[1] Tsinghua Univ, Dept Phys, Beijing 100084, Peoples R China
[2] Tsinghua Univ, Ctr Astrophys, Beijing 100084, Peoples R China
[3] Univ Alabama, Dept Phys, Huntsville, AL 35899 USA
[4] NASA, George C Marshall Space Flight Ctr, Space Sci Lab, Huntsville, AL 35812 USA
[5] Chinese Acad Sci, Inst High Energy Phys, Key Lab Particle Astrophys, Beijing 100039, Peoples R China