arXiv subjects

Yutaro Nagai

Publications and source records attributed to Yutaro Nagai.

5 recordsLinked to original sources

arXiv2026

XRISM/Resolve observations of Hercules X-1: a pulsating, highly broadened Fe K emission line from the neutron star accretion column

The study of X-ray pulsar accretion columns helps us characterize accretion physics in this extreme regime of strong gravity and strong magnetic fields. Previous observations of the X-ray pulsar Hercules X-1 revealed a highly broadened Fe K emission line, associated with Doppler motions exceeding 0.1c, suggesting its origin in the accretion column. We obtained a high-spectral resolution view of the Fe K energy band of Hercules X-1 thanks to a 200 ks observation with the XRISM observatory. The XRISM/Resolve microcalorimeter spectra allow us to separate the different spectral components and accurately model them with phenomenological models. We confirm the presence of a broad line near 6.5 keV with a typical $1σ$ width of 1 keV. Performing a pulse-phase-resolved analysis, we find that the feature is strongly variable with Her X-1 pulse phase. This is consistent with the proposed origin due to collisional recombination or by reprocessing of the primary X-ray emission in the accretion column, where strong variability with pulse phase is expected due to the rotation of the columns alongside with the neutron star. Additionally, the Fe K line pulsation pattern evolves with the 35-day cycle of Hercules X-1, supporting the scenario that the neutron star and its accretion columns undergo precession, in agreement with recent polarimetric results from the IXPE observatory. We discuss the future applications of modeling of this broad line in X-ray pulsars with physical spectral models. This could be used to detect and track neutron star precession, advancing our understanding of neutron star interiors.

astro-ph.HE↗

arXiv2026

Distinct Velocity Components in the Absorption Lines of the Neutron Star X-ray Binary AX J1745.6-2901

Accretion disks in X-ray binaries regulate mass transfer onto compact objects and drive radiative and kinetic feedback to their surroundings. Here we report X-ray spectroscopy of the eclipsing neutron star low-mass X-ray binary AX J1745.6-2901 with XRISM/Resolve. The phase-averaged Fe XXVI Lyα absorption profile exhibits two absorption minima with relative depths that are inconsistent with the theoretical Lyα1/Lyα2 doublet ratio expected from a single velocity component. We demonstrate that this profile is well described by two discrete velocity components: a blueshifted component at v~ -160 km/s and a redshifted component at v ~ +590 km/s. The significance of the redshifted component is more than 3 sigma based on a Monte-Carlo calculation. This velocity structure persists across orbital phases, disfavoring a localized origin such as a bulge or dip. The blueshifted component, well below the outer-disk escape velocity, is consistent with a slow outflow or disk atmosphere. The redshifted absorber can be explained either by infalling gas from a failed wind or by a gravitational redshift, and the present data cannot rule out either possibility. Regardless of its origin, the redshifted component is kinematically separate from the disk atmosphere and outflow. The absence of absorption at intermediate velocities further indicates a genuinely bimodal velocity distribution rather than the two ends of a single continuous flow, offering a new view of the absorbing-gas kinematics.

astro-ph.HE↗

arXiv2026

XRISM/Resolve observations of Hercules X-1: vertical structure and kinematics of the disk wind

X-ray binary accretion disk winds can carry away a significant fraction of the matter transferred from the companion and hence strongly affect the accretion flow and the long-term evolution of the binary. However, accurate mass outflow rate measurements are challenging due to uncertainties in our understanding of the 3D wind structure. Most studies employ absorption line spectroscopy that only gives us a single sightline through the wind streamlines. Hercules X-1 is a peculiar X-ray binary which allows us to avoid this issue, as its warped, precessing accretion disk naturally presents a range of sightlines through the vertical structure of its disk wind. Here we present the first results from a large, coordinated campaign on Her X-1 led by the new XRISM observatory (with an exposure of 210 ks) and supported by XMM-Newton, NuSTAR and Chandra. We perform a time-resolved analysis and constrain the wind properties. With XRISM/Resolve, we directly detect the Her X-1 orbital motion with an amplitude of 170 km/s in the evolution of the wind velocity. After correcting for this effect, we observe an increase in wind velocity from 250 km/s to 600 km/s as the wind rises to greater heights above the disk. The wind column density decreases with increasing height, as expected, but its ionization parameter log($ξ$/erg cm s$^{-1}$) evolves only weakly from 3.65 to 3.9 as the wind expands away. Additionally, we detect a new orbital dependence of the wind properties, revealing a likely second component that appears only briefly after the eclipse by the secondary star.

