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NuSTAR X-ray observations help understanding of strange supernova

NuSTAR: NASA's Chandra X-ray Observatory image of spiral galaxy NGC 7331

This image from NASA’s Chandra X-ray Observatory shows spiral galaxy NGC 7331, center, in a three-color X-ray image. Red, green, and blue colors are used for low, medium, and high-energy X-rays, respectively. An unusual supernova called SN 2014C has been spotted in this galaxy, indicated by the box. Image & Caption Credit: NASA/CXC/CIERA/R.Margutti et al

NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) satellite, which is capable of observing the highest energy X-rays, is helping scientists unravel the secrets of a puzzling supernova.

Discovered in 2014, SN 2014C is the remnant of a massive star that had exploded in the spiral galaxy NGC 7331 somewhere between 36 and 46 million light-years away.

Supernovae are classified into two categories based on the presence or absence of hydrogen inside them. Those with very small amounts of hydrogen are designated as Type I, while those with abundant hydrogen are categorized as Type II.

Supernova enigma

NuSTAR: Supernova SN 2014C (Optical and X-Ray) in spiral galaxy NGC 7331

This image shows spiral galaxy NGC 7331, center, where astronomers observed the unusual supernova SN 2014C. Image & Caption Credit: X-ray images – NASA/CXC/CIERA/R.Margutti et al; Optical image – SDSS

SN 2014C baffled astronomers, who observed it in optical wavelengths using several ground-based telescopes, because, over the course of a year, it seemed to transform from a Type 1 to a Type II, leading scientists to dub it the “chameleon supernova“.

The dramatic change in its appearance did not match anything seen previously in supernovae and indicated the star expelled significant amounts of material in the latter part of its life before it exploded.

In a 2015 study, Dan Milisavljevic of the Harvard-Smithsonian Center for Astrophysics reported no hydrogen was initially seen in the supernova. However, about a year later, researchers detected shock waves coming from the supernova and impacting a shell of material outside the stellar remnant composed mostly of hydrogen.

Follow-up observations of SN 2014C using NuSTAR enabled scientists to monitor temperature changes in the electrons that make up the shock wave. With this data, they were able to determine the rate at which the supernova expanded and the amount of material inside the shell.

The research team also observed the supernova using NASA’s Chandra and Swift observatories, which helped them piece together its history.

From the NuSTAR observations, they determined the precursor star expelled both hydrogen and heavier elements with the equivalent of one solar mass, possibly centuries before exploding as a supernova.

Challenging established theories

“The notion that a star could expel such a huge amount of matter in a short interval is completely new,” explained NuSTAR principal investigator Fiona Harrison. “It is changing our fundamental ideas about how massive stars evolve, and eventually explode, distributing the chemical elements necessary for life.”

Two possible explanations for the phenomenon have been proposed. The first suggests scientists are missing a piece of the puzzle when it comes to understanding the nuclear reactions that occur in massive stars before they explode.

Alternatively, this star, like approximately 70 percent of massive stars, may have been part of a binary system. Interaction with a companion star could have caused the unusual supernova.

Chandra and Swift data confirmed the supernova brightened in X-rays immediately after exploding, meaning a shell of material previously thrown off by the star had to already be present. The brightening was produced when the supernova’s shock wave impacted that shell.

“This ‘chameleon supernova’ may represent a new mechanism of how massive stars deliver elements created in their cores to the rest of the universe,” noted Raffaella Margutti of Northwestern University in Evanston, Illinois, who authored a paper about SN 2014C published in The Astrophysical Journal this week.

Scientists hope to learn more by observing older massive stars before they die in supernova explosions. They will also continue to monitor SN 2014C for any further changes.



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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