Sagittarius A East: Supernova Type 1ax in our Galaxy

In the galaxy, astronomers know about 350 supernova relics – the glowing remains of an exploded star. But not a single one of them came from a rare exceptional case, the so-called Supernova Typ1ax – until now.

Researchers have now tracked down the first remnant of such a particularly slow explosion of a white dwarf in our galaxy in the heart of the Milky Way.

Because the relic of this supernova, known as Sagittarius A East, is only around 25,000 light-years away from us, it opens up completely new opportunities to get to the bottom of this previously hardly explored type of star explosion.

A “normal” supernova occurs while a massive star has reached the end of its life cycle. Its nuclear fusion subsides, and it comes to a collapse caused by gravity, in which the star virtually collapses. But there are also supernovae in which the starting star has already become a white dwarf – a very compact remnant of a star that has shed a large part of its shell.

When this white dwarf orbits in a binary star system, it often sucks material from its stellar companion. If its mass then exceeds a critical limit, a type 1a supernova occurs. Because these star explosions show a brightness that can be standardized, they are often used as “standard candles” in astronomy – they can be used to measure the distance of distant galaxies and the expansion of the cosmos.

X-ray view into the heart of the Milky Way

But as necessary as the type 1a supernovae are for astronomy, the sequence of these explosions has only been partially clarified. This is especially true for a subspecies of these star explosions, the supernova type 1ax. Astrophysicists suspect that the thermonuclear reactions in these white dwarf explosions are slower, creating less heavy elements.

Besides, remnants of the white dwarf could still be preserved in these supernovae. According to theoretical calculations, there are around three Type 1ax explosions for every ten standard type 1a supernovae. So far, however, astronomers have hardly had a chance to investigate the events of this rare type of supernova in more detail. “The origin of these star explosions is highly controversial, also because the knowledge about this type comes from extragalactic cases alone,” explain Ping Zhou from the University of Amsterdam and his colleagues. Of the 290 to 380 known supernova relics in the Milky Way, not a single remnant has come from a Type 1ax supernova.

This has changed now. Because Zhou and his team have now found out that an object in the heart of the Milky Way that has been a mystery for some time could be the relic of such a type 1ax supernova, it is about Sagittarius A East (Sgr A East), a luminous structure made of dust and gas, which lies near the central black hole of our galaxy.

It is, therefore, only around 25,000 light-years away from us. To determine how this relic was created, the astronomers have now examined it more closely using the Chandra X-ray telescope. Using X-ray spectroscopy, they were able to decide how the gases are composed and how fast they move.

The first intragalactic representative of a type 1ax supernova

The images and spectral data revealed that this supernova relic’s elemental composition differs significantly from regular star explosions and common type 1a supernovae. “Our data show a small proportion of medium-weight elements compared to iron and large proportions of manganese and nickel – these frequencies match neither the models of core-collapse supernovae nor normal type 1a supernovae,” the researchers report. “This pattern is surprisingly different from that of other supernova relics.” With the help of astrophysical models, the astronomers, therefore, next examined which type of explosion could most likely have produced this basic pattern.

The result: Sagittarius A East could likely be the remains of a rarity – a Type 1ax supernova. “Sgr A East is the first galactic supernova relic for which a Type 1ax origin is likely,” write Zhou and his colleagues. When it is confirmed, it would be the first nearby representative of these largely puzzling stellar explosions. Sagittarius A East could thus make a decisive contribution to clearing up the Type 1ax supernovae’s mysteries. “This discovery is important to gain a better understanding of the various ways in which white dwarfs can explode,” explains Zhou.

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