Testing the magnetar model with the first low-B soft gamma repeater

Soft gamma repeaters (SGRs) are part of a rapidly increasing group of xraysources exhibiting sporadic and powerful emission of short bursts and outbursts,believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields(Bvirgul10^1415 Gauss). We have recently discovered an SGR with a low magneticfield, SGR 0418+57, during outburst and after it emitted bursts similar to thoseof magnetars. Xray observations show that its dipolar magnetic field cannot begreater than 7.5x10^12 Gauss, well in the range of ordinary radio pulsars. Weask here for a joint XMM/Chandra monitoring of the first lowB magnetar SGR0418+57 in order to have a final measure of its period derivative, hence of itsurface dipolar B field. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]