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1. Polarized radiation transfer in neutron star surface layers

   https://doi.org/10.1093/mnras/staa3541  

   Barchas, Joseph A; Hu, Kun; Baring, Matthew G (December 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The study of polarized radiation transfer in the highly magnetized surface locales of neutron stars is of great interest to the understanding of accreting X-ray pulsars, rotation-powered pulsars, and magnetars. This paper explores scattering transport in the classical magnetic Thomson domain that is of broad applicability to these neutron star classes. The development of a Monte Carlo simulation for the polarized radiative transfer is detailed: it employs an electric field vector formalism to enable a breadth of utility in relating linear, circular, and elliptical polarizations. The simulation can be applied to any neutron star surface locale, and is adaptable to accretion column and magnetospheric problems. Validation of the code for both intensity and Stokes parameter determination is illustrated in a variety of ways. Representative results for emergent polarization signals from surface layers are presented for both polar and equatorial magnetic locales, exhibiting contrasting signatures between the two regions. There is also a strong dependence of these characteristics on the ratio of the frequency $$\, \omega \,$$ of a photon to the cyclotron frequency $$\, \omega _{\mathrm{B}}=eB/mc\,$$. Polarization signatures for high-opacity domains are presented, highlighting compact analytical approximations for the Stokes parameters and anisotropy relative to the local field direction for an extended range of frequencies. These are very useful in defining injection conditions deep in the simulation slab geometries, expediting the generation of emission signals from highly opaque stellar atmospheres. The results are interpreted throughout using the polarization characteristics of the magnetic Thomson differential cross-section. 

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2. X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?

   https://doi.org/10.1093/mnras/stad1961  

   Mushtukov, A A; Tsygankov, S S; Poutanen, J; Doroshenko, V; Salganik, A; Costa, E; Marco, A Di; Heyl, J; Monaca, F La; Lutovinov, A A; et al (July 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT X Persei is a persistent low-luminosity X-ray pulsar of period of ≈ 835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above 1013 G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3–8 keV band varied from several to ∼20 per cent over the pulse with a phase dependence resembling the pulse profile. The polarization angle shows significant variation and makes two complete revolutions during the pulse period, resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky, as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination, reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008−57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered, with its value increasing from 0 at ∼2 keV to 30per cent at 8 keV. 

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3. X-ray pulsar GRO J1008−57 as an orthogonal rotator

   https://doi.org/10.1051/0004-6361/202346134  

   Tsygankov, Sergey S.; Doroshenko, Victor; Mushtukov, Alexander A.; Poutanen, Juri; Di Marco, Alessandro; Heyl, Jeremy; La Monaca, Fabio; Forsblom, Sofia V.; Malacaria, Christian; Marshall, Herman L.; et al (July 2023, Astronomy & Astrophysics)

   X-ray polarimetry is a unique way to probe the geometrical configuration of highly magnetized accreting neutron stars (X-ray pulsars). GRO J1008−57 is the first transient X-ray pulsar observed at two different flux levels by the Imaging X-ray Polarimetry Explorer (IXPE) during its outburst in November 2022. We find the polarization properties of GRO J1008−57 to be independent of its luminosity, with the polarization degree varying between nondetection and about 15% over the pulse phase. Fitting the phase-resolved spectro-polarimetric data with the rotating vector model allowed us to estimate the pulsar inclination (130°, which is in good agreement with the orbital inclination), the position angle (75°) of the pulsar spin axis, and the magnetic obliquity (∼74°). This makes GRO J1008−57 the first confidently identified nearly orthogonal rotator among X-ray pulsars. We discuss our results in the context of the neutron star atmosphere models and theories of the axis alignment of accreting pulsars. 

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4. Mutual spin-phonon driving effects and phonon eigenvector renormalization in nickel (II) oxide

   https://doi.org/10.1073/pnas.2120553119  

   Sun, Qiyang; Wei, Bin; Su, Yaokun; Smith, Hillary; Lin, Jiao Y.; Abernathy, Douglas L.; Li, Chen (July 2022, Proceedings of the National Academy of Sciences)

   The physics of mutual interaction of phonon quasiparticles with electronic spin degrees of freedom, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interests for decades. Despite its pivotal role in transport processes, the effect of spin-phonon coupling on the phonon system, especially acoustic phonon properties, has so far been elusive. By means of inelastic neutron scattering and first-principles calculations, anomalous scattering spectral intensity from acoustic phonons was identified in the exemplary collinear antiferromagnetic nickel (II) oxide, unveiling strong spin-lattice correlations that renormalize the polarization of acoustic phonon. In particular, a clear magnetic scattering signature of the measured neutron scattering intensity from acoustic phonons is demonstrated by its momentum transfer and temperature dependences. The anomalous scattering intensity is successfully modeled with a modified magneto-vibrational scattering cross-section, suggesting the presence of spin precession driven by phonon. The renormalization of phonon eigenvector is indicated by the observed “geometry-forbidden” neutron scattering intensity from transverse acoustic phonon. Importantly, the eigenvector renormalization cannot be explained by magnetostriction but instead, it could result from the coupling between phonon and local magnetization of ions. 

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5. The X-Ray Polarimetry View of the Accreting Pulsar Cen X-3

   https://doi.org/10.3847/2041-8213/aca486  

   Tsygankov, Sergey S.; Doroshenko, Victor; Poutanen, Juri; Heyl, Jeremy; Mushtukov, Alexander A.; Caiazzo, Ilaria; Di Marco, Alessandro; Forsblom, Sofia V.; González-Caniulef, Denis; Klawin, Moritz; et al (December 2022, The Astrophysical Journal Letters)

   Abstract The first X-ray pulsar, Cen X-3, was discovered 50 yr ago. Radiation from such objects is expected to be highly polarized due to birefringence of plasma and vacuum associated with propagation of photons in the presence of the strong magnetic field. Here we present results of the observations of Cen X-3 performed with the Imaging X-ray Polarimetry Explorer. The source exhibited significant flux variability and was observed in two states different by a factor of ∼20 in flux. In the low-luminosity state, no significant polarization was found in either pulse phase-averaged (with a 3 σ upper limit of 12%) or phase-resolved (the 3 σ upper limits are 20%–30%) data. In the bright state, the polarization degree of 5.8% ± 0.3% and polarization angle of 49.°6 ± 1.°5 with a significance of about 20 σ were measured from the spectropolarimetric analysis of the phase-averaged data. The phase-resolved analysis showed a significant anticorrelation between the flux and the polarization degree, as well as strong variations of the polarization angle. The fit with the rotating vector model indicates a position angle of the pulsar spin axis of about 49° and a magnetic obliquity of 17°. The detected relatively low polarization can be explained if the upper layers of the neutron star surface are overheated by the accreted matter and the conversion of the polarization modes occurs within the transition region between the upper hot layer and a cooler underlying atmosphere. A fraction of polarization signal can also be produced by reflection of radiation from the neutron star surface and the accretion curtain. 

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