[1] UNAM, Inst Radioastron & Astrofis, Campus Morelia,AP 3-72, Morelia 58089, Michoacan, Mexico
[2] Univ Veracruzana, Fac Fis, Xalapa 91000, Veracruz, Mexico
[3] Univ Valencia, Dr Moliner 50, E-46100 Valencia, Spain
[4] Inst Theoret Phys, Max von Laue Str 1, D-60438 Frankfurt, Germany
[5] Univ Ind Santander, Grp Invest Relatividad & Gravitac, Escuela Fis, AA 678, Bucaramanga 680002, Colombia
[6] Univ Michoacana, Inst Fis & Matemat, Edificio C3, Morelia 58040, Michoacan, Mexico