[1] Univ Calif Santa Barbara, Dept Phys, Santa Barbara, CA 93106 USA
[2] Univ Calif, White Mt Res Stn, CA 93514 USA
[3] Univ Calif Santa Barbara, Ctr High Altitude Astrophys, Santa Barbara, CA 93106 USA
[4] Univ Milan, I-20133 Milan, Italy
[5] Univ Fed Itajuba, Dept Quim & Fis, BR-37500903 Itajuba, MG, Brazil
[6] CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA
[7] Univ Illinois, Dept Astron, Urbana, IL 61801 USA
[8] Univ Illinois, Dept Phys, Urbana, IL 61801 USA
[9] CNR, IASF, Sez Bologna, I-40129 Bologna, Italy
[10] Inst Nacl Pesquisas Espaciais, Div Astrofis, BR-12210070 Sao Jose Dos Campos, SP, Brazil
[11] Univ Roma Tor Vergata, Dipartimento Fis, I-00173 Rome, Italy
[12] Univ Roma Tor Vergata, Sez INFN, I-00173 Rome, Italy
[13] CALTECH, Dept Appl Phys, Pasadena, CA 91125 USA