Sagittarius A East (Sgr A East), a supernova remnant located strikingly close to the central supermassive black hole of our Milky Way Galaxy, resulted from a rare type of stellar explosion known as Type Iax supernova, according to a new analysis of data gathered by NASA’s Chandra X-ray Observatory. The discovery adds to the understanding of how some stars shatter and seed the Universe with elements critical for life as we known it.
This composite image contains data from NASA’s Chandra X-ray Observatory (blue) and the NSF’s Very Large Array (red) of Sgr A East. This object is located very close to the supermassive black hole in the Milky Way’s center, and likely overruns the disk of material surrounding the black hole. Image credit: NASA / CXC / Nanjing University / Zhou et al. / NSF / NRAO / VLA.
Type Ia supernovae are produced by white dwarfs, the condensed remnant of what used to be Sun-like stars.
A white dwarf, a dense ball primarily composed of carbon and oxygen atoms, is intrinsically the most stable of stars, as long as its mass remains below the so-called Chandrasekhar limit of 1.4 solar masses.
If, however, accretion of matter from a companion star or the merger with another white dwarf pushes a white dwarf over the limit of 1.4 solar masses, the temperature in its core will rise, triggering explosive nuclear fusion reactions that release an enormous amount of energy.
The star explodes in about ten seconds, leaving no remnant. The expanding cloud of ejecta glows brightly for many weeks as radioactive nickel produced in the explosion decays into cobalt and then iron.
Type Iax supernovae are less powerful, and produce different relative amounts of elements than traditional Type Ia supernovae do.
They show lower luminosities and ejecta velocities and masses, implying that the Type Iax and Type Ia classes are created from different explosion mechanisms.
The leading theory is that Type Iax supernovae involve thermonuclear reactions that travel much more slowly through the star than in normal Type Ia events. This relatively slow walk of the blast leads to weaker explosions and, hence, different amounts of elements produced in the explosion.
Nanjing University astronomer Ping Zhou and colleagues found this distinctive pattern of elements in the Chandra observations of Sgr A East.
Previous studies had argued that the object was the remnant from the collapse of a massive star.
“While we’ve found Type Iax supernovae in other galaxies, we haven’t identified evidence for one in the Milky Way until now,” Dr. Zhou said.
“This discovery is important for getting a handle of the myriad ways white dwarfs explode.”
“This result shows us the diversity of types and causes of white dwarf explosions, and the different ways that they make these essential elements,” said Dr. Shing-Chi Leung, an astronomer at Caltech.
“If we’re right about the identity of this supernova’s remains, it would be the nearest known example to Earth.”
The astronomers were able to use Chandra observations targeting the supermassive black hole and the region around it for a total of about 35 days to study Sgr A East.
“This supernova remnant is in the background of many Chandra images of our Galaxy’s supermassive black hole taken over the last 20 years,” said Dr. Zhiyuan Li, an astronomer at Nanjing University.
“We finally may have worked out what this object is and how it came to be.”
The team’s results will be published in the Astrophysical Journal.
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Ping Zhou et al. 2021. Chemical abundances in Sgr A East: evidence for a Type Iax supernova remnant. ApJ, in press; arXiv: 2006.15049
