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The Forefront of Space Science

Physics of Fermi Acceleration Explored by Fermi Gamma-ray Space Telescope
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Observation of supernova remnant with Fermi satellite

The main LAT detector of the Fermi satellite is called a pair-conversion telescope. By measuring tracks and all energy of the electron-positron pairs that are created from gamma-rays coming from the universe in the LAT, it is possible to measure the gamma-rays with energy of 20 Mega to 300 Giga eV. The new LAT has far superior performance compared to its predecessor, i.e., the EGRET detector onboard the Compton satellite and, accordingly, gamma-ray observation of supernova remnants has advanced greatly.

One important outcome provided by LAT observation is that it allows us to estimate the energy amount to be distributed to high-energy protons in shock waves of supernova remnants. The energy amount transferred to high-energy protons is not determined by the current Fermi acceleration theory because of the “entranceEproblem above. When a high-energy proton with energy of more than approx. 300 Mega eV collides with a hydrogen nucleus in the interstellar gas, neutral meson is produced and immediately decays into two gamma-ray photons. If we can observe the meson decay gamma-ray, the energy amount of high-energy protons could be estimated. By combining this method with the results obtained before by X-ray observation, it was concluded that gamma-rays from the famous Tycho's supernova remnant were meson decay gamma-rays. It was also revealed that the large energy amount was distributed to the high-energy protons. In the past, estimates of high-energy electrons have been made, but this is the first time it has become possible to estimate protonsEenergy. Observational results showed that the protons bore far more energy than electrons. This is an important step to elucidate the origin of the cosmic rays and will contribute to understanding shock-wave acceleration in a variety of celestial bodies.

Escape from supernova remnant

For particles to be accelerated in the Fermi acceleration process, they must be fully scattered even upstream of the shock wave, i.e., just outside the supernova remnant, and return to the shock wave. Turbulent magnetic fields, which work as scatterers in the shock-wave upstream, are thought to be generated by the accelerated particles themselves. Because of this non-linear nature, a theoretical solution to the above “exitEproblem is still difficult. In order to understand this issue, it is important to observe the particles that pass through the acceleration process and “escapeEfrom the supernova remnants.

For the first time, we were able to find an observational example of such an event by the Fermi satellite. As shown in Fig. 2, when observing the supernova remnant W44, we identified that high-energy particles having “escapedEupstream of the shock wave then collided with the nearby giant molecular cloud to emit gamma-rays. Particle acceleration, especially its maximum attainable energy, is inextricably linked to their escape. Thus, we successfully obtained new information from the gamma-ray observation toward the elucidation of the Fermi acceleration mechanism.

Figure 2
Figure 2. Gamma-ray emission map around supernova remnant W44 shot by Fermi satellite
W44 (magenta color contour) is a strong source of radio waves and also has strong gamma-ray emissions. In order to see the gamma-ray distribution around W44, gamma-ray radiation from it is erased in this figure. Green contours are carbon monoxide (CO) line radio wave emission map, which suggests that a huge molecular cloud of 300 light years in diameter exists as if surrounding W44.


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