COSMIC RAY ACCELERATION IN SHOCK-CLOUD INTERACTIONS
Can we establish the Vela SNR and its shock-cloud interaction as a possible site where cosmic rays are produced?
- Look for asymmetries in the shape of the formed supernova remnants. Dents and “break out regions” (i.e. where the shock wave appears to have expanded much farther in one direction than another) can be good indicators of how the surrounding matter is impacting the shock wave expansion. Essentially anything that is not perfectly circular gives you indicators of the density of the ambient material and how it changes in any direction of the supernova remnant. The shape of a supernova remnant can change dramatically by what wavelength you are looking at. The wavelength can also give you different information!
- Radio: Radio emission is one of the best regimes to identify supernova remnants and classify them. Radio emission is often coupled to the X-ray radiation present and can give information on the electrons responsible for emission in both wavelengths. Radio waves can also map the supernova remnants’ surroundings, showing what pre-existing neutral matter might be lurking in the vicinity of nearby shockwaves.
- Infrared (IR): IR emission can show dust from the surroundings that has been swept up by the supernova remnant shock wave and has been shocked and heated.
- Optical: Filaments visible in the optical regime can tell you how the shock wave has impacted pre-existing clouds of material. Filaments form when the ambient material is shocked and heated. A really good indicator of a shock-cloud interaction is confirming an optical filament that coincides with a bright X-ray boundary. This is supportive that you have found the position of the shock wave boundary of the remnant and its surroundings, indicating it is pushing up against something.
- X-ray: This regime alone tells you a lot about the morphology of a supernova remnant and potentially even what type of explosion occurred that gave rise to this emission. X-rays do a great job of showing you where the shockwave is in space because the material that is swept up, shocked, and heated, will be excited enough to radiate a lot in this regime. Finding those boundaries and comparing across wavelengths can make a more complete picture.
- Gamma-ray: Gamma-rays tell you an environment has been disturbed so aggressively, the particles are accelerated to very high energies. Gamma-rays are also the product of cosmic rays interacting with ambient material. Because cosmic rays are observed to be mostly protons or ions, we look for gamma-ray signatures that indicate a high proton population presence and acceleration mechanism.
Tying all of the information from each wavelength together creates a robust picture in order to determine if a shock-cloud is in fact happening. When gamma-ray emission is present as well, this presents the possibility for particles to be efficiently accelerated at the shock-cloud boundary to cosmic ray (CR) energies that then decay to gamma-rays when interacting with their dense surroundings.
The pion bump is the result of CR protons’ interaction with other ambient, less energetic protons. The protons collide and decay to neutral pions which then decay into gamma-rays. The gamma-ray signature will show up at the rest mass of the decayed neutral pion. On a standard spectral energy distribution (SED) plot, that would be around 200MeV. Here is a really great article discussing observations and modeling looking for the pion bump.
The first line indicated is showing you the best fit model for the gamma-ray emission, regardless of the particle population, and only dependent on the energy distribution. The data points are taken from two different telescopes 1) Fermi and 2) AGILE, another gamma-ray telescope. The solid purple-blue line models the emission if it were from pion decay (i.e. the pion bump). The dashed purple-blue line corresponds to the electron population generating the observed emission via nonthermal Bremsstrahlung. The dash-dotted line is a modified electron population generating the observed emission via nonthermal Bremsstrahlung. A careful inspection of this plot shows that the data points follow the pion decay model the best, thus confirming that the gamma-ray emission for this supernova remnant is dominated by proton-proton collisions, and hence makes W44 another candidate for fresh CR acceleration. This means the environment is energetic enough to produce their own CRs, as opposed to just boosting up pre-existing CRs that get tangled in the region.
With our SED, things are not so clear. It is shown below and is also discussed in the paper.
But it is not a lost cause. There are other things to look for. This requires a deeper investigation into the properties of the shock itself. If we can determine the interaction is relatively new we might be able to say fresh CRs are produced here. However, if the shock has pushed into this cloud for a while, it has probably lost a lot of speed with respect to the rest of the shockwave and thus, loses more opportunity to accelerate particles to CR energies. Ways we can do this are by looking back in the optical and looking for tracers that tell us if the shock has gone radiative.
When a shock has gone radiative, this is when rapid cooling takes place and will dominate with time, sucking energy away from the shock and dispensing it into its surroundings. When rapid cooling starts, elements can “recombine” and will radiate via optical radiation, further instigating the cooling mechanism. The first elements to show up are oxygen, silicon, and nitrogen. Specifically, O [III], Si [II], and N [II]. These are ions of the elements, where the numbers indicate the loss of electrons. For example, oxygen should have 8 electrons (just by looking at the periodic table), but its doubly ionized in O III, which means it only has 6 electrons (and therefore has a positive charge of 2+ because it now has 2 more protons than there are electrons). Therefore, observing the shock location in the optical range where these ions radiate when present will tell us if the shock is radiative, and whether it can freshly produce cosmic rays by itself in the shock-cloud boundary.
So we ask for time on an optical telescope, the Gemini telescope in Chile. It is an 8-meter telescope with spectacular angular resolution so we can probe the shock with sub-arcminute resolution to find radiative tracers. We ask for both imaging and spectroscopy to do a thorough study in this band. We got both! In my next post on this, I will share with you the preliminary findings from the imaging, which are spectacular to look at. Optical astronomers are so lucky – they produce such beautiful, whimsical, and informative images. The spectroscopy, on the other hand, is amazingly hard to reduce. Currently, I have no results from the spectroscopy (that make sense). This should excite you as I have yet to publish any results on these optical images but yet, you will be able to see them here first! 🙂