BGU Physics Department

Colloquium, Dec. 1st, 2011

---

Magnetars: the Strongest Magnets in Nature

Jonathan Granot, University  of Hertfordshire

Magnetars are highly magnetized neutron stars, with surface dipole magnetic fields that can reach 10^{15} Gauss - over a million-billion
times stronger than that of the Earth within a region the size of a big city. They are detected mainly in our own Galaxy, through their
bursting activity or quiescent x-ray emission that is modulated at the rotational period of the neutron star. The two main classes of
magnetars are anomalous x-ray pulsars (AXPs) and soft-gamma repeaters (SGRs), which occasionally produce short (sub-second) bursts of soft
gamma-rays (and hard x-rays) and very rarely also emit spectacular giant flares that last for a few minutes. Their quiescent emission and
bursting activity are thought to be powered by the decay of their huge magnetic fields, but the details as well as the dominant field decay
mechanism are uncertain. I will review some of the recent progress in our understanding of magnetars in light of the flurry of interesting
new observations that have become available in the last several years. The rare and unique 27 December 2004 giant flare from SGR 1806-20 was
the most extreme so far, and provided us with a wealth of data that has led to exciting new insight on how magnetars operate.  New
space-born missions (most noticeably Fermi/GBM) have led to the discovery of new magnetar candidates, and to great improvement in the
quality of our data on their bursting activity, leading to interesting new results. In particular, the observed sample of magnetars is by now
large enough that a global modeling of the population can teach us a lot about their properties. We find strong direct observational
evidence for the decay of their dipole magnetic field, and that the dipole field decay alone is not enough to power their quiescent x-ray
emission. This suggests an even larger energy reservoir - most likely an even stronger magnetic field in the interior of the neutron star,
which must have an initial strength of at least 10^{16} Gauss, over an order of magnitude higher than the initial surface dipole field.  We
derive constraints on the required dominant field decay mechanism for both the dipole filed and the internal field. Tentative evolutionary
sequences are outlined, in which objects evolve between different phenomenological classes as they age and their magnetic field decays.