erupt multiple times. Astronomers first discovered such a nova in 1968, identifying LMC 1968-12a (LMC68), which has now produced unexpected readings in its latest outburst.
Recurrent Novae LMC68
When the dense remnant of a dead star siphons material from its binary companion, the surface temperature rises until it reaches a critical threshold, triggering a nova explosion. In most cases, novae are one-time events. However, in rare instances, they can erupt multiple times, with intervals ranging from years to decades. Astronomers have only identified about a dozen such recurrent novae in our Milky Way galaxy.
Most recurrent novae are extragalactic—located outside the Milky Way—such as LMC68. This particular nova resides in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, and has the third shortest known recurrence interval, erupting approximately every four years. LMC68 consists of a small white dwarf paired with a larger red subgiant star, larger than our Sun. While astronomers first detected the recurrent nova in 1968, its eruptions were not consistently observed until 1990.
A Recent Eruption
The latest eruption occurred in August 2024 and was monitored by several research institutions. Since LMC68’s previous eruption in 2020, NASA’s Neil Gehrels Swift Observatory has been tracking the nova monthly, anticipating its next explosion within the expected four-year cycle. Nine days after the initial outburst, the Carnegie Institution’s Magellan Baade Telescope observed the event, followed by the International Gemini Observatory’s Gemini South Telescope 22 days later.
These observations resulted in the first-ever near-infrared spectroscopy of an extragalactic recurrent nova’s eruption. By capturing the ultra-hot phase of the explosion in near-infrared light, astronomers could analyze highly energized elements, offering new insights into the extreme conditions driving these recurrent outbursts.
Analyzing a Recurrent Eruption
Observations from Magellan detected light from ionized silicon shining 95 times brighter than the Sun when measured across all wavelengths. Meanwhile, Gemini South’s FLAMINGOS-2 instrument recorded fading ionized silicon signals days later, showing that the element had lost nine out of 14 electrons—evidence of the immense radiation or collision energy involved in the eruption. Even in its faded state, silicon remained the dominant element across the entire spectrum of the explosion.
“The ionized silicon shining at almost 100 times brighter than the Sun is unprecedented,” said 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.”
A Strange Cosmic Signature
A single element dominating a nova’s spectrum is highly unusual. Novae observed in the Milky Way typically contain a mix of elements, such as sulfur, phosphorus, calcium, and aluminum.
“This surprising absence, combined with the presence and great strength of the silicon signature, implied an unusually high gas temperature, which our modeling confirmed,” said co-author Sumner Starrfield, Regents Professor of Astrophysics at Arizona State University.
That modeling revealed that the expelled gas in the early post-explosion phase reached an astonishing 5.4 million °F, making LMC68 one of the hottest novae ever recorded. Researchers theorize that the extreme environmental conditions surrounding the nova contributed to such a violent eruption.
Creating Cosmic Violence
LMC68 resides in the Large Magellanic Cloud, a galaxy with a lower metallicity compared to the Milky Way. In astronomy, “metallicity” refers to the abundance of elements heavier than hydrogen and helium. High-metallicity environments allow heavy elements to trap heat on the white dwarf’s surface, causing eruptions earlier in the accretion process. However, in low-metallicity environments like the Large Magellanic Cloud, eruptions occur later in the accretion cycle, allowing for more violent explosions with greater fuel accumulation.
Stargazer
“With only a small number of recurrent novae detected within our own galaxy, understanding of these objects has progressed episodically,” said 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,” Still said.
The paper “Near-infrared Spectroscopy of the LMC Recurrent Nova LMCN 1968-12a” appeared on March 5, 2025 in Monthly Notices of the Royal Astronomical Society.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted atryan@thedebrief.org, and follow him on Twitter@mdntwvlf.