Let’s get comfortable with the systems that house pulsar wind nebulae. Those systems are what we call composite supernova remnants.
What are composite supernova remnants?
First, let’s recap: supernova remnants occur when massive stellar systems collapse under their own gravity and then rebound back from their inert cores in a violent explosion, ejecting tons of mass with it, in all directions through space at thousands of kilometers per second. Note: Get comfortable with the metric here! All scientists, including Americans, use this measurement system (it’s way more convenient for a lot of reasons).
It’s a pretty violent event, right? There are two common supernova events you should be familiar with: Type Ia supernovae and Core Collapse (CC) supernovae.
Type Ia supernovae are distinct from CC supernovae both in their appearance, the light they give off, the elements present in their spectra, and in their origin.
Type Ia supernova remnants typically arise from a binary system of two massive stars (but not more massive than, say, 4 times the mass of the Sun). Both of these stars evolve very similarly: first on the main sequence branch where they burn hydrogen for several billion years, then they swell into a puffy and bright red giant as they exhaust their hydrogen, fusing the element into helium in a shell around a helium core. Eventually, the puffy red giant says “ENOUGH!” of all the nasty heavier elements accumulating around its inert core and the star expels all of the layers, leaving behind a smaller core of a star known as a white dwarf. (The now expelled layers are called a planetary nebula).
The white dwarf that is left very, very dense. The size of the star is comparable to the size of Earth but packed with a mass that is comparable to the Sun!
Take one massive stellar companion and pair it up with one comparable in size, but smaller. Such that the more massive star will reach the white dwarf stage first. In this scenario, you have a white dwarf evolving around a middle-aged star that begins to enter its red giant phase. That means the white dwarf star is orbiting a star that is beginning to expand significantly into the orbit of the white dwarf. If the two stars are close enough, the white dwarf can actually start capturing matter from the surface of the red giant! We call this accretion. A white dwarf can accrete matter from its stellar companion if they are close enough in space. This causes a problem though. The white dwarf is already SO HEAVY. IT’S SO DENSE. IF IT TAKES ON TOO MUCH MASS IT WILL LITERALLY EXPLODE.
And that, my friends, is a Type Ia supernova.
Answer: Nothing. The white dwarf explodes so violently, it obliterates the entire stellar system, leaving behind no core, no pulsar, nothing but the ejected mass that has been sent hurling through space at monstrous speeds. I guess I’m lying to you a little though because it’s not nothing. What is left behind is a bright and energetic supernova remnant left to its own demise as its mingles with the interstellar medium (i.e. the space and gas and dust that exists between star A and star B).
But, I still haven’t even touched on CC supernova remnants which is a great segue into composite supernova remnants, as composite remnants describe many CC supernovae. But since we already know what causes a Type Ia supernova, it’ll be easy to describe:
Take one really massive star that is at least 8 times the mass of the Sun. When the star runs out of hydrogen to burn in its red giant phase, the only thing left keeping the star from collapsing in on itself is the electron degeneracy pressure (oh, fancy talk!). The star is too massive at this stage to withhold the outer layers of the star against the iron core so, what happens? Kaboom!! A CC supernova occurs. This time what is left behind is a supernova remnant and the remaining core of the progenitor star (more fancy talk!). The core of the progenitor star is the neutron star and often powers a pulsar wind nebula.
And just like that, we are familiar with the two most common types of supernova remnants found in our Galaxy. CC supernova remnants are the remnants that can be considered composite.
A composite supernova remnant is one that has all three components from a CC supernova: a pulsar, a pulsar wind nebula, and a SNR shell. All of these things have signatures in the light they give off and can be identified this way.
Let’s briefly discuss the other two types of remnants and how we can distinguish between them all!
Shell remnants: Observed emission from these systems is dominated by only the shell of the remnant. Remnants of either class could be categorized as such (i.e. Type Ia or CC). A ring like structure that often radiates in radio and X-ray wavelengths is commonly seen with shell remnants.
Crab-like or plerionic remnants: Aptly named after the famous Crab nebula, Crab-like or plerionic remnants are CC supernova remnants that have observed emission coming mainly from the pulsar and/or pulsar wind nebula. The emission that comes from the pulsar and pulsar wind nebula is mostly attributed to highly relativistic electrons and positrons.
It is sometimes observed for an SNR to have a filled center as opposed to a ring like structure. Depending on the nature of the emission, it could be thermal X-rays from hot supernova ejecta or non-thermal emission from the pulsar wind nebula.
In general, emission from the shell of the SNR can be thermal or non-thermal in nature; thermal emission can be radiated by hot gas that is along the edge of the SNR’s front shock and being close to thermal equilibrium, i.e, all of the particles here emit radiation at the same temperature. If the plasma is not in thermal equilibrium, then it is non-thermal in origin. We often see non-thermal emission from the plerion. In both instances, we can model with good precision the temperature (for thermal) and the behavior of the particles and thus understand where the emission (whether it’s X-ray or else) is coming from. A composite supernova remnant has emission from both the shell and the plerion.
Finally, in summary, we can also identify Type Ia and CC SNe based on the elements present in their spectrum. This is a very basic rule but it is a good one for Type Ia and Type II supernovae (note: CC supernovae are a type of Type II supernovae. For now just know that – we can revisit other types of Type II SNe later):
If you see hydrogen emission lines in the light spectrum of the SNR, it is likely a Type II supernova remnant. If you do not see hydrogen emission lines in the light spectrum of the SNR, it is likely a Type Ia supernova remnant. I use the word “likely” because its probable based on this rule but, there are other indicators! For example, the location of the SNR, the density of the surrounding region, and other emission lines in their spectra can provide clues to the event that occurred for the SNR to exist.