arXiv subjects

N. Sameshima

Publications and source records attributed to N. Sameshima.

3 recordsLinked to original sources

arXiv2026

3D Radiative Transfer of Lyman-series Lines with SKIRT

Context. High-resolution X-ray spectroscopy and polarimetry provided by XRISM and IXPE offer new diagnostics of the geometry and kinematics of photo-ionised plasmas around compact objects. Interpreting reprocessed X-ray emission in such systems requires full three-dimensional radiative transfer (3D RT) including photon-ion interactions. Aims. We extend the Monte Carlo (MC) RT code SKIRT by implementing the Lyman-series lines of H-like ions, enabling self-consistent modelling of resonance scattering, radiative recombination, and polarisation of these lines in X-ray photo-ionised plasmas. Methods. We implemented Lyman-series transitions (up to $n=10$) for ions with $Z=$1--30, including fine-structure splitting and linear polarisation in resonance scattering. Two channels for the production of the Lyman-series lines (resonance scattering and radiative recombination) are considered. Results. The implementation reproduces analytical expectations and shows good agreement with Cloudy. The SKIRT simulations naturally capture RT effects such as P Cygni profiles and line-profile distortion in optically thick media, which are inaccessible to the conventional 1D RT codes commonly used in X-rays. In 3D geometries, we find that anisotropic illumination and velocity fields significantly modify the Lyman series line ratios and profiles, all of which are observable with XRISM. Conclusions. The extended version of SKIRT provides a powerful framework for interpreting X-ray line spectra and polarisation from photo-ionised plasmas. It is particularly suited for constraining the geometry and velocity structure in the vicinity of compact objects in the XRISM and IXPE era.

astro-ph.HE↗

arXiv2026

X-ray Doppler tomography of Fe K$\alpha$ emission in a low-mass X-ray binary 4U 1822-371 - a localized reflector at the accretion stream-disk overflow

We present the X-ray Doppler tomography of the Fe K$\alpha$ (6.4 keV) fluorescence line of the low-mass X-ray binary 4U 1822-371 obtained with XRISM. Eleven orbits of this short period (5.57 hr) binary were covered. The Doppler shift of the line shows clear modulation with the orbital period, motivating us to apply the Doppler tomography in the X-ray band for the first time. The resulting velocity map reveals a compact feature at ($v_{\mathrm{x}}$, $v_{\mathrm{y}}$) $\sim$ ($-$550, $+$125) km s$^{-1}$. This is inconsistent with the emission originating from a symmetric accretion disk, an extended corona around the neutron star, or the surface of the neutron or companion star. Instead, it suggests that the emission originates from the accretion stream-disk overflow. Remarkably, the Fe K$\alpha$ velocity map closely resembles that of the O VI 3811 {\AA}, indicating that both X-ray and optical lines arise from the same site irradiated by the central X-ray source. These results provide the first velocity-resolved X-ray map of the fluorescent line, directly localizing the major reflector in an X-ray binary and establishing X-ray Doppler tomography as a new probe of the structures of accreting systems.

astro-ph.HE↗

arXiv2026

JWST-DECO: Temporal Variations in the Mid-IR Silicate Features of Two T Tauri Discs Based on Spitzer and JWST observations

Mid-infrared spectra of planet-forming discs commonly show prominent silicate emission, whose spectral shape is sensitive to the disc temperature distribution as well as its mineralogical composition. We report new James Webb Space Telescope (JWST) observations of the discs around Sz 96 and IP Tau and find that their silicate emission significantly changes in the 20 years since they were observed with the Spitzer Space Telescope (SST). Significant differences between the SST and JWST spectra are found for both sources, with flux variations of 10--15\% in Sz~96 and 30--35\% in IP Tau. Sz 96 dimmed at $\le$ 18~$\mathrm{\mu m}$ and did not change significantly at longer wavelengths, whereas IP Tau became brighter across the entire wavelength range, with a particularly strong enhancement around 10~$\mathrm{\mu m}$ in the JWST data compared to the SST data. We propose that this large degree of variability is explained by structural changes in the inner regions of the discs. Specifically, we also find that crystalline silicates exhibit lower temperatures than amorphous silicates in the JWST data of both sources. This result supports the idea that crystalline grains, formed through high-temperature annealing in the inner disc regions, have been transported outward, leading to their presence in cooler regions of the disc. While similar behavior had been reported in previous SST-based studies, the much higher spectral resolution of JWST enables clearer identification of the crystalline features.

astro-ph.SR↗