astro-ph.HE↗

arXiv2026

Energy shift of Fe-K fluorescence lines due to low ionization demonstrated with XRISM in Centaurus X-3

The Fe K$α$ fluorescence line at 6.4 keV is a powerful probe of cold matter surrounding X-ray sources and has been widely used in various astrophysical contexts. The X-ray microcalorimeter spectrometer onboard XRISM can measure line shifts with unprecedented precision of $\sim$0.2 eV, equivalent to a line-of-sight velocity of $\sim$10 km s$^{-1}$. At this level of accuracy, however, several factors that influence the line energy must be carefully considered prior to astrophysical interpretation. One such important factor is the ionization degree, Fe$^{q+}$. The K$α$ line shifts redward by $\sim$4 eV as $q$ increases from 0 (neutral) to 8 (Ar-like). Additionally, the accompanying Fe K$β$ line at 7.06 keV shifts blueward by $\sim$30 eV from $q=0$ to 8. We demonstrate that this effect is actually observable in the XRISM data of the high-mass X-ray binary Centaurus X-3 (Cen X-3). We advocate that the differential energy shift between the K$α$ and K$β$ line provides a robust estimate of $q$ by decoupling from other effects that shift the two lines in the same direction. We derived $q \sim 5$ (Sc-like) for the fluorescing matter by comparing the observation with atomic structure calculations of our own and in the literature. By accounting for the derived charge state and the corresponding shift in the rest-frame line energy, we made corrections for this effect and reached a consistent residual shift among the K$α$, K$β$, and the optical measurement attributable to the systemic velocity of the system. Consequently, we obtained a new constraint on the location of the cold matter. This ionization effect needs to be assessed in all use cases of the Fe K$α$ line shift beyond Cen X-3, and the proposed metric is generally applicable to all of them.

astro-ph.HE↗

arXiv2025

Detection of the orbital modulation of Fe K$α$ fluorescence emission in Centaurus X-3 using the high-resolution spectrometer Resolve onboard XRISM

The Fe K$α$ fluorescence line emission in X-ray spectra is a powerful diagnostic tool for various astrophysical objects to reveal the distribution of cold matter around photo-ionizing sources. The advent of the X-ray microcalorimeter onboard the \textit{XRISM} satellite will bring new constraints on the emission line. We present one of the first such results for the high-mass X-ray binary Centaurus X-3, which is composed of an O-type star and a neutron star (NS). We conducted a 155 ks observation covering an entire binary orbit. A weak Fe K$α$ line was detected in all orbital phases at an equivalent width (EW) of 10--20 eV. We found for the first time that its radial velocity (RV) is sinusoidally modulated by the orbital phase. The RV amplitude is 248 $\pm$ 13 km s$^{-1}$, which is significantly smaller than the value (391 km s$^{-1}$) expected if the emission is from the NS surface, but is consistent if the emission takes place at the O star surface. We discuss several possibilities of the line production site, including the NS surface, O star surface, O star wind, and accretion stream from the O star to the NS. We ran radiative transfer calculation for some of them assuming spherically-symmetric density and velocity profiles and an isotropic distribution of X-ray emission from the NS. None of them explains the observed EW and velocity dispersion dependence on the orbital phase, suggesting that more elaborated modeling is needed. In other words, the present observational results have capability to constrain deviations from these assumptions.

astro-ph.HE↗