Supernovae are some of the most energetic events in the Universe, sending massive shock waves out into the interstellar medium. And there's every reason to think those shock waves run into things before they've had much of a chance to dissipate. Many stars have companions, either planets or other stars that orbit in reasonable proximity. In fact, there's an entire subtype of supernova that appears to require a nearby companion.

So what happens to these objects when the shock wave hits? With our improved ability to rapidly identify supernovae, we may be on the cusp of finding out. Several times recently, researchers have spotted an extra blue glow to the burst of light from a supernova. And, in the most detailed observations yet, they make the case this glow comes from the supernova debris slamming into a companion star.

A supernova explosion that envelopes a nearby star is an inevitability. Eta Carinae, for example, is a system with two stars that are at least 30 times the Sun's mass, meaning they'll both eventually explode as a type-II supernova. Whichever goes first will undoubtedly send debris into the second. But there's a different class of supernova, type-Ia, which requires the presence of a nearby star.

Exploding dwarfs

Type-Ia supernovae involve the death of a white dwarf, the carbon/oxygen rich remains of a star like the Sun. White dwarfs have burnt through all their hydrogen and helium fuel but don't have the gravity to create conditions where carbon and oxygen can fuse. If these cross a certain mass threshold, however, fusion will suddenly restart, blowing the white dwarf to pieces in what's a staggeringly large nuclear explosion.

There are two ways for that additional mass to reach the white dwarf. The first is a collision of two white dwarfs, both of which end up destroyed by the blast. The second is when there's a nearby companion star. In this case, the white dwarf can draw off matter from its companion, slowly growing until it crosses the critical threshold and explodes. By necessity, there's going to be a star nearby when the explosion takes place. So what happens to it?

Supernovae are rather notoriously bright, which makes detailed observations of anything nearby more than a bit challenging. But we've seen a lot of type-Ia supernovae over the years, and, since they all explode through a similar mechanism, the light they produce shares a number of common features. So, the trick is to identify deviations from these features.

In a new paper, a large team of researchers notes that a couple of recent supernovae have shown an unexpected excess of blue light. They then go on to describe the supernova SN 2017cbv, which became visible in March of this year. In this case, the supernova was spotted only a day after the explosion took place. Follow-up observations over the next six weeks allowed the team to build up a detailed record of the aftermath of the supernova (called a light curve). This record was then compared to what we'd expect from a typical type-Ia supernova.

Photon surplus

Compared to a standard type-Ia supernova, SN 2017cbv showed an excess of light at all wavelengths, including the blue and UV regions. This was mostly pronounced during the first week after the explosion, as the light ramped up toward its peak. At the peak, there was an excess in redder wavelengths than you'd see in a typical type-Ia supernova.

What could cause this? To find out, the team modified a model of type-Ia supernovae to include a shockwave running into a nearby star. The modified model fits the data well and suggests that somewhere between five and 15 percent of the light during the peak came from the shockwave hitting a nearby star.

The model, however, suggests that the companion star was nearly 40 million kilometers from the white dwarf (which is a bit closer than Mercury is to our Sun). That's right on the far edge of where you'd expect the companion star to be if the two were close enough to share material. However, the authors point out that this model assumes that they're looking at the event-companion with an ideal viewing angle. If the angle is off, it would look dimmer, which would make the companion star appear to be farther away.

The authors don't think this is a conclusive demonstration that we're watching a companion star, since there are a number of other potential explanations that haven't been ruled out. These include an irregular shaped debris cloud, which could allow more light to escape, or a blob of highly radioactive material to be visible sooner after the explosion. There's also the prospect that some material was ejected in advance of the explosion, and the extra light was the debris of the explosion running into this earlier material.

The best way to sort this out is, of course, more observations. This wasn't the first supernova we've seen that's a bit brighter than expected. Adding a few more detailed views of the early supernova debris could help us sort out what, precisely, we're looking at.

The Astrophysical Journal Letters, 2017. DOI: Not yet available.