Magnetar

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

Particle-in-cell simulations of the twisted magnetospheres of magnetars. I

Abstract: The magnetospheres of magnetars are believed to be filled with electron–positron plasma generated by electric discharge. We present a first numerical experiment demonstrating this process in an axisymmetric magnetosphere with a simple threshold prescription for pair creation, which is applicable to the inner magnetosphere with an ultrastrong field. The ${e}^{\pm }$ discharge occurs in response to the twisting of the closed magnetic field lines by a shear deformation of the magnetar surface, which launches electric currents into the magnetosphere. The simulation shows the formation of an electric “gap” with an unscreened electric field (${\boldsymbol{E}}\cdot {\boldsymbol{B}} e 0$) that continually accelerates particles along the magnetic field lines and sustains pair creation. The accelerating voltage is self-regulated to the threshold of the ${e}^{\pm }$ discharge. It controls the rate of energy release and the lifetime of the magnetic twist. The simulation follows the global evolution of the twisted magnetosphere over a long time and demonstrates its gradual resistive untwisting. A vacuum cavity forms near the star and expands, gradually erasing magnetospheric electric currents j. The active j-bundle shrinks with time and its footprints form shrinking hot spots on the magnetar surface bombarded by the created particles.

Phase-resolved NuSTAR and Swift-XRT Observations of Magnetar 4U 0142+61

Abstract: We present temporal and spectral analysis of simultaneous 0.5-79 keV Swift-XRT and NuSTAR observations of the magnetar 4U 0142+61. The pulse profile changes significantly with photon energy between 3 and 35 keV. The pulse fraction increases with energy, reaching a value of ~20%, similar to that observed in 1E 1841-045 and much lower than the ~80% pulse fraction observed in 1E 2259+586. We do not detect the 55-ks phase modulation reported in previous Suzaku-HXD observations. The phase-averaged spectrum of 4U 0142+61 above 20 keV is dominated by a hard power law with a photon index, Γ ~ 0.65, and the spectrum below 20 keV can be described by two blackbodies, a blackbody plus a soft power law, or by a Comptonized blackbody model. We study the full phase-resolved spectra using the electron-positron outflow model of Beloborodov (2013). Our results are consistent with the parameters of the active j-bundle derived from INTEGRAL data by Hascoet et al. (2014). We find that a significant degeneracy appears in the inferred parameters if the footprint of the j-bundle is allowed to be a thin ring instead of a polar cap. The degeneracy is reduced when the footprint is required to be the hot spot inferred from the soft X-ray data.

Thermoplastic waves in magnetars

Abstract: Magnetar activity is generated by shear motions of the neutron star surface, which relieve internal magnetic stresses. An analogy with earthquakes and faults is problematic, as the crust is permeated by strong magnetic fields, which greatly constrain crustal displacements. We describe a new deformation mechanism that is specific to strongly magnetized neutron stars. The magnetically stressed crust begins to move because of a thermoplastic instability, which launches a wave that shears the crust and burns its magnetic energy. The propagating wave front resembles the deflagration front in combustion physics. We describe the conditions for the instability, the front structure and velocity, and discuss implications for observed magnetar activity.

New Limits on Axionic Dark Matter from the Magnetar PSR J1745-2900

Abstract: Axions are a promising dark matter candidate that were motivated to solve the strong CP problem and that may also address the cosmological matter-antimatter asymmetry. Axion-photon conversion is possible in the presence of the strong magnetic fields, and the photon so produced will have energy equal to the axion mass. Here we report new limits on axionic dark matter obtained from radio spectra of the Galactic Center magnetar PSR J1745-2900. The magnetar has a magnetic field of $1.6\times10^{14}$ G that interacts with a dark matter density $2\times10^5$ to $2\times10^9$ times greater than the local dark matter encountered by terrestrial haloscopes, depending on the Galactic dark matter profile. No significant spectral features are detected across 62% of the axion mass range 4.1-165.6 $\mu$eV (1-40 GHz). The interpretation of flux limits into limits on the two-photon coupling strength $g_{a\gamma\gamma}$ depends on the magnetospheric conversion model and on the dark matter density at the Galactic Center. For a standard dark matter profile, we exclude axion models with $g_{a\gamma\gamma}> $ 6-34 $\times 10^{-12}$ GeV$^{-1}$ with 95% confidence over the mass ranges 4.2-8.4, 8.9-10.0, 12.3-16.4, 18.6-26.9, 33.0-62.1, 70.1-74.3, 78.1-80.7, 105.5-109.6, 111.6-115.2, 126.0-159.3, and 162.5-165.6 $\mu$eV. For the maximal dark matter cusp allowed by stellar orbits near Sgr A*, these limits reduce to $g_{a\gamma\gamma} > $ 6-34 $ \times10^{-14}$ GeV$^{-1}$, which exclude some theoretical models for masses $> 33$ $\mu$eV. Limits may be improved by modeling stimulated axion conversion, by ray-tracing conversion pathways in the magnetar magnetosphere, and by obtaining deeper broad-band observations of the magnetar.

Resonant Inverse Compton Scattering Spectra from Highly Magnetized Neutron Stars

Abstract: Hard, non-thermal, persistent pulsed X-ray emission extending between 10 keV and $\sim 150$ keV has been observed in nearly ten magnetars. For inner-magnetospheric models of such emission, resonant inverse Compton scattering of soft thermal photons by ultra-relativistic charges is the most efficient production mechanism. We present angle-dependent upscattering spectra and pulsed intensity maps for uncooled, relativistic electrons injected in inner regions of magnetar magnetospheres, calculated using collisional integrals over field loops. Our computations employ a new formulation of the QED Compton scattering cross section in strong magnetic fields that is physically correct for treating important spin-dependent effects in the cyclotron resonance, thereby producing correct photon spectra. The spectral cut-off energies are sensitive to the choices of observer viewing geometry, electron Lorentz factor, and scattering kinematics. We find that electrons with energies $\lesssim 15$ MeV will emit most of their radiation below 250 keV, consistent with inferred turnovers for magnetar hard X-ray tails. More energetic electrons still emit mostly below 1 MeV, except for viewing perspectives sampling field line tangents. Pulse profiles may be singly- or doubly-peaked dependent upon viewing geometry, emission locale, and observed energy band. Magnetic pair production and photon splitting will attenuate spectra to hard X-ray energies, suppressing signals in the Fermi-LAT band. The resonant Compton spectra are strongly polarized, suggesting that hard X-ray polarimetry instruments such as X-Calibur, or a future Compton telescope, can prove central to constraining model geometry and physics.