A gigantic explosion may be the end of a massive star's life, but it is by no means the end of its story. Take Cassiopeia A (Cas A for short), the remnants of the most recent known core-collapse supernova in our Galaxy—a stellar explosion about 11,000 light-years away that would have been visible to Earth around 350 years ago. Since then, debris from the explosion has been blasting into the universe as fast-moving material plows into its slow-moving surroundings and creates powerful shockwaves.
These shockwaves heat the gas to millions of degrees, causing it to glow brightly in optical, ultraviolet, and even X-ray light. These shocks can also accelerate particles like electrons to nearly the speed of light, becoming what are known as cosmic rays. These high-energy cosmic rays also emit X-rays, which carry information about the heating and cooling processes happening within the remnant.
Low-energy X-ray observations taken by NASA’s Chandra X-ray Observatory over the past two decades have shown that the Cas A supernova remnant is expanding and slowly cooling down. However, since low-energy X-rays are produced by both hot gas and high-energy cosmic rays, it is difficult to determine which of these light sources contribute the most to these changes.
That's where NASA’s NuSTAR satellite comes in. With its ability to detect the high-energy X-rays that are only produced by the high-energy cosmic rays, NuSTAR can produce maps of the most energetic regions of the supernova remnant and watch how these regions evolve over time.
In a recent paper led by Dr Jooyun Woo, then a graduate student at Columbia University, astronomers used new NuSTAR observations of Cas A and compared them with observations taken ten years ago. If the electrons had been accelerated all at once in the initial shock wave, then we would have expected them to have cooled down and become dimmer. In comparing the two images, Woo and her co-authors found that the X-ray brightness of these shock regions did not decrease as much as expected. This tells us that cosmic ray heating is still taking place, keeping the supernova remnant bright in the latest NuSTAR image. Studying such changes in brightness over time allows astronomers to compare different models of electron acceleration, enabling the remnants of the relatively nearby and recent Cas A supernova to act as a laboratory in which we can test physical theories in environments that we can't reproduce in labs on Earth.
Even with its slower-than-expected rate of dimming, one day Cas A will fade away and become too faint for a telescope like NuSTAR to detect, possibly within a century. It is incredible to think of how many advances in astronomy have taken place over the last 350 years to allow us to see the high-energy emission from this explosion before it vanishes!
