Magnetar

About: Magnetar is a research topic. Over the lifetime, 2905 publications have been published within this topic receiving 106806 citations.

On the radiation mechanism of repeating fast radio bursts

Abstract: Recent observations show that fast radio bursts (FRBs) are energetic but probably non-catastrophic events occurring at cosmological distances. The properties of their progenitors are largely unknown in spite of many attempts to determine them using the event rate, duration and energetics. Understanding the radiation mechanism for FRBs should provide the missing insights regarding their progenitors, which is investigated in this paper. The high brightness temperatures (>10^{35} K) of FRBs mean that the emission process must be coherent. Two general classes of coherent radiation mechanisms are considered --- maser and the antenna mechanism. We use the observed properties of the repeater FRB 121102 to constrain the plasma conditions needed for these two mechanisms. We have looked into a wide variety of maser mechanisms operating in either vacuum or plasma and find that none of them can explain the high luminosity of FRBs without invoking unrealistic or fine-tuned plasma conditions. The most favorable mechanism is antenna curvature emission by coherent charge bunches where the burst is powered by magnetic reconnection near the surface of a magnetar (B > 10^{14} G). We show that the plasma in the twisted magnetosphere of a magnetar may be clumpy due to two-stream instability. When magnetic reconnection occurs, the pre-existing density clumps may provide charge bunches for the antenna mechanism to operate. This model should be applicable to all FRBs that have multiple outbursts like FRB 121102.

An upper limit to the energy of gamma-ray bursts indicates that GRBs/SNe are powered by magnetars

Abstract: The kinetic energy of supernovae (SNe) accompanied by gamma-ray bursts (GRBs) tends to cluster near 1052 erg, with 2 × 1052 erg an upper limit to which no compelling exceptions are found (assuming a certain degree of asphericity), and it is always significantly larger than the intrinsic energy of the GRB themselves (corrected for jet collimation). This energy is strikingly similar to the maximum rotational energy of a neutron star rotating with period 1 ms. It is therefore proposed that all GRBs associated with luminous SNe are produced by magnetars. GRBs that result from black hole formation (collapsars) may not produce luminous SNe. X-ray flashes, which are associated with less energetic SNe, are produced by neutron stars with weaker magnetic field or lower spin.

Thermal structure and cooling of neutron stars with magnetized envelopes

Abstract: The thermal structure of neutron stars with magnetized envelopes is studied using modern physics input. The relation between the internal (Tint) and local surface temperatures is calculated and tted by analytic expressions for magnetic eld strengths B from 0 to 10 16 G and arbitrary inclination of the eld lines to the surface. The luminosity of a neutron star with dipole magnetic eld is calculated and tted as a function of B, Tint, stellar mass and radius. In addition, we simulate cooling of neutron stars with magnetized envelopes. In particular, we analyse ultramagnetized envelopes of magnetars and also the eects of the magnetic eld of the Vela pulsar on the determination of critical temperatures of neutron and proton superfluids in its core.

Atmospheres and spectra of strongly magnetized neutron stars

Abstract: We construct atmosphere models for strongly magnetized neutron stars with surface fields B � 10 12 10 15 G and effective temperatures Teff � 10 6 10 7 K. The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars, including radio pulsars, soft gamma-ray repeaters, and anomalous Xray pulsars. In our models, the atmosphere is composed of pure hydrogen or helium and is assumed to be fully ionized. The radiative opacities include free-free absorption and scattering by both electrons and ions computed for the two photon polarization modes in the magnetized electron-ion plasma. Since the radiation emerges from deep layers in the atmosphere with � > 10 2 g/cm 3 , plasma effects can significantly modify the photon opacities by changing the properties of the polarization modes. In the case where the magnetic field and the surface normal are parallel, we solve the full, angle-dependent, coupled radiative transfer equations for both polarization modes. We also construct atmosphere models for general field orientations based on the diffusion approximation of the transport equations and compare the results with models based on full radiative transport. In general, the emergent thermal radiation exhibits significant deviation from blackbody, with harder spectra at high energies. The spectra also show a broad feature (�E/EBi � 1) around the ion cyclotron resonance EBi = 0.63(Z/A)(B/10 14 G) keV, where Z and A are the atomic charge and atomic mass of the ion, respectively; this feature is particularly pronounced when EBi > 3kTeff. Detection of the resonance feature would provide a direct measurement of the surface magnetic fields on magnetars.

On the theory of magnetar QPOs

Abstract: 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 magnetohydrodynamic (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. First, 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 ∼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.