Astronomers have found the gamma-ray-emitting remains of three exploded stars, and the remnants might reveal the origin of cosmic rays.
Astronomers are hoping to learn more about the mysterious nature of cosmic rays by scrutinizing three possible supernova remnants that are emitting very high-energy gamma rays.
A new Astronomy & Astrophysics paper presents a meta-analysis of the potential supernova remnants using data from the High Energy Stereoscopic System (HESS) telescope array in Namibia. While much of the study expands on previous research, the findings provide a point of comparison for scientists studying the origin of cosmic rays, energetic particles that fly through the Milky Way at relativistic speeds.
Very high-energy gamma rays are the most energetic form of radiation and have the smallest wavelengths on the electromagnetic spectrum. They are emitted during extremely energetic events, such as the aftermath of supernova explosions. HESS is optimized to find the most energetic gamma rays, with energies between 0.03 and 100 teraelectronvolts (TeV). (One TeV is equivalent to the energy carried in a flying mosquito — a whopping amount when you consider that a proton is a trillion times smaller than a mosquito.)
As the expanding layers of an exploded star slam into the surrounding medium, the shocked gas emits radio waves, and sometimes gamma rays, too, though a supernova remnant has never been seen that solely radiates gamma rays. Yet the astronomers couldn’t pinpoint a radio source for two of three suspected remnants, possibly because they lie in the crowded plane of the Milky Way. More study will be needed to better understand the nature of these sources, says HESS collaborator Gerd Pühlhofer (University of Tübingen, Germany).
One of the larger goals of the HESS Galactic Plane Survey, from which this study comes, is to better understand the origin of cosmic rays. Cosmic rays are unstable, gradually decaying into other subatomic particles and emitting gamma rays in the process. Supernova remnants are the main source of the speedy particles, but exactly what processes accelerate them is still unclear.
The outward-expanding shock waves in supernova remnants ought to accelerate both electrons and protons, but electrons are easier to detect since they emit over the entire electromagnetic spectrum, from radio waves to gamma rays. Protons, which make up 89% of all cosmic rays that reach Earth, reveal themselves only by gamma-ray radiation. That makes gamma rays ideal for detecting both types of cosmic rays. More observations of gamma-ray sources will help astronomers find the conditions needed to accelerate cosmic rays.
Bradley Schaefer (Louisiana State University), who did not participate in the research, cautions that the new paper is but one step toward understanding cosmic rays — it provides the data theorists need to move forward.
H.E.S.S. Collaboration. "A search for new supernova remnant shells in the Galactic plane with H.E.S.S." Astronomy & Astrophysics 2018 April 9.