Speaker
Description
Cosmic rays (CRs) with energies around 10$^{18}$ eV are generally believed to originate outside our galaxy. If such extragalactic CRs are accelerated by first-order Fermi acceleration, due to their large energy, the shock velocity must be very close to the speed of light, implying relativistic shock waves. However, previous studies have pointed out that the acceleration efficiency at relativistic shocks is typically very low, making it difficult to reach ultra-high energies.
To accelerate CRs by relativistic shocks, we need strong turbulence. We consider that such turbulence can be generated when a shock propagates into the inhomogeneous media. To test this scenario, we performed 3D relativistic MHD simulations and confirmed that turbulence is indeed produced under these conditions. In addition, by performing the test-particle simulations, we found that CRs are accelerated by a relativistic shock wave thanks to the turbulence. We will discuss the conditions for efficient CR acceleration.