Plasma Dynamics of Radiative Cooling Accretion Flow in AM Herculis with XRISM

Abstract We present XRISM/Resolve high-resolution X-ray spectroscopy of the prototypical magnetic cataclysmic variable AM Herculis. All satellite lines of highly ionized Fe are fully resolved. Lighter-element lines (Si, S, Ca) show 2–3 eV widths consistent with purely thermal broadening, while the broader 6–7 eV Fe lines require additional bulk Doppler broadening. Spin-phase-resolved modulations are clearly detected in the Fe XXV and Fe XXVI lines, with semiamplitudes of 81.8 ± 6 km s −1 and 132.5 ± 9 km s −1 , and mean velocities of 143.6 ± 6 km s −1 and 225.6 ± 8 km s −1 , respectively. After removing these bulk Doppler shifts, we obtain intrinsic Doppler widths of 5.2 3 − 0.15 + 0.16 eV for Fe XXV and 6.2 3 − 0.18 + 0.19 eV for Fe XXVI , directly revealing gradients of bulk velocity and temperature in the cooling-flow plasma. We additionally examined the resonance anisotropy predicted by Terada et al.; the equivalent