• Gamma-ray emission uncovered to the west of the Vela SNR
• Investigations of this region in the X-ray uncover soft, diffuse X-ray emission overlapping with the gamma-ray position
• We now look across the light spectrum using Aladin (free for everyone and you don’t need to download anything. It’s truly amazing – See over a dozen sky maps across the light spectrum right now: https://aladin.u-strasbg.fr/AladinLite/)

​Click the link above. I’ll show you how we found our optical counterpart. Type in “08 26 07 -45 00 00” verbatim, double check you have “DSS2” selected. Hit ENTER. Do you see a thin film of material move downwards from the crosshair in the center of the screen? Yeah – that’s our optical counterpart and you just found it using the gamma-ray emission’s coordinates.

Now the real fun sets in: I begin digging. Digging for information about the Vela SNR. How old is it? i.e. When did the progenitor star explode? What type of supernova was it? How far away is it? What available information is known, specifically coinciding with this gamma-ray emission?

Here’s what I found out. 

Vela is about 10,000 years old descending from a Type II supernova which we have discussed before here. Because it is a type II supernova remnant, it houses a pulsar, the remnant of the star’s explosion which powers a pulsar wind nebula that is quite large. Vela as it turns out is the closest composite SNR to Earth which means there are tons of literature about the region. I read dozens of papers studying Vela from the radio to the gamma-ray. It became apparent that the pulsar wind nebula is huge and is seen brightly in all wavelengths and so is the pulsar. The remnant itself was only known to generate gamma-rays 1.2 degrees away from the pulsar or about 15 parseconds (50 light years) but in the opposite direction of where we found the new gamma-ray emission and we are still not really sure where the gamma-rays in this other region come from. It’s possible it could be another interaction site (which I’ll get to).

Because Vela is a Type II supernova remnant, this means the area surrounding the remnant is quite dense compared to other regions of space. As a refresher, let’s remind ourselves that Type II supernovae occur when a seriously massive star explodes. Massive stars have shorter lifespans than smaller stars because the larger the star is, the faster it burns its fuel. Once it runs out, it explodes. The largest stars have lives as short as a few million years old. For comparison, our star, the Sun, is very “average” in size and will not explode at end of life but will rather complete a few evolutionary stages before shedding it’s layers and retiring to the white dwarf stage – which will take our Sun a total of 10 billion years. So, when a massive star is born, it is born into a dense environment of stellar gas, a stellar nursery, if you will. The most massive stars die off and explode the quickest and thus never leave the dense region of stellar gasses, so when it explodes, the explosion is typically very asymmetrical and quite complicated due to the complexity of the region. As the initial blast wave plows into interstellar space, it will readily interact with material already here like gas, dust, molecular clouds and clumps, etc. Depending on how fast the initial blast wave is and how dense the material is, it interacts differently. If the shock wave is still hugely fast, it will sweep up this material, shock it, and move it with the blast wave itself. If it is has slowed down considerably, it will run into the material and slowly shock and heat it as the shock wave dissipates into​ the material. The timescales of these interactions depend on the density of the material. 

Why am I talking so much about a remnant’s surroundings? Oh right, so, Vela is a Type II SNR which means that its surroundings are probably pretty clumpy. In fact, in 1998, when I was 4 years old, radio astronomers pointed their telescopes at Vela and saw an abundance of neutral hydrogen out here. They came to the conclusion that these structures were pre-Vela, or pre-existing. What likely happened is another massive star, possibly millions of years ago or more, exploded and its SNR expanded to be this entire huge neutral hydrogen bubble expanding into deep space and by the time Vela came along, the medium was made up of dense, clumpy clouds from the leftover material of this long-ago supernova, leaving plenty of opportunity for a fresher remnant to interact with it. 

This was the same paper I noticed our optical counterpart was being studied. Like, our exact optical filament, believed to be associated in some way to the mysterious gamma-ray emission, was being discussed and implicated in a theory in this paper, that was published over 20 years ago. 

I remember it vividly – figure 7 of her paper – showing me that our instincts were, in fact, right. We suspected the blast wave of the remnant to be running into something, generating the emission we were observing. This 1998 paper confirmed what we thought and provided the last piece of the puzzle: the something it was running into. A small neutral hydrogen cloud, at the same position of the optical filament. What’s more is that they shared not only the same region of space, but also opposite curvature. You can see it in the picture above. See how the filament seems to trace the contours of the hydrogen cloud? Finding “puzzle pieces” like this is actually pretty substantial evidence that these two are not only connected, but are telling you an interaction between some type of shock wave and a cloud is occurring. 

​We found older X-ray maps from a previous mission, ROSAT (The Roentgen satellite, named after the scientist who discovered X-rays), that mapped the entire Vela region in the soft X-rays. You can see in the image, appended below, where our source lies with respect to the entire remnant. Let’s spend a few minutes dissecting the image for a more wholesome understanding, then we will loop back to the important piece of evidence here.

We see the entire remnant right – looks almost fully spherical in the X-rays. Compared with Vela in the optical here, it looks quite different, doesn’t it? In the X-rays, it’s nature is obvious. This emission is found to be mostly from really hot particles from the blast wave and from stellar ejecta that have been swept up as the supernova expanded into its surroundings. You can identify a few “break out” regions, one is directly to the left (east) and the other is more subtle, and fainter, directly to the west (right) and much farther out. These break out regions are indicators for how the density in the interstellar medium changes. Where the blast wave has been able to break into indicates less dense regions. Where the bright boundaries exist, this indicates where the blast wave has coincided with denser material, shocking and heating it as it begins to slow down due to the blockage.

So, essentially the bright X-ray boundaries give you a little information about the 1) density of the surrounding medium and 2) speed of the shock there and 3) where the remnant ends and the surroundings begin.
​
In summary, we have an optical filament attached to both the X- and gamma-ray emission to the west, that is now also associated with a neutral hydrogen cloud and they share opposite curvature, and all of this coincides with a bright X-ray boundary. 

…. What? You thought this is where the story ends? Heck no! This is just the beginning. Now that we have established that the SNR is interacting with its surroundings – we just confirmed the Vela SNR as a candidate for fresh cosmic ray acceleration.

Recall that SNRs and PWNe are some of the most extreme objects in our galaxy and are thought to generate the bulk of Galactic cosmic rays. So a new challenge has revealed itself to us: Can we establish the Vela SNR and its shock-cloud interaction as a possible site where cosmic rays are produced?