Calcium-rich supernova examined with x-rays for first time

Unprecedented observations shine light on a compact star’s final moments

spiral galaxy host of supernova
Hubble Space Telescope image of SN 2019ehk in its spiral host galaxy Messier 100. The red arrow points to the supernova. (Image credit: Charlie Kilpatrick, UCSC)
supernova illustration
An artist’s interpretation of the calcium-rich supernova 2019ehk. Shown in orange is the calcium-rich material created in the explosion. Purple coloring represents gas shed by the star before the explosion, which then produced bright X-ray emission when the material collided with the supernova shockwave. (Credit: Aaron M. Geller, Northwestern University)

Half of all the calcium in the universe, including the very calcium in our teeth and bones, was created in the last gasp of dying stars. Called “calcium-rich supernovae,” these stellar explosions are so rare that astrophysicists have struggled to find and study them. As a result, the nature of these supernovae and their mechanism for creating calcium have remained elusive.

Now an international team has potentially uncovered the true nature of these rare, mysterious events. For the first time ever, the researchers examined a calcium-rich supernova with x-ray imaging, which provided an unprecedented glimpse into the star during the last month of its life and ultimate explosion.

The new findings, published August 5 in the Astrophysical Journal, revealed that a calcium-rich supernova is a compact star that sheds an outer layer of gas during the final stages of its life. When the star explodes, its matter collides with the loose material in that outer shell, emitting bright x-rays. The overall explosion causes intensely hot temperatures and high pressure, driving a chemical reaction that produces calcium.

“These events are so few in number that we have never known what produced calcium-rich supernova,” said first author Wynn Jacobson-Galan, who began working on the study at UC Santa Cruz and is currently a graduate student at Northwestern University. “By observing what this star did in its final month before it reached its critical, tumultuous end, we peered into a place previously unexplored, opening new avenues of study within transient science.”

Nearly 70 co-authors from more than 15 countries contributed to the paper, including Ryan Foley, assistant professor of astronomy and astrophysics at UC Santa Cruz, UCSC postdoctoral researchers Charles Kilpatrick and Georgios Dimitriadis, and UCSC graduate students Dave Coulter, César Rojas-Bravo, and Matt Siebert. Jacobson-Galan earned his B.S. in astrophysics at UCSC in 2018 and then worked as a junior specialist on Foley’s research team for a year before starting his graduate studies at Northwestern.

“This supernova was in Messier 100, a beautiful nearby spiral galaxy, so we have superb data,” Foley said. “In addition to the first x-ray detection of a supernova of this class, we had really deep pre-explosion Hubble images, and we caught it right after explosion.”

Some of the most critical observations were obtained at UC’s Lick Observatory by Foley’s team during the first few days after the supernova was detected. “In the Lick spectra, we saw strong emission from ionized helium in the stellar environment that matches up with the x-ray data,” Foley said. “The combination of everything really constrains the nature of the progenitor system and how the star exploded.”

“Before this event, we had indirect information about what calcium-rich supernovae might or might not be,” added senior author Raffaella Margutti, assistant professor of physics and astronomy at Northwestern. “Now, we can confidently rule out several possibilities.”

Amateur astronomer Jaroslaw Grzegorzek was the first to announce the news of a new supernova, dubbed SN 2019ehk, to the astronomical community. Amateur astronomer Joel Shepherd also spotted the bright burst while stargazing in Seattle and acquired the first image of the new source in the sky. Both astronomers had been viewing Messier 100 (M100), a spiral galaxy located 55 million light years from Earth, and noticed that a bright orange dot appeared in the frame.

“As soon as the world knew that there was a potential supernova in M100, a global collaboration was ignited,” Jacobson-Galan said. “Every single country with a prominent telescope turned to look at this object.”

This included leading observatories in the United States such as NASA’s Swift Satellite, W.M. Keck Observatory in Hawaii, and Lick Observatory. The UCSC team obtained data from Lick’s 3-meter Shane Telescope, the 10-meter Keck I Telescope, and the 1-meter Swope Telescope at Las Campanas Observatory in Chile. The also worked closely with the Thacher School, a high school in Ojai, California, to use their 0.7-meter telescope to make observations.

UC Santa Barbara graduate student Daichi Hiramatsu was the first to trigger Swift to study SN 2019ehk in the X-ray and ultraviolet. Hiramatsu also is a staff scientist at Las Cumbres Observatory, which played a crucial role in monitoring the long-term evolution of this supernova with its global telescope network.

The worldwide follow-up operation moved so quickly that the supernova was observed just 10 hours after explosion. The x-ray emission detected with Swift only lingered for five days and then completely disappeared.

“In the world of transients, we have to discover things very, very fast before they fade,” Margutti said. “Initially, no one was looking for x-rays. Daichi noticed something and alerted us to the strange appearance of what looked like x-rays. We looked at the images and realized something was there. It was much more luminous than anybody would have ever thought. There were no preexisting theories that predicted calcium-rich transients would be so luminous in x-ray wavelengths.”

While all calcium comes from stars, calcium-rich supernovae pack the most powerful punch. Typical stars create small amounts of calcium slowly through burning helium throughout their lives. Calcium-rich supernovae, on the other hand, produce massive amounts of calcium within seconds.

“The explosion is trying to cool down,” Margutti explained. “It wants to give away its energy, and calcium emission is an efficient way to do that.”

Using Keck, the Northwestern team discovered that SN 2019ehk emitted the most calcium ever observed in a singular astrophysical event. “It wasn’t just calcium rich,” Margutti said. “It was the richest of the rich.”

SN 2019ehk’s brief luminosity told another a story about its nature. The researchers believe that the star shed an outer layer of gas in its final days. When the star exploded, its material collided with this outer layer to produce a bright, energetic burst of x-rays.

“The luminosity tells us how much material the star shed and how close that material was to the star,” Jacobson-Galan said. “In this case, the star lost a very small amount of material right before it exploded. That material was still nearby.”

Although the Hubble Space Telescope had been observing M100 for the past 25 years, the powerful telescope never registered the star responsible for SN 2019ehk, which was experiencing its final evolution. UCSC’s Kilpatrick analyzed the supernova site in the Hubble images taken before the explosion occurred, providing yet another clue to the star’s true nature.

“The Hubble data from before the explosion are incredibly deep, so we would have seen any type of star at the site of the supernova unless it was almost as small as the sun or something even smaller, like a white dwarf,” Kilpatrick said.

“Without this explosion, you wouldn’t know that anything was ever there,” Margutti added. “Not even Hubble could see it.”

This research was supported by the National Science Foundation, the David and Lucile Packard Foundation, the Gordon & Betty Moore Foundation, and the Heising-Simons Foundation.