Princeton University Astroplasmas Seminar - TITLE UPDATED

ABSTRACT: Important questions have been raised by the recent discovery of the ``weak field, old magnetar", the soft gamma repeater SGR 0418+5729, with a surface dipole magnetic field of $B_{dip} < 7.5 \times 10^{12}$ G: How can the neutron star produce SGR-like bursts with such a low magnetic field? What powers the observed X-ray emission as both the rotational energy and the magnetic dipole energy are insufficient? These observations suggest either a much larger internal magnetic field or a much younger true age (or both). We examined a phenomenological model for the magnetic field decay: $\dot{B}_{dip} \propto B_{dip}^{1+\alpha}$ and compare its predictions with the observed period, $P$, period derivative, $\dot{P}$, and X-ray luminosity, $L_X$, of magnetar candidates. We found strong evidence for a dipole field decay on a timescale of $\sim 10^3$ years 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 $1.5\lesssim\alpha\lesssim 1.8$. This field decay implies a younger true age than what is implied by $P/2 \dot{P}$. Surprisingly, even with this younger age the energy released in the decay of the dipole field is insufficient to power $L_X$, suggesting the existence of a stronger internal field, $B_{\rm int}$. We examining several models for the internal magnetic field decay and found 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.