“Heavy Metal” Supernova Discovery

Building on this discovery, Nicholl’s team found that SN 2017egm’s host galaxy has a high concentration of elements heavier than hydrogen and helium, which astronomers call “metals.” This is the first clear evidence for a metal-rich birthplace for a superluminous supernova. The dwarf galaxies that usually host superluminous supernovas are known to have a low metal content, which was thought to be an essential ingredient for making these explosions.

“Superluminous supernovas were already the rock stars of the supernova world,” Nicholl said. “We now know that some of them like heavy metal, so to speak, and explode in galaxies like our own Milky Way.”

“If one of these went off in our own galaxy, it would be much brighter than any supernova in recorded human history and would be as bright as the full moon,” said co-author Edo Berger, also of the CfA. “However, they’re so rare that we probably have to wait several million years to see one.”

The CfA researchers also found more clues about the nature of SN 2017egm. In particular, their new study supports the idea that a rapidly spinning, highly magnetized neutron star, called a magnetar, is likely the engine that drives the incredible amount of light generated by these supernovas.

While the brightness of SN 2017egm and the properties of the magnetar that powers it overlap with those of other superluminous supernovas, the amount of mass ejected by SN 2017egm may be lower than the average event. This difference may indicate that the massive star that led to SN 2017egm lost more mass than most superluminous supernova progenitors before exploding. The spin rate of the magnetar may also be slower than average.

These results show that the amount of metals has at most only a small effect on the properties of a superluminous supernova and the engine driving it. However, the metal-rich variety occurs at only about 10 percent of the rate of the metal-poor ones. Similar results have been found for bursts of gamma rays associated with the explosion of massive stars. This suggests a close association between these two types of objects.

From July 4 until Sept. 16 of this year, the supernova is not observable because it is too close to the sun. After that, detailed studies should be possible for at least a few more years.

“This should break all records for how long a superluminous supernova can be followed,” Margutti said. “I’m excited to see what other surprises this object has in store for us.”

Nicholl’s team observed SN 2017egm on June 18 with the 60-inch telescope at the Smithsonian Astrophysical Observatory’s Fred Lawrence Whipple Observatory in Arizona.

In addition to Nicholl, Berger and Margutti, the co-authors of the paper are Peter Blanchard, James Guillochon and Joel Leja, all of the CfA, and Ryan Chornock of Ohio University in Athens, Ohio.

A paper by Matt Nicholl describing these results was recently accepted for publication in The Astrophysical Journal Letters, and is available online. In addition to Berger and Margutti, the co-authors of the paper are Peter Blanchard, James Guillochon, and Joel Leja, all of the CfA, and Ryan Chornock of Ohio University in Athens, Ohio.

A copy of the paper is available online.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Contacts and sources:
Megan Watzke
Harvard-Smithsonian Center for Astrophysics