Figuring out what powers the Universe's largest explosions can be a real challenge, as the explosion wipes out evidence of what caused it. Archival data can sometimes provide hints of what was in the area where things went boom, but a lot of the progress we've made comes down to physicists modeling some of the more extreme objects out there and seeing if they can recapitulate the details of the explosion.

That's where we're at with long gamma ray bursts (where "long" in this case means a couple of seconds). We've seen them happen, and astrophysicists have calculated that they could be emitted from a rapidly rotating, massive star. But we don't have a lot of examples of this sort of star to study in order to see if the physics of their explosions match up with our models. Now, a team of researchers thinks it has spotted one that, in combination with a second massive star, created the fantastic-looking pinwheel shown above. But detailed observations of the system suggest that the pinwheel is formed by materials that originated on a single star yet are moving at two different speeds—something we can't explain.

The serpent god

Technically, the new object goes by the absurdly memorable name 2XMM J160050.7–514245. Surveys spotted it because it was an oddity: unusually bright at certain infrared wavelengths. Follow-up observations revealed its sinuous form, which led the researchers to rename it from the "cumbersome" 2XMM J160050.7–514245 to Apep, which is the name of a serpent deity in Egyptian mythology.

Apep turns out to be a binary star system, with the main star being the source of most of the infrared emissions and the second star a fainter companion. Details of the light coming from the star suggested it lacked hydrogen; that, combined with its apparent brightness, indicate it's a Wolf-Rayet star. Wolf-Rayets are so massive that, as their core shifts to burning heavier elements, they produce enough energy to expel their outer hydrogen-rich layers. The result is a massive bright star with a spectrum that's dominated by heavier elements (in this case, carbon).

Rapidly rotating Wolf-Rayet stars are thought to give rise to long gamma ray bursts, in part because the heat of the core and their rapid rotation leave them on the cusp of bursting apart.

In the case of Apep, a Wolf-Rayet's tendency to shed its outer layers has helped create the pinwheel-shaped cloud of material that surrounds it. Other known pinwheels are formed by a binary system of two Wolf-Rayet stars, with the elaborate shapes crafted by a combination of winds of material blasting off both stars and the gravitational interactions of the stars with that material. Apep's companion is far enough away that the pinwheel is largely shaped by the interactions of the winds coming off the two stars, so the researchers focused on characterizing those.

And that's where things got a bit weird. The dust itself was tracked by observations made a year apart, which allowed the researchers to figure out its relative motion. Based on indications that Apep is a bit under 8,000 light years away, measurements indicate the dust is moving at about 570 kilometers a second. They also used the Doppler shift of the gas being ejected from the primary star to measure its velocity. That came up with a much higher number: 3,400 kilometers a second.

Bad explanations

That's strange, because the dust is formed when the material in the two stellar winds interact. Even if the dust doesn't start out moving at the same speed as the wind, collisions with it will quickly bring the two up to the same speed. So it's not at all clear why the two are moving at such radically different speeds.

The researchers consider a number of possible explanations, but most of them are bad. The discrepancy would go away if Apep were much farther away, but the distances involved aren't consistent with the observations and would mean that Apep is actually the brightest object in our galaxy at several wavelengths. So that's unlikely. Another option would be that the speed of the material ejected by the star underwent an acceleration at some point in the recent past. This doesn't work because it would only take a decade for the fast-moving material to catch up with the slower stuff, a collision that would be obvious.

Instead, the researchers favor the idea that the star is ejecting a fast-moving wind from its poles and slower-moving material along its equator. Given the position of its companion, this would bias dust formation to the slow-moving material. A simple model of the geometry suggests that this is compatible with the pinwheel shape of the visible material, although a more detailed physics-based model would need to be tested.

In any case, the most likely explanation for the pinwheel structure has a rapidly rotating Wolf-Rayet star at its center, which makes this a possible long-duration gamma ray burst precursor. That and the relatively short distance to it make it a great laboratory to test some of the models that theoretical astrophysicists have come up with.

Nature Astronomy, 2017. DOI: 10.1038/s41550-018-0617-7 (About DOIs).