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Shock Waves Power an Exploding Star

Roughly 50 times each year, a star nearing the end of its life accretes too much material from a close companion star and erupts in a violent display of light — shedding its outer surface and propelling shock waves into our galaxy — only to recover and smolder as it did before. This event is called a ‘nova.’

This ability to “reprocess” sets novae apart from their rare supernovae counterparts, which occur only several times per century and self-destruct amid an even greater celestial outburst. And yet, it’s the more common novae that could hold the answers to essential questions regarding our universe.

Nova illustration

As this nova illustration shows, when a white dwarf accretes too much material from its companion star, it ejects material in two district winds. Shock waves are created as the two winds collide, producing gamma rays (magenta). Research now suggests this may be the primary source of visible light as well. Credit: NASA’s Goddard Space Flight Center/S. Wiessinger

Given that these events usually occur thousands of light years away, I went to discuss their significance with two more accessible sources: Laura Chomiuk and Kwan Lok Li of Michigan State University. Both scientists recently allied with Columbia University and the ASAS-SN project (of which Chomiuk is a key member) in hopes of untangling the fundamental processes underlying the lifecycle of stars.

They have turned their attention to one classical nova in particular, ASASSN-16ma, located within the “archer” constellation Sagittarius. Their paper, recently published in Nature Astronomy, suggests a novel source for the nova’s post-outburst glow — offering insight into other outer-space explosions.

The authors explained that ASASSN-16ma is among the brightest novae ever detected by the Fermi Large Area Telescope — a satellite instrument built to observe cosmic events releasing a high-energy form of light called gamma rays. The Fermi Mission, spearheaded by NASA’s Goddard Space Flight Center, also discovered back in 2010 that novae generate these high-energy rays.

Gamma rays are emitted as material is ejected from the star in two distinct winds or “outflows.” A slow burst is followed by a faster one, which then crashes into the first and creates a shock wave. Visible light also radiates from the same explosion, although until now most scientists believed it stemmed from nuclear reactions on the surface of the star.

This animation shows the first nova outburst, from a white dwarf called V407 Cygni, detected by Fermi’s Large Area Telescope (LAT) back in 2010. Gamma rays (magenta) arise when accelerated particles in the explosion’s shock wave crash into the red giant’s stellar wind. Credit: NASA/Conceptual Image Lab/Goddard Space Flight Center

But something wasn’t right about the traditional explanation. Novae shouldn’t have enough power to produce gamma rays — nor release as much visible light as they do given their calculated luminosity limit.

This inherent contradiction confounded astrophysicists and theorists alike, until last year when Chomiuk requested that Fermi focus on ASASSN-16ma for an extended duration to collect more sensitive data. As luck would have it, Fermi was already observing another nova in the same neighborhood, so Goddard’s team was more than happy to oblige, tilting their telescope slightly. When scientists began to sift through the data transmitted from the cosmos to Earth, one specific trend became abundantly clear.

“Our results were telling us our previous assumption that all the luminosity comes from the surface of the star was flawed,” Chomiuk explained to me from her office in East Lansing. “A lot of it actually comes from the same place as the gamma rays,” she continued. That is, the colliding shock waves.

Seeking an outsider’s perspective, I made my way to the second floor of Goddard’s astrophysics building to chat with one of the center’s Fermi aficionados, David Thompson. Eager to lend guidance, Thompson presented his miniature replica of the spacecraft, and indicated the boxy Large Area Telescope seated atop the winged satellite. “We’ve been seeing similar novae for years, but this was unexpected,” he said. “The authors have enough detail in their data to challenge the conventional wisdom about what makes novae bright.”

But I soon learned these ideas weren’t entirely unprecedented. During every interview I conducted, one name kept coming up: collaborator and lead theorist, Brian Metzger of Columbia University. Metzger had been crunching the numbers on novae for several years, but lacked substantive data to bolster his hypotheses. That is, until now.

“Novae have been observed by the naked eye since well before the modern era,” Metzger told me, “and yet our view of what is producing these bright outbursts continues to change.”

Fermi LAT 60-month image

The Fermi LAT 60-month image, constructed from front-converting gamma rays with energies greater than 1 GeV. The most prominent feature is the bright band of diffuse glow along the map’s center, which marks the central plane of our Milky Way galaxy. Image credit: NASA/DOE/Fermi LAT Collaboration

Fermi’s Deputy Project Scientist, Elizabeth Hays, said Metzger’s paradigm had already garnered some buzz throughout the scientific community, although at the time the universe had yet to reveal an event that clearly reflected his calculations. Now, thanks to ASASSN-16ma, she affirmed his conjectures may expose what powers these nova outbursts — processes which might also extend to other kinds of stellar explosions, like dazzling supernovae as well as star mergers.

“We’ve been ignoring this whole piece of the puzzle,” she said. “When we come across high-energy processes like these novae, it becomes clear just how much more work we have left to do.”

Understanding novae shock and gamma ray production could also explain certain properties of accelerating particles traveling close to the speed of light, as well as the subsequent magnetic fields.

When I asked Li what was next, he said he plans to continue using Fermi data to monitor nearby novae. Ultimately, he hopes to pinpoint similarly strong correlations between gamma ray and visible light emissions within additional novae. “We’re always looking for new sources to test our model, and ensure it truly and accurately describes gamma-ray phenomena in classical novae,” he added.

Back at Goddard, Thompson repositioned his Fermi model on the shelf among his books, and agreed a single example alone does not constitute proof. But it’s certainly a good place to start. “These results prompt us to think about things in a new way,” he said, “and that’s what science is about.”

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