The center of our Milky Way Galaxy contains a ‘trap’ that concentrates some of the highest-energy cosmic rays, according to a new analysis of data from the High Energy Stereoscopic System (H.E.S.S.) and NASA’s Fermi Gamma-ray Space Telescope.
NASA’s Spitzer Space Telescope, Chandra X-ray Observatory, and the NASA/ESA Hubble Space Telescope produced this composite image of the central region of our Milky Way Galaxy. Note that the center of the Galaxy is located within the bright white region to the right of and just below the middle of the image. Image credit: NASA / JPL-Caltech / ESA / CXC / STScI.
Cosmic rays are high-energy particles moving through space at almost the speed of light. About 90% are protons, with electrons and the nuclei of various atoms making up the rest.
In their journey across our Galaxy, these electrically charged particles are affected by magnetic fields, which alter their paths and make it impossible to know where they originated. But astrophysicists can learn about these cosmic rays when they interact with matter and emit gamma rays, the highest-energy form of light.
In 2016, researchers with the H.E.S.S. Collaboration reported gamma-ray evidence of the extreme activity in a small region surrounding Sgr A*, the supermassive black hole at the center of our Galaxy.
They 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.
In the new analysis, an international team led by University of Amsterdam researcher Daniele Gaggero 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.
“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,” Dr. Gaggero said.
“Those interactions produce much of the gamma-ray emission observed by Fermi and H.E.S.S.”
“Once we subtracted bright point sources, we found good agreement between the LAT and H.E.S.S. data, which was somewhat surprising due to the different energy windows and observing techniques used,” said co-author Dr. Marco Taoso, from the Institute of Theoretical Physics in Madrid and Italy’s National Institute of Nuclear Physics (INFN).
“This agreement indicates that the same population of cosmic rays — mostly protons — found throughout the rest of the Galaxy is responsible for gamma rays observed from the Galactic center.”
“But the highest-energy share of these particles, those reaching 1,000 TeV, move through the region less efficiently than they do everywhere else in the Galaxy.”
“This result in a gamma-ray glow extending to the highest energies H.E.S.S. observed.”
“The most energetic cosmic rays spend more time in the central part of the Galaxy than previously thought, so they make a stronger impression in gamma rays,” said co-author Dr. Alfredo Urbano, of INFN Trieste and CERN.
This effect is not included in conventional models of how cosmic rays move through the Galaxy.
But the researchers show that simulations incorporating this change display even better agreement with Fermi data.
“The same breakneck particle collisions responsible for producing these gamma rays should also produce neutrinos, the fastest, lightest and least understood fundamental particles,” said co-author Dr. Antonio Marinelli, of INFN Pisa.
“Neutrinos travel straight to us from their sources because they barely interact with other matter and because they carry no electrical charge, so magnetic fields don’t sway them.”
The results were published online this week in the journal Physical Review Letters.
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D. Gaggero et al. 2017. Diffuse Cosmic Rays Shining in the Galactic Center: A Novel Interpretation of H.E.S.S. and Fermi-LAT γ-Ray Data. Phys. Rev. Lett 119 (3): 031101; doi: 10.1103/PhysRevLett.119.031101
This article is based on text provided by the National Aeronautics and Space Administration.
