Constraining the Origin of Magnetar Flares

Sudden relaxation of the magnetic field in the core of a magnetar produces mechanical energy primarily in the form of shear waves which propagate to the surface and enter the magnetosphere as relativistic Alfv\'en waves. Due to a strong impedance mismatch, shear waves excited in the star suffer many reflections before exiting the star. If mechanical energy is deposited in the core and is converted {\em directly} to radiation upon propagation to the surface, the rise time of the emission is at least seconds to minutes, and probably minutes to hours for a realistic magnetic field geometry, at odds with observed rise times of \lap 10 ms for both and giant flares. Mechanisms for both small and giant flares that rely on the sudden relaxation of the magnetic field of the core are rendered unviable by the impedance mismatch, requiring the energy that drives these events to be stored in the magnetosphere just before the flare. ends, unless the waves are quickly damped.Comment: Final version in Monthly Notices of the Royal Astronomical Society. 13 pages, 5 figure

Torsional oscillations of a magnetar with a tangled magnetic field

We propose a scenario for the quasi-periodic oscillations observed in magnetar flares wherein a tangled component of the stellar magnetic field introduces nearly isotropic stress that gives the fluid core of the star an effective shear modulus. In a simple, illustrative model of constant density, the tangled field eliminates the problematic Alfv\'en continuum that would exist in the stellar core for an organized field. For a tangled field energy density comparable to that inferred from the measured dipole fields of $\sim 10^{15}$ G in SGRs 1806-20 and 1900+14, torsional modes exist with fundamental frequencies of about 20 Hz, and mode spacings of $\sim 10$ Hz. For fixed stellar mass and radius, the model has only one free parameter, and can account for {\em every} observed QPO under 160 Hz to within 3 Hz for both SGRs 1806-20 and 1900+14. The combined effects of stratification and crust stresses generally decrease the frequencies of torsional oscillations by $<10$% for overtones and increase the lowest-frequency fundamentals by up to 50%, and so the star can be treated as having constant density to a generally good first approximation. We address the issue of mode excitation by sudden readjustment of the stellar magnetosphere. While the total energy in excited modes is well within the energy budget of giant flares, the surface amplitude is $< 10^{-3}$ of the stellar radius for global oscillations, and decreases strongly with mode frequency. The 626 Hz QPO reported for SGR 1806-20 is particularly problematic to excite beyond a surface amplitude of $10^{-6}$ of the stellar radius.Comment: 14 pages, 3 figure

Flux predictions of high-energy neutrinos from pulsars

Young, rapidly rotating neutron stars could accelerate ions from their surface to energies of $\sim 1$ PeV. If protons reach such energies, they will produce pions (with low probability) through resonant scattering with x-rays from the stellar surface. The pions subsequently decay to produce muon neutrinos. Here we calculate the energy spectrum of muon neutrinos, and estimate the event rates at Earth. The spectrum consists of a sharp rise at $\sim 50$ TeV, corresponding to the onset of the resonance, above which the flux drops with neutrino energy as $\epsilon_\nu^{-2}$ up to an upper-energy cut-off that is determined by either kinematics or by the maximum energy to which protons are accelerated. We estimate event rates as high as 10-100 km^${-2}$ yr$^{-1}$ from some candidates, a flux that would be easily detected by IceCube. Lack of detection would allow constraints on the energetics of the poorly-understood pulsar magnetosphere.Comment: MNRAS, 6 pages, 3 figures, 1 table. Minor editorial changes and typos correcte

Gamma Ray Bursts with peculiar temporal asymmetry

Based on the study of temporal asymmetry of 631 gamma ray bursts from the BATSE 3B catalog by Link and Epstein [Ap J 466, 764 (1996)], we identify the population of bursts whose rising times are longer than their decays, thus showing atypical profiles. We analyse their sky distribution, morphology, time-space clustering and other average properties and compare them with those associated with the bulk of the bursts. We show how most of the peculiar bursts analysed are consistent with recent fireball models, but a fraction of bursts ($\sim 4$% of the total sample) appear to be inconsistent.Comment: mn style (included in the submission), 4 figures that must be printed separately. Submitted to Monthly Notices of RA

Are We Seeing Magnetic Axis Reorientation in the Crab and Vela Pulsars?

Variation in the angle $\alpha$ between a pulsar's rotational and magnetic axes would change the torque and spin-down rate. We show that sudden increases in $\alpha$, coincident with glitches, could be responsible for the persistent increases in spin-down rate that follow glitches in the Crab pulsar. Moreover, changes in $\alpha$ at a rate similar to that inferred for the Crab pulsar account naturally for the very low braking index of the Vela pulsar. If $\alpha$ increases with time, all pulsar ages obtained from the conventional braking model are underestimates. Decoupling of the neutron star liquid interior from the external torque cannot account for Vela's low braking index. Variations in the Crab's pulse profile due to changes in $\alpha$ might be measurable.Comment: 14 pages and one figure, Latex, uses aasms4.sty. Accepted to ApJ Letter

Evidence for Heating of Neutron Stars by Magnetic Field Decay

We show the existence of a strong trend between neutron star surface temperature and the dipolar component of the magnetic field extending through three orders of field magnitude, a range that includes magnetars, radio-quiet isolated neutron stars, and many ordinary radio pulsars. We suggest that this trend can be explained by the decay of currents in the crust over a time scale of few Myr. We estimate the minimum temperature that a NS with a given magnetic field can reach in this interpretation.Comment: 4 pages, 1 figures, version accepted for publication in Phys. Rev. Let

The Crustal Rigidity of a Neutron Star, and Implications for PSR 1828-11 and other Precession Candidates

We calculate the crustal rigidity parameter, b, of a neutron star (NS), and show that b is a factor 40 smaller than the standard estimate due to Baym & Pines (1971). For a NS with a relaxed crust, the NS's free-precession frequency is directly proportional to b. We apply our result for b to PSR 1828-11, a 2.5 Hz pulsar that appears to be precessing with period 511 d. Assuming this 511-d period is set by crustal rigidity, we show that this NS's crust is not relaxed, and that its reference spin (roughly, the spin for which the crust is most relaxed) is 40 Hz, and that the average spindown strain in the crust is 5 \times 10^{-5}. We also briefly describe the implications of our b calculation for other well-known precession candidates.Comment: 44 pages, 10 figures, submitted to Ap