Browsing by Author "Hu, Kun"

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    Intensity and Polarization Characteristics of Extended Neutron Star Surface Regions
    (IOP Publishing, 2022) Hu, Kun; Baring, Matthew G.; Barchas, Joseph A.; Younes, George
    The surfaces of neutron stars are sources of strongly polarized soft X-rays due to the presence of strong magnetic fields. Radiative transfer mediated by electron scattering and free–free absorption is central to defining local surface anisotropy and polarization signatures. Scattering transport is strongly influenced by the complicated interplay between linear and circular polarizations. This complexity has been captured in a sophisticated magnetic Thomson scattering simulation we recently developed to model the outer layers of fully ionized atmospheres in such compact objects, heretofore focusing on case studies of localized surface regions. Yet, the interpretation of observed intensity pulse profiles and their efficacy in constraining key neutron star geometry parameters is critically dependent upon adding up emission from extended surface regions. In this paper, intensity, anisotropy, and polarization characteristics from such extended atmospheres, spanning considerable ranges of magnetic colatitudes, are determined using our transport simulation. These constitute a convolution of varied properties of Stokes parameter information at disparate surface locales with different magnetic field strengths and directions relative to the local zenith. Our analysis includes full general relativistic propagation of light from the surface to an observer at infinity. The array of pulse profiles for intensity and polarization presented highlights how powerful probes of stellar geometry are possible. Significant phase-resolved polarization degrees in the range of 10%–60% are realized when summing over a variety of surface field directions. These results provide an important background for observations to be acquired by NASA’s new Imaging X-ray Polarimetry Explorer X-ray polarimetry mission.
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    Polarized Radiative Transfer in the Magnetospheres and Atmospheres of Neutron Stars
    (2022-11-29) Hu, Kun; Baring, Matthew G.
    Neutron stars are sources of strongly polarized emission in X-rays or soft gamma-rays due to the presence of strong magnetic fields. Radiation transport of soft X-rays in neutron star surface layers is critical to the determination of the emergent anisotropy of light intensity and polarization signatures. Additionally high-energy photons propagating in neutron star magnetospheres can be attenuated by QED processes like photon splitting and magnetic pair creation. In this thesis, I explore the scattering transport in the classical magnetic Thomson domain using Monte Carlo technique. Representative results for emergent polarization signals from surface layers are presented for both localized and extended surface regions with magnetic field strengths that are of broad applicability to different neutron star classes. These results provide an important background for observations acquired by polarimetry missions like IXPE. I also explore polarization-dependent opacities for the two QED processes in static dipolar or twisted magnetospheres of highly magnetized neutron stars like magnetars, calculating attenuation lengths and determining escape energies, which are the maximum photon energies for transparency out to infinity. These opacity calculations put constraints on the possible emission locales and the strengths of the magnetospheric twists, and apply not only to magnetar flares but also to their quiescent hard X-ray tail emission. An exploration of photon splitting attenuation in the context of a resonant inverse Compton scattering model for the hard X-ray tails derives distinctive phase-resolved spectroscopic and polarimetric signatures, of significant interest for future MeV-band missions such as AMEGO and e-ASTROGAM.