When observing AR Scorpii, researchers noticed that its brightness varied over a 3.5 hour period. So they labelled it a periodic variable and paid it no further attention. Now, however, a large international team of astronomers has gone back and taken a more careful look at the star. The astronomers found that AR Scorpii is much more variable than first thought, with 400 percent changes in brightness occurring within only 30 seconds. The reason for this? AR Scorpii is actually two stars, and one of them is launching relativistic electrons at the other.

The paper describing these results was published this week in Nature.

The researchers were drawn to AR Scorpii because of seven years of archival images that revealed a lot of additional variability layered on top of its well-described 3.5 hour period. Rather than peaking at a similar level each time, the output could vary by as much as a factor of four.

This caused the astronomers to look at the M-class star's output more carefully. They found that the light alternated being red- and blue-shifted over the same 3.5 hour period that its brightness varies. This typically means that the star is being pulled around by something orbiting it, which causes it to accelerate towards and away from Earth, accounting for the Doppler shift. "The 3.56 hour period is therefore the orbital period of a close binary star," the authors conclude.

Based on the strength of the red- and blue-shifts, that companion must be roughly a third of the mass of the Sun, which places it squarely in white dwarf territory.

Since the changes in brightness line up so nicely with the orbit, the authors looked more carefully at those, too. And once again, something strange was up. We know the sorts of radiation that M dwarfs and white dwarfs produce, but AR Scorpii produces more than that. "In the infrared and radio in particular," the authors write, "is orders of magnitudes brighter than the thermal emission from its component stars." While the combined luminosities of the two stars should be about 1024 Watts, the maximum luminosity of the system is over 1025 Watts.

That's a lot of Watts to account for. But the authors determined that the white dwarf is spinning incredibly quickly, with a "day" on it lasting just under two minutes, based on pulses in the light output. And, if that rotation is slowing down, it could easily provide enough power to account for the extra energy.

But how does that energy get converted into light? The broad spectrum of light produced by the white dwarf suggests that it's being produced by accelerated particles, which lose energy at a variety of wavelengths when they're twisted through a curved path. In addition to that, at visible and UV wavelengths, most of the emissions appear to come from the face of the M dwarf, suggesting that the electrons eventually end up bombarding it.

So, we now have a rough idea of why the system behaves as it does. But we don't know many of the finer details about the mechanics of all of this, such as where the electrons come from, how they're accelerated, or how they end up producing these bursts of radiation. As the researchers put it, "the exact emission mechanism operating in AR Scorpii is perhaps its most mysterious feature." That's understandable, given that they also note that we've never seen anything like this before.

Nature, 2016. DOI: 10.1038/nature18620 (About DOIs).