A strange and sneaky type of supernova has exploded in a galaxy not so far away.
Called a type Iax, the supernova belongs to a class of explosions astronomers affectionately call “weirdos,” because they look a lot like standard supernovae but come with a twist. It likely exploded earlier this month, and was spotted on Oct.29 by amateur astronomer Koichi Itagaki.
This morning, a team in Italy identified the explosion as a peculiar type Ia supernova.
“We’re interested in the weirdos in part because they’re fun, and in part because they’re extreme cases of physics going on that are easily noticeable,” UC-Berkeley astronomer Alexei Filippenko said, when we spoke about these explosions a bit ago.
The new supernova doesn’t have an official name yet. It’s located about 55 million light-years from Earth in a beautiful spiral galaxy known as M61, and could be the nearest type Iax supernova spotted so far (though two explosions in 2010 are close contenders).
Type Iax explosions are a recent discovery, and often masquerade as the more common — and much more well-known — cosmic firecrackers known as a type Ia supernovae. It’s only by looking closely at the light produced by the explosions that astronomers can distinguish between the two.
“The spectra of supernovae Ia and supernovae Iax are very similar. In fact, people often mistakenly classify supernovae Iax as supernovae Ia, and we have identified really old supernovae — all the way back to 1991 — as Iax by looking carefully,” says astronomer Ryan Foley of the University of Illinois at Urbana-Champaign.
In 2013, Foley and his colleagues identified Iax explosions as a distinct class of stellar explosion.
Type Iax supernovae are kind of like mini-Ias. A normal type Ia occurs when a white dwarf steals a bit of material from a companion star, eventually growing massive enough to blow itself to bits. Because of their predictable brightness, type Ia supernovae are used to measure cosmic distances, and played a crucial role in the late 1990’s discovery that the expansion of the universe is speeding up.
Type Iax explosions, however, are a less energetic form of Ia that might not completely obliterate the white dwarf star. Their explosions are weaker, don’t shine as brightly, and produce ever so slightly different ratios of chemical elements than a normal Ia. Foley and his colleagues suspect that a Iax ignites when a white dwarf steals a bit of material from a companion star that’s rich in helium; earlier this year, he and others successfully identified the stars involved in a 2012 Iax explosion as being a white dwarf star and a luminous blue helium star.
Now, astronomers are wondering whether the white dwarfs might survive these explosions. Foley says the close proximity of the Iax in M61 should help answer that question. “We’ll be able to monitor this supernova for at least a couple of years,” he says. “This will help us determine if the star was disrupted.”
Scientists are still struggling to understand the mechanics of type Ia supernovae and their weirdo cousins. In particular, the issue of which stars are involved has been the source of much recent controversy. The good news with this explosion is not only that it’s nearby, but that there are pre-supernova Hubble Space Telescope images of the galaxy. That means it should be possible to get a good look at the pair of stars that contributed to the explosion, before they got all roughed up.
By studying the weird supernovas, astronomers can gain a better understanding of what the normal type Ias are — and what they are not. “We have no idea what a normal Ia is,” says astrophysicist Brad Tucker of both UC-Berkeley and Australian National University. “It will be quite ironic if the first two supernova Ia progenitors we detect are of this small weirdo class, while with the other thousands of normal Ias, we detected nothing!”