Magnetar

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

Resonant Cyclotron Scattering in Magnetars’ Emission

Abstract: We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X-ray data of 10 magnetars, including canonical and transient anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs). In this scenario, nonthermal magnetar spectra in the soft X-rays (i.e., below ~10 keV) result from resonant cyclotron scattering of the thermal surface emission by hot magnetospheric plasma. We find that this model can successfully account for the soft X-ray emission of magnetars, while using the same number of free parameters as in the commonly used empirical blackbody plus power-law model. However, while the RCS model can alone reproduce the soft X-ray spectra of AXPs, the much harder spectra of SGRs below 10 keV require the addition of a power-law component (the latter being the same component responsible for their hard X-ray emission). Although this model in its present form does not explain the hard X-ray emission (i.e., above ~20 keV) of a few of these sources, we took this further component into account in our modeling not to overlook its contribution in the ~4-10 keV band. We find that the entire class of sources is characterized by magnetospheric plasma with a density which, at resonant radius, is about 3 orders of magnitude higher than the Goldreich-Julian electron density. The inferred values of the intervening hydrogen column densities are also in better agreement with more recent estimates. Although the treatment of the magnetospheric scattering used here is only approximated, its successful application to all magnetars shows that the RCS model is capable of catching the main features of the spectra observed below ~10 keV.

Period Clustering of the Anomalous X-Ray Pulsars and Magnetic Field Decay in Magnetars

Abstract: We confront theoretical models for the rotational, magnetic, and thermal evolution of an ultramagnetized neutron star, or magnetar, with available data on the anomalous X-ray pulsars (AXPs). We argue that, if the AXPs are interpreted as magnetars, their clustering of spin periods between 6 and 12 s (observed at present in this class of objects), their period derivatives, their thermal X-ray luminosities, and the association of two of them with young supernova remnants can only be understood globally if the magnetic field in magnetars decays significantly on a timescale of the order of 104 yr.

A flaring magnetar in FRB 121102

Abstract: The persistent radio counterpart of FRB 121102 is estimated to have $N\sim 10^{52}$ particles, energy $E_N\sim 10^{48}$ erg, and size $R\sim 10^{17}$ cm. The source can be a nebula inflated and heated by an intermittent outflow from a magnetar --- a neutron star powered by its magnetic (rather than rotational) energy. The object is young and frequently liberating energy in magnetic flares driven by accelerated ambipolar diffusion in the neutron star core, feeding the nebula and producing bright millisecond bursts. The particle number in the nebula is consistent with ion ejecta from giant flares. The nebula may also contain the freeze-out of electron-positron pairs $N_\pm\sim 10^{51}$ created months after the neutron star birth; the same mechanism offers an explanation for $N_\pm$ in the Crab nebula. The persistent source around FRB 121102 is likely heated by magnetic dissipation and internal waves excited by the magnetar ejecta. The volumetric heating by waves explains the nebula's enormous efficiency in producing radio emission. The repeating radio bursts are suggested to occur much closer to the magnetar, whose flaring magnetosphere drives ultrarelativistic internal shocks into the magnetar wind. The shocks are mediated by Larmor rotation that forms a GHz maser with the observed ms duration. Furthermore, the flare ejecta can become charge-starved and then convert to electromagnetic waves.

Magnetorotational Effects on Anisotropic Neutrino Emission and Convection in Core-Collapse Supernovae

Abstract: We perform a series of two-dimensional hydrodynamic simulations of the magnetorotational collapse of a supernova core. We employ a realistic equation of state and take into account electron capture and neutrino transport by the so-called leakage scheme. Recent stellar evolution calculations imply that the magnetic fields of the toroidal components are much stronger than the poloidal ones at the presupernova stage. In this study we systematically investigate the effects of the toroidal magnetic fields on the anisotropic neutrino radiation and convection. Our results show that the shapes of the shock wave and the neutrino spheres generally become more oblate for the models whose profiles of rotation and the magnetic field are shell type and become, in contrast, more prolate for the models whose profiles of rotation and the magnetic field are cylindrical than for the corresponding models without the magnetic fields. Furthermore, we find that magnetorotational instability induced by nonaxisymmetric perturbations is expected to develop within the prompt-shock timescale. Combined with the anisotropic neutrino radiation, which heats matter near the rotational axis preferentially, the growth of the instability may enhance the heating near the axis. This might suggest that magnetar formation is accompanied by a jetlike explosion.