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SYFY WIRE Bad Astronomy

Is this cosmic sprinkler surrounding galaxy’s next gamma-ray burst?

By Phil Plait
The astonishing dust spiral around 2XMM J160050.7-514245, aka Apep — a star that may become a powerful gamma-ray burst. Credit: ESO/Callingham et al.

Astronomers have found a star that may produce the next gamma-ray burst in the Milky Way.

What's a gamma-ray burst, you ask? Well, it's a cosmic explosion so overwhelmingly, ridiculously powerful that, compared to it, the sweatiest biblical apocalypse looks like a Victorian lady's polite belch.

Let's just say I'm very, very glad this thing is nearly 8,000 light years away and not aimed at us. Otherwise, I might be concerned.

In other words, I'm reasonably sure it won't kill us. And bonus: It's extremely cool. I mean truly, extraordinarily cool.

The star in question is called 2XMM J160050.7-514245 — that's its catalog designation, after the orbiting observatory that discovered it (XMM-Newton) and its coordinates on the sky. That's a mouthful, so the astronomers who studied it gave it the nickname Apep, after an Egyptian deity: a god of chaos, who appears as a serpent. Both of those characteristics are fairly appropriate.

Why? Take a look, and note that this is an actual, real image of the system:

The astonishing dust spiral around 2XMM J160050.7-514245, aka Apep — a star that may become a powerful gamma-ray burst. Credit: ESO/Callingham et al.

Holy herpetological heavens! This is combination of two infrared images; what you see as blue is at a wavelength of 2.24 microns, about three times the reddest light your eye can see. The red is at 8.9 microns, where warm dust tends to glow. Both were taken using the Very Large Telescope in Chile.

That incredible structure is a pinwheel, a spiral of dust being blasted out from the bright star you can see in the center. But there is a lot more going on than you can see here.

As you can see, there are two stars at the center of the spiral. It's known that some binary stars can blow out pinwheels like this, but there's a problem: That second, fainter star is way too far away to be the culprit. At 250 billion kilometers out from the bright star (about ten times the distance of Neptune from the Sun), it would take more than 10,000 years to circle it once, much too far out to be creating this structure.

It turns out that bright star is actually two stars. A tighter binary, so close that they appear as one star in the image. They orbit each other probably once every century or so. Both are monsters, and at least one of them truly earns that moniker.

It's what we call a Wolf-Rayet star, one of the most terrifying beasts in the galaxy's zoo. These are extremely massive stars, 15 times the mass of the Sun at least, with some much more massive even than that. Stars like that burn through their core fuel rapidly, living only a few million years. As they cycle through nuclear fusion in their cores, going from hydrogen to helium to carbon fusion and beyond, they can become unstable. They become extremely luminous — hundreds of thousands times brighter than the Sun — and blow off a dense and very fast wind of gas particles, sometimes creating huge and gorgeous nebulae around them.

The huge nebula M1-67 around the Wolf-Rayet star WR124. Credit: ESA/Hubble & NASA / Judy Schmidt

Sometimes, if they are in a tight binary, you get a pinwheel. The most famous example of that is WR 104, a binary Wolf-Rayet star roughly the same distance away from us as Apep. The motion of the two stars acts like a sprinkler, sending dust straight out from the star, but in a wave that looks like a pinwheel. WR 104 has a much shorter period than Apep, less than a year, so we see the motion of the ejected dust much more clearly.

However, Apep has an obviously much more complicated structure than a simple pinwheel. What's going on?

It's likely that both stars are really massive, with both possibly being Wolf-Rayets. The astronomers who observed it think that the primary (brighter) one is spinning extremely rapidly, so fast it's nearly at the breakup rate — in other words, spinning so fast that the gravity of the star at the surface is nearly balanced by the centrifugal force outwards, making it much easier for the intense heat and light to blow particles away. Because of the rapid rotation, the gas is blown out strongly along the equator, forming a thick disk.

The second star orbits the first, but not in the same plane as the first star's spin. So twice per orbit it plunges through that gas disk. When that happens the two winds interact in complicated ways, forming huge amounts of dust (bigger grains of material) that puff up and out, forming the complex structure seen.

They created a simple physical model of this, and it mimics the behavior of Apep's spiral uncannily well:

An animation created using a physical model for Apep: Two massive stars orbiting one another, their winds colliding and producing a huge spiral of dust. The loop pauses when it matches the configuration seen in the actual observations.

The dust is only created when the second star slams into the first's disk, so you get an episodic pulse of dust blown out.

The star's rapid rotation solves another mystery they found when they measured the speeds of the winds. Using spectra, they found the wind is blowing off the star extremely rapidly, at a staggering 3,400 kilometers per second. That's more than 1% the speed of light!

However, the dust in the structure itself is expanding at only ("only") 570 kps. All sorts of physics says the two winds should expand at the same rate, but they don't. However, if the primary star is spinning madly, then it ends up emitting essentially two winds: A slow one around its equator, where material is piling up and slowing expansion, and a much faster one along its poles, where there's less material so the wind can expand more freely. The rapid rotation therefore solves the mystery of the two mismatched winds.

But how did the star get spinning so rapidly? The best way to do this is if there's yet another star in the system… or there used to be. Maybe it wasn't always a binary, but used to have a third star in the system, one orbiting the primary closer yet, so close they almost touched. Over time, forces conspire such that they could actually spiral together and merge into a single star. We do know this can happen — the famous V838 Monocerotis is the prime example of it — and would vastly spin up the merged star.


This may be why systems like Apep are so rare. You need a binary star of two massive components, plus a third that merges with one of the other two and spins it up. That can't happen very often.

But this introduces another issue. Gamma-ray bursts are tremendous explosions, dwarfing even a supernova. They come in two flavors: Short ones that last from milliseconds up to about 2 seconds in duration (caused in general by neutron stars colliding), and longer ones that last for more than two seconds. The latter are thought to be from Wolf-Rayet stars, but not just any Wolf-Rayet star: It has to be rapidly rotating to be able to efficiently create the massive explosion. Otherwise it just explodes as a normal supernova (which is still an underpants-changingly terrifying event).

So you need a massive, evolved star spinning at near its breakup rate to create a gamma-ray burst. And um, here we are with Apep.

So will it actually become a GRB? That's hard to say. If it does decide to let go, the good news is that it's far enough away to have a minimal effect on us. Also, GRBs tend to focus their energy into beams, like a lighthouse. The best fitting model by the astronomers has the axis of the star tipped away from us by about 30°, so we'd miss the full brunt of the blast.

Also, it's likely it won't explode either way for tens or hundreds of thousands of years. In the what-keeps-me-up-at-night category, this object ranks pretty low. That dumb thing I said to a celebrity I met once rates way higher on that list, if that makes you feel better.

I'm actually quite happy this system has been found. If it's dangerous I'd rather know about it than not know. And since it probably isn't dangerous, that just leaves it being one of the most incredible, over-the-top, fascinating and gorgeous objects in the sky.

I can live with both of those things.