NASA | GSFC | Fermi | HESS | 2017 Jul 18
[img3="The five telescopes of the High Energy Stereoscopic System (H.E.S.S.), located in Namibia, capture faint flashes that occur when ultrahigh-energy gamma rays are absorbed in the upper atmosphere. A new study of the galactic center combines high-energy observations from H.E.S.S. with lower-energy data from NASA's Fermi Gamma-ray Space Telescope to show that some of the speediest particles become trapped there. (Credits: H.E.S.S., MPIK/Christian Foehr)"]https://www.nasa.gov/sites/default/file ... 818385.jpg[/img3][hr][/hr]A combined analysis of data from NASA's Fermi Gamma-ray Space Telescope and the High Energy Stereoscopic System (H.E.S.S.), a ground-based observatory in Namibia, suggests the center of our Milky Way contains a "trap" that concentrates some of the highest-energy cosmic rays, among the fastest particles in the galaxy.
"Our results suggest that most of the cosmic rays populating the innermost region of our galaxy, and especially the most energetic ones, are produced in active regions beyond the galactic center and later slowed there through interactions with gas clouds," said lead author Daniele Gaggero at the University of Amsterdam. "Those interactions produce much of the gamma-ray emission observed by Fermi and H.E.S.S."
Cosmic rays are high-energy particles moving through space at almost the speed of light. About 90 percent are protons, with electrons and the nuclei of various atoms making up the rest. In their journey across the galaxy, these electrically charged particles are affected by magnetic fields, which alter their paths and make it impossible to know where they originated.
But astronomers can learn about these cosmic rays when they interact with matter and emit gamma rays, the highest-energy form of light.
In March 2016, scientists with the H.E.S.S. Collaboration reported gamma-ray evidence of the extreme activity in the galactic center. The team found a diffuse glow of gamma rays reaching nearly 50 trillion electron volts (TeV). That's some 50 times greater than the gamma-ray energies observed by Fermi's Large Area Telescope (LAT). To put these numbers in perspective, the energy of visible light ranges from about 2 to 3 electron volts.
The Fermi spacecraft detects gamma rays when they enter the LAT. On the ground, H.E.S.S. detects the emission when the atmosphere absorbs gamma rays, which triggers a cascade of particles resulting in a flash of blue light.
In a new analysis an international team of scientists combined low-energy LAT data with high-energy H.E.S.S. observations. The result was a continuous gamma-ray spectrum describing the galactic center emission across a thousandfold span of energy. ...
Diffuse Cosmic Rays Shining in the Galactic Center:
A Novel Interpretation of H.E.S.S. and Fermi-LAT γ-Ray Data - D. Gaggero et al
- Physical Review Letters 119:031101 (17 Jul 2017) DOI: 10.1103/PhysRevLett.119.031101
arXiv.org > astro-ph > arXiv:1702.01124 > 03 Feb 2017 (v1), 16 Jun 2017 (v2)