In 2016 an amateur astronomer was testing his camera — and captured the first flash of a supernova.
Observing the exact moment that a supernova explodes has long been a challenge for astronomers. Initiatives are underway to catch this fleeting moment, but it’s an amateur astronomer who now claims the earliest detection of a supernova.
On September 20, 2016, amateur astronomer Víctor Buso was testing a new CCD camera in the homemade observatory he had built atop his home in Rosario, Argentina. He pointed his 40-centimeter Newtonian telescope to the spiral galaxy NGC 613 because it was near zenith and he knew it had a beautiful shape with an interesting structure full of bright and dark clouds — and also because he could see it without moving his heavy dome late at night, which would disturb the neighbors.
Buso observed the galaxy for approximately 90 minutes, taking 20-second exposures to avoid saturation by the bright city sky. The first series of images didn’t show anything unusual, but after a 45-minute break, Buso noticed that a pixel near the end of one of the galaxy’s spiral arms had brightened and was becoming brighter with each shot.
The pixel was initially so faint that Buso didn´t recognize it as a supernova right away. Nevertheless suspicious that the bright spot might be something interesting, he reached out to some professional astronomers — only to find that none were available. Then he called another amateur astronomer, Sebastian Otero, a member of the American Association of Variable Star Observers (AAVSO). Otero helped Buso send an international warning for other astronomers to follow up. Both amateurs are receiving credit as coauthors of the research article published in the February 22nd Nature.
Once the supernova was confirmed, receiving the official designation SN 2016gkg, extensive monitoring began, including with the Neil Gehrels Swift Observatory, which took X-ray, ultraviolet, and visible-light observations. However, Buso had captured the most valuable information in the earliest hours of the stellar explosion. What he imaged was what astronomers call the shock-breakout phase of the supernova, the moment when the shockwave traveling from the collapsing core of the star reaches the outer layers and breaks through the surface. The star’s outer layers of gas heat up as they’re ejected, and they brighten rapidly — in this case, at a rate of 40 magnitudes per day!
Until now, the shock-breakout phase was largely theoretical. Although hints of the phenomenon had been observed, it had never been definitively detected at visible wavelengths due in large part to its ephemeral nature — the shock wave only takes two to five hours to break out of the star. Moreover, much of the immediate emission is at high-energy wavelengths rather than visible light — that’s probably why Buso didn’t realize he had discovered a supernova and stopped observations before dawn.
“Actually, some researchers were starting to question if the models were right,” says lead author Melina Bersten (Instituto de Astrofísica de La Plata, Argentina). “[Buso’s] observations are extremely invaluable; winning the lottery is more likely to happen than what he did.” Bersten estimated the chance of such a serendipitous observation at 1 in a million, and given the bright city lights and other observing conditions, the chances might have been even lower than that.
Based on the discovery and follow-up observations, Bersten and her team determined that the exploding object had been a star in a binary system that had lost its outer layers of gas, leaving behind a helium-dominated core. This star, once about 20 times as massive as our Sun, had lost most of its mass to its companion star. By the time it exploded, it had shrunk to around five solar masses. Further analysis of the data will enable researchers to learn more about the structure of the star before it exploded, and about the physical processes that occur during supernovae.
Buso, who works as a locksmith, affirms that the excitement of finding something that none had observed before has brought him and his family and friends extreme joy. “Sometimes I wonder why I do this, why I put so many hours and so much passion into this . . . Now, I have found the answer.”