Photon Splitting in Magnetar Models of Soft Gamma Repeaters

The recent association of soft gamma repeaters (SGRs) with counterparts in other wavebands has sparked much interest in these sources. One of the recent models for these objects is that they originate in the environs of neutron stars with fields much stronger than the quantum critical field \teq{B_{cr}=4.413\times 10^{13}} Gauss. Near such neutron stars, dubbed magnetars, the exotic quantum process of magnetic photon splitting becomes prolific. Its principal effect is to degrade photon energies and thereby soften gamma-ray spectra from neutron stars; it has recently been suggested that splitting may be responsible for limiting the hardness of emission in SGRs, if these sources originate in neutron stars with supercritical surface fields. Seed photons in supercritical fields efficiently generate soft gamma-ray spectra, typical of repeaters. In this paper, the influence of the curved dipole field geometry of a neutron star magnetosphere on the photon splitting rate is investigated. The dependence of the attenuation length on the location and angular direction of the seed photons is explored.Comment: 5 pages including 3 encapsulated figures, as a compressed, uuencoded, Postscript file. To appear in Proc. of the 1995 La Jolla workshop ``High Velocity Neutron Stars and Gamma-Ray Bursts'' eds. Rothschild, R. et al., AIP, New Yor

High Energy Neutrinos and Photons from Curvature Pions in Magnetars

We discuss the relevance of the curvature radiation of pions in strongly magnetized pulsars or magnetars, and their implications for the production of TeV energy neutrinos detectable by cubic kilometer scale detectors, as well as high energy photons.Comment: 19 pages, 4 figures, to appear in JCA

Hard X-ray Quiescent Emission in Magnetars via Resonant Compton Upscattering

Non-thermal quiescent X-ray emission extending between 10 keV and around 150 keV has been seen in about 10 magnetars by RXTE, INTEGRAL, Suzaku, NuSTAR and Fermi-GBM. For inner magnetospheric models of such hard X-ray signals, inverse Compton scattering is anticipated to be the most efficient process for generating the continuum radiation, because the scattering cross section is resonant at the cyclotron frequency. We present hard X-ray upscattering spectra for uncooled monoenergetic relativistic electrons injected in inner regions of pulsar magnetospheres. These model spectra are integrated over bundles of closed field lines and obtained for different observing perspectives. The spectral turnover energies are critically dependent on the observer viewing angles and electron Lorentz factor. We find that electrons with energies less than around 15 MeV will emit most of their radiation below 250 keV, consistent with the turnovers inferred in magnetar hard X-ray tails. Electrons of higher energy still emit most of the radiation below around 1 MeV, except for quasi-equatorial emission locales for select pulse phases. Our spectral computations use a new state-of-the-art, spin-dependent formalism for the QED Compton scattering cross section in strong magnetic fields.Comment: 5 pages, 2 figures, to appear in Proc. "Physics of Neutron Stars - 2017," Journal of Physics: Conference Series, eds. G. G. Pavlov, et al., held in Saint Petersburg, Russia, 10-14 July, 201

Nonlinear wave interaction and spin models in the MHD regime

Here we consider the influence on the electron spin in the MHD regime. Recently developed models which include spin-velocity correlations are taken as a starting point. A theoretical argument is presented, suggesting that in the MHD regime a single fluid electron model with spin correlations is equivalent to a model with spin-up and spin-down electrons constituting different fluids, but where the spin-velocity correlations are omitted. Three wave interaction of 2 shear Alfven waves and a compressional Alfven wave is then taken as a model problem to evaluate the asserted equivalence. The theoretical argument turns out to be supported, as the predictions of the two models agree completely. Furthermore, the three wave coupling coefficients obey the Manley-Rowe relations, which give further support to the soundness of the models and the validity of the assumptions made in the derivation. Finally we point out that the proposed two-fluid model can be incorporated in standard Particle-In-Cell schemes with only minor modifications.Comment: 8 page

Photon splitting in soft gamma repeaters

The exotic quantum process of photon splitting has great potential to explain the softness of emission in soft gamma repeaters (SGRs) if they originate in neutron stars with surface fields above the quantum critical field B_{\rm cr}=4.413\times 10^{13}Gauss. Splitting becomes prolific at such field strengths: its principal effect is to degrade photon energies, initiating a cascade that softens gamma-ray spectra. Uniform field cascade calculations have demonstrated that emission could be softened to the observed SGR energies for fields exceeding about 10^{14}Gauss. Recently, we have determined splitting attenuation lengths and maximum energies for photon escape in neutron star environments including the effects of magnetospheric dipole field geometry. Such escape energies \erg_{esc} suitably approximate the peak energy of the emergent spectrum, and in this paper we present results for \erg_{esc} as a function of photon emission angles for polar cap and equatorial emission regions. The escape energy is extremely insensitive to viewing perspective for equatorial emission, arguing in favour of such a site for the origin of SGR activity

Compton Scattering in Ultra-Strong Magnetic Fields: Numerical and Analytical Behavior in the Relativistic Regime

This paper explores the effects of strong magnetic fields on the Compton scattering of relativistic electrons. Recent studies of upscattering and energy loss by relativistic electrons that have used the non-relativistic, magnetic Thomson cross section for resonant scattering or the Klein-Nishina cross section for non-resonant scattering do not account for the relativistic quantum effects of strong fields ($> 4 \times 10^{12}$ G). We have derived a simplified expression for the exact QED scattering cross section for the broadly-applicable case where relativistic electrons move along the magnetic field. To facilitate applications to astrophysical models, we have also developed compact approximate expressions for both the differential and total polarization-dependent cross sections, with the latter representing well the exact total QED cross section even at the high fields believed to be present in environments near the stellar surfaces of Soft Gamma-Ray Repeaters and Anomalous X-Ray Pulsars. We find that strong magnetic fields significantly lower the Compton scattering cross section below and at the resonance, when the incident photon energy exceeds $m_ec^2$ in the electron rest frame. The cross section is strongly dependent on the polarization of the final scattered photon. Below the cyclotron fundamental, mostly photons of perpendicular polarization are produced in scatterings, a situation that also arises above this resonance for sub-critical fields. However, an interesting discovery is that for super-critical fields, a preponderance of photons of parallel polarization results from scatterings above the cyclotron fundamental. This characteristic is both a relativistic and magnetic effect not present in the Thomson or Klein-Nishina limits.Comment: AASTeX format, 31 pages included 7 embedded figures, accepted for publication in The Astrophysical Journa

Spin induced nonlinearities in the electron MHD regime

We consider the influence of the electron spin on the nonlinear propagation of whistler waves. For this purpose a recently developed electron two-fluid model, where the spin up- and down populations are treated as different fluids, is adapted to the electron MHD regime. We then derive a nonlinear Schrodinger equation for whistler waves, and compare the coefficients of nonlinearity with and without spin effects. The relative importance of spin effects depend on the plasma density and temperature as well as the external magnetic field strength and the wave frequency. The significance of our results to various plasmas are discussed.Comment: 5 page

Full polar cap cascade scenario: γ\gamma-ray and X-ray luminosities from spin-powered pulsars

We modify polar cap cascade picture to include the ICS of the higher generation pairs. In such a ``full-cascade'' scenario, not only the perpendicular portion of the energy of the pairs goes to high energy radiation via SR, but the parallel portion of the energy of the pairs can also contribute to high energy emission via ICS with the soft thermal photons from either the full neutron star surface or the hot polar cap. An important output of such a scenario is that the soft tail of the ICS spectrum can naturally result in a non-thermal X-ray component which can contribute to the luminosities observed by ROSAT and ASCA. Here we present an analytic description of such a full polar cap cascade scenario within the framework of Harding & Muslimov acceleration model. We present the theoretical predictions of the $\gamma$-ray luminosities, the thermal and non-thermal X-ray luminosities for the known spin-powered X-ray pulsars. Our results show that the observed different dependences of the high energy luminosities on the pulsar spin-down luminosities, i.e., $L_\gamma \propto (L_{\rm sd})^{1/2}$ and $L_x \sim 10^{-3} L_{\rm sd}$, are well reproduced. Our model predicts that the {\em pulsed} soft X-rays in the ROSAT band from most of the millisecond pulsars might be of thermal origin if there is no strong multipole field components near the surfaces of these pulsars.Comment: 23 pages, emulateapj style, final version to appear in the Astrophysical Journa