Magnetar Crusts Research Articles

Comparative Study of the Initial Spikes of Soft Gamma-Ray Repeater Giant Flares in 1998 and 2004 Observed withGeotail: Do Magnetospheric Instabilities Trigger Large-Scale Fracturing of a Magnetar's Crust?

We present the unsaturated peak profile of the giant flare from SGR 1900+14 on 1998 August 27. This was obtained by the particle counters of the Low Energy Particles instrument on board the Geotail spacecraft. The observed peak profile revealed four characteristic features: an initial steep rise, an intermediate rise to the peak, an exponential decay, and a small hump in the decay phase. From this light curve, we found that the isotropic peak luminosity was 2.3 × 1046 ergs s-1 and that the total energy was 4.3 × 1044 ergs s-1 (E 50 keV), assuming that the distance to SGR 1900+14 is 15 kpc and that the spectrum is optically thin thermal bremsstrahlung with kT = 240 keV. These values are consistent with the previously reported lower limits derived from Ulysses and Konus-Wind observations. A comparative study of the initial spikes of the SGR 1900+14 giant flare in 1998 and of the SGR 1806-20 giant flare in 2004 is also presented. The timescale of the initial steep rise shows a magnetospheric origin, while the timescale of the intermediate rise to the peak indicates that it originates from crustal fracturing. Finally, we argue that the four features and their corresponding timescales provide us with a clue to identify extragalactic soft gamma-ray repeater giant flares among short gamma-ray bursts.

On the theory of magnetar QPOs

We consider torsional oscillations of magnetars. This problem features rich dynamics due to the strong interaction between the normal modes of a magnetar's crust and a continuum of Magneto-Hydro-Dynamic (MHD) modes in its fluid core. We study the dynamics using a simple model of a magnetar possessing a uniform magnetic field and a thin spherical crust. Firstly, we show that global torsional modes only exist when one introduces unphysically large dissipative terms into the equations of motion; thus global modes are not helpful for understanding the magnetar Quasi-Periodic Oscillations (QPOs). Secondly, we solve the initial-value problem by simulating the sudden release of an initially strained crust and monitoring the subsequent crustal motion. We find that the crustal torsional modes quickly exchange their energy with the MHD continuum in the core, and decay by several orders of magnitude over the course of about 10 oscillation periods. After the initial rapid decay, the crustal motion is stabilized and several time-varying QPOs are observed. The dynamical spectrum of the simulated crustal motion is in qualitative agreement with that of the x-ray light-curve in the tail of a giant magnetar flare. The asymptotic frequencies of some of the QPOs are associated with the special spectral points--the turning points or edges--of the MHD continuum, and are not related to those of the crust. The observed steady low-frequency QPO at 18 Hz is almost certainly associated with the lowest frequency of the MHD continuum, or its first overtone. We also find that drifting QPOs get amplified when they come near the frequencies of the crustal modes. This explains why some of the observed QPOs have frequencies close to the expected crustal frequencies, and why these QPOs are highly variable with time.

Magnetars as cooling neutron stars with internal heating

We study thermal structure and evolution of magnetars as cooling neutron stars with a phenomenological heat source in a spherical internal layer. We explore the location of this layer as well as the heating rate that could explain high observable thermal luminosities of magnetars and would be consistent with the energy budget of neutron stars. We conclude that the heat source should be located in an outer magnetar's crust, at densities rho < 5e11 g/cm^3, and should have the heat intensity of the order of 1e20 erg/s/cm^3. Otherwise the heat energy is mainly emitted by neutrinos and cannot warm up the surface.