The recent discovery of the “ weak field , old magnetar ” , the soft gamma repeater SGR 0418+5729 , whose dipole magnetic field , B _ { dip } , is less than 7.5 \times 10 ^ { 12 } G , has raised perplexing questions : How can the neutron star produce SGR-like bursts with such a low magnetic field ?
What powers the observed X-ray emission when neither the rotational energy nor the magnetic dipole energy are sufficient ?
These observations , that suggest either a much larger energy reservoir or a much younger true age ( or both ) , have renewed the interest in the evolutionary sequence of magnetars .
We examine , here , a phenomenological model for the magnetic field decay : \dot { B } _ { dip } \propto B _ { dip } ^ { 1 + \alpha } and compare its predictions with the observed period , P , the period derivative , \dot { P } , and the X-ray luminosity , L _ { X } , of magnetar candidates .
We find a strong evidence for a dipole field decay on a timescale of \sim 10 ^ { 3 } yr for the strongest ( B _ { dip } \sim 10 ^ { 15 } G ) field objects , with a decay index within the range 1 \leq \alpha < 2 and more likely within 1.5 \lesssim \alpha \lesssim 1.8 .
The decaying field implies a younger age than what is implied by P / 2 \dot { P } .
Surprisingly , even with the younger age , the energy released in the dipole field decay is insufficient to power the X-ray emission , suggesting the existence of a stronger internal field , B _ { int } .
Examining several models for the internal magnetic field decay we find that it must have a very large ( \gtrsim 10 ^ { 16 } G ) initial value .
Our findings suggest two clear distinct evolutionary tracks – the SGR/AXP branch and the transient branch , with a possible third branch involving high-field radio pulsars that age into low luminosity X-ray dim isolated neutron stars .