  Soft gamma-ray repeaters and anomalous X-ray pulsars are a small (but
growing) group of X-ray sources characterized by the emission of short bursts
and by a large variability in their persistent flux. They are believed to be
magnetars, i.e. neutron stars powered by extreme magnetic fields 1E14-1E15 G).
We found evidence for a magnetar with a low magnetic field, SGR 0418+5729,
recently detected after it emitted bursts similar to those of soft gamma-ray
repeaters. New X-ray observations show that its dipolar magnetic field cannot
be greater than 8E12 G, well in the range of ordinary radio pulsars, implying
that a high surface dipolar magnetic field is not necessarily required for
magnetar-like activity. The magnetar population may thus include objects with a
wider range of magnetic-field strengths, ages and evolutionary stages than
observed so far.
