[1] Univ Fed Rio Grande do Sul, Inst Fis, Caixa Postal 15051, BR-91501970 Porto Alegre, RS, Brazil
[2] Univ Fed Ciencias Saude Porto Alegre, Dept Ciencias Exatas & Sociais Aplicada, BR-90050170 Porto Alegre, RS, Brazil
[3] Max Planck Inst Phys Komplexer Syst, Nothnitzer Str 38, D-01187 Dresden, Germany
[4] Univ Florence, Dipartimento Fis Astron, Via G Sansone 1, I-50019 Sesto Fiorentino, Italy
[5] Univ Florence, CSDC, Via G Sansone 1, I-50019 Sesto Fiorentino, Italy
[6] INFN, Sez Firenze, Via G Sansone 1, I-50019 Sesto Fiorentino, Italy
[7] INAF, Osservatorio Astrofis Arcetri, Largo E Fermi 5, I-50125 Florence, Italy