Here I’m going to break down my Master’s thesis with you. Let’s introduce ourselves to cosmic rays first.
Cosmic rays are a wild phenomenon. They are relativistic electrons, protons, and other heavier nuclei that are traveling almost at the speed of light. About 90% of all cosmic rays are known to be protons while the rest make up the other 10%. It’s amazing to see these massive particles moving so close to the speed of light because it means they must have come from an immensely violent origin for the particles to have attained such high energy. It’s important to note that cosmic rays are distinct from light in that light particles are completely massless and neutral in charge (i.e. they do not have a charge).
We know these particles exist – they bombard Earth from all directions and from a variety of sources, one including our Sun. High energy astrophysicists want to know what sources are capable of producing such high energy particles.
However, this question is not a straight forward one to answer. These particles not only have a mass but, they also have a charge. A charged particle readily interacts with magnetic fields which is a prominent feature throughout the Galaxy and the objects it contains. This makes tracing a cosmic ray’s path back to its origin, like we are able to do with light particles, incredibly difficult. The image below (right panel) illustrates a complicated cosmic ray path, rendering it impossible to know where in the Galaxy this cosmic ray formed. The left image shows us real data measured from Earth, showing us the spectrum of cosmic rays we have detected. Don’t freak out – a spectrum is simply a graph that shows you which energy range contributes the most to the flux we observe. The flux you may think of as the emitted power of the object as observed from Earth. For example, we see the lowest energy cosmic rays contribute the most to the overall flux by studying the observed spectrum (displayed in the left image below) because the peak of the spectral flux (on the y-axis) occurs at the lowest energies (x-axis). Additionally, right around the 10^6GeV (or 10^15eV) mark on the x-axis roughly tells us where Galactic cosmic rays cap off; cosmic rays with greater energy than 10^15eV must be explained by something even more energetic than any object known in our Galaxy. We therefore identify cosmic rays with energy less than or equal to 10^15eV Galactic cosmic rays and cosmic rays with energy greater than 10^15eV extragalactic cosmic rays.
The cosmic ray spectrum as measured from Earth, identifying contributions from different particle populations where applicable.
Illustration of the path a cosmic ray takes as it travels in the Galaxy. Upon detection on Earth, we cannot reconstruct its path to understand its origin.
The spectrum to the left I took from here
and is cited to be from the original experiment here
. Unfortunately, the first source requires you to have credentials or pay $$ but the original experiment is available on the arXiv, a free service for all.
Now that we have a good grasp on what cosmic rays are and why we are so interested in them, I present to you the first source I extensively studied. It started out as an unidentified gamma-ray source discovered by Fermi at very high energies. We’re talking about energies above 50GeV which is on the more energetic side of the gamma-ray energy range.
A reminder of the type of light we are talking about when I say gamma-rays. These are the highest energy rays of light!
The unidentified gamma-ray source led to my first published paper and was the basis of my Master’s thesis. I will link both at the end of this blog post!
In addition to a new, very high energy, gamma-ray source being discovered, this unusual guy also has some interesting characteristics. It’s spectrum in the high energy range (above 50GeV) is what we describe as hard. This means the majority of the flux is dominated at seriously high energies and increases rapidly with energy. We can describe its spectrum in this range with a power law. A power law models the observed flux assuming the energy falls off with a given power law index. For example, gravity behaves like a power law – its strength decreases with increasing radius with a power law index of 2. The smaller the power law index, the more dramatic this behavior would become. So in the gamma-ray energy range, when we see a very hard spectrum with a small photon index, this indicates there is efficient particle acceleration occurring here – i.e. the environment generating these gamma-rays is also probably accelerating massive particles close to the speed of light. This further implies that this environment could be generating its own cosmic rays and injecting them into the Galaxy. This was intriguing to discover so the first thing we did was see where in the sky it is located. We look at the objects around it, determining if this is truly a newly discovered candidate accelerator or if a known source in this region of space is now being detected at such high energies. I should mention that the special thing about this source is that it was reported in a catalog of objects using the Fermi-LAT with a brand new software update that enabled the instrument, for the very first time, to detect gamma-rays at energies above 50GeV. The official name of this source is 2FHL J0826.1-4500 which you see in the images below. For the purposes of this blog post, I will name it “Geronimo“.
The spectral energy distribution (SED or the spectrum) for the unidentified gamma-ray source. See how it quickly increases in energy? The shaded area marks the range of power law indices that are in agreement with the three data points. The best fit index is 1.3 for this source.
Where in the gamma-ray sky (with Fermi) this source is located. The very bright, extended source marks the Vela complex – a composite SNR.
2FHL in the name stands for the second (2) F
igh Energy Catalog. The numbers following represent its equatorial coordinates (in J2000). In other words, any astronomer anywhere in the world can find where this source is located in the sky using its Right Ascension (R.A.) of 08:26:07 and declination (Dec.) of -45:00:00. Almost all astronomical objects are given official names based on their coordinates in the sky. You will typically see either equatorial or Galactic coordinates used.
You see in the image to the right above that the source lies exactly west of the Vela complex which is a composite SNR. The Vela SNR is the closest composite SNR to Earth at only 290 parseconds or almost 1,000 light years (or 5.879×10^15 miles). This might sound incredibly far to you but in the grand scheme of things, the Sun and the Vela SNR are almost right on top of each other when you look at where they lie with respect to the rest of the Galaxy. I have a really cool schematic from a published article that I’ll try to find to illustrate this!
When we saw how close in the sky this unfamiliar gamma-ray source was to the Vela complex, we began to dig deeper into the Vela region to try to understand the possibility of this source being associated to Vela in some way. This required a thorough multi-wavelength analysis. This entailed grabbing all information known about Vela in all wavelengths of light: radio, microwave, infrared, visible, ultraviolet, gamma-ray, you name it. I tracked all published work on the Vela region and read it. What we found is that this source is indeed a by-product coming from an interaction of the Vela SNR moving into its surroundings. Before we go any further, how about a quiz to make sure we’re all on the same page? Then, we can move onto Part II: how the different wavelength information helped us piece together what is happening to the west of the Vela remnant!
The published work can be found here. My Master’s thesis is linked here.