Where were we?
Ah, right. The new gamma-ray emission on the west of the Vela supernova remnant. After retrieving all of the known data available to us about this region, we started to piece together the puzzle. Here’s what we know!
1. New gamma-ray emission is uncovered directly to the West of the Vela SNR.
2. The gamma-ray emission is very high in energy, that is, all of the energy is detected above 50GeV.
But that’s it. We don’t know why gamma-rays are emitting at this one small section of the remnant. We don’t know what event occurred for this to happen. We don’t know what particles are responsible for the emission. Instead, we used this information and what it implies to make the next step: submit a proposal to view the region in X-rays. X-rays are also pretty high in energy but what makes this wavelength regime appealing to us is that present telescopes that can image in the X-ray have spectacular angular resolution compared to gamma-ray telescopes right now, i.e. we can see more features and distinguish between sources easier in the X-ray than in the gamma-ray. For example, the XMM-Newton X-ray space telescope has an angular resolution on the order or arcseconds. This is 1/3600 of one degree. In comparison, a full moon is roughly 1/2 a degree in our sky so 1 arcsecond of the moon would be 1.388×10^(-4) (or 1/7200) of the Moon we see which is a really tiny, tiny, tiny part of the moon. We would not be able to resolve 1 arcsecond with our own eyes. Nor would we be able to resolve 1 arcminute (which is 1/60 of a degree or 1/120 of the full Moon) with our own eyes. The Fermi-LAT, on the other hand, can resolve very high energy (VHE) sources on the order of arcminutes. So, the angular resolution in the X-ray regime is much more attractive in our endeavor to try to find more information about this new gamma-ray source.
Furthermore, the X-ray sky is somewhat less crowded. The gamma-ray sky has a lot of diffuse, or spread out, emission across the sky. A lot of this comes from our Galaxy as well as extragalactic sources (sources that are not in this Galaxy) and this can be especially distracting near the Galactic plane (see image below). We now know that the entire gamma-ray sky is full of gamma-ray emission coming from all over the Universe! The X-ray sky also has a diffuse background but it is a little easier to work with.
The gamma-ray sky at energies from 50GeV and up to 2TeV as seen with Fermi. This was first reported in Ackermann et al. 2016; the 2FHL catalog. Our source of interest in indicated by the magenta circle. The center of the image shows you the Galactic center with the Galactic Bulge dimly visible in gamma-ray emission. There are “Fermi bubbles” or arms reaching out from either side of the center. The long line of gamma-ray emission shows you our Galactic plane, where nearly all of our Galaxy sits. The rest of the emission is coming largely from extragalactic objects.
So for this reason, we asked for time on the XMM-Newton X-ray space telescope to observe our peculiar source and we got it! After cleaning the data we received from the telescope, we were able to study the X-ray emission that exists at the same location in space as the gamma-ray emission we see. We indeed found an X-ray counterpart which is a compelling overlap in both shape and position for the X-ray and gamma-ray emission. The other nice thing about looking in other wavelengths, if you can find one positive counterpart, you can then use this new information to look further into other wavelengths to see what other emission this region might be giving off. That’s exactly what we did! The images below reveal the X-ray emission we see after cleaning up the data and the first counterpart we found by using the position of our gamma- and X-ray data and the shape we resolved with XMM-Newton.
This is the X-ray emission we see with XMM-Newton coming from the 2FHL position. The circle is 5 arcminutes in radius and denotes where the gamma-ray emission is observed. We used this odd shape of X-ray emission to try to find other counterparts.
This is an optical image, specifically looking at the Hydrogen (-alpha) emission at 656 nm. You see a filament, or very thin structure, that traces the X-ray emission we see, with the 2FHL position again indicated by the white circle.
A picture is starting to form in our minds… We have gamma-ray emission that is very concentrated to the west of the Vela supernova remnant that has soft, or low-energy, X-rays tracing out an optical boundary or filament. Something must be happening at the edge of the remnant here for it to be so energetic with so many puzzle pieces. We started leaning towards the idea that maybe the remnant is running into something here and is shocking
it. Imagine a hot, really fast-moving wave of heavy mass hitting a cold, slow clump of gas. A lot of mixture, turbulence, and violent disruption happens on many scales. You would expect the cold, slow clump of gas to ignite in some way; the gas particles responding rapidly and enthusiastically with its new momentum from the collision, colliding into each other and gaining massive amounts of energy at the collision boundary. It begins to illuminate its surroundings as these interactions take place, heating the gas and shocking it further. The gas clump likely feels an incredible increase in its temperature.
It seems reasonable then that this is in fact what we are seeing but we cannot say for sure. If this work will be worthy to publish, we need concrete evidence that a shock-cloud interaction is taking place and causing the emission we see. So, we keep digging for more puzzle pieces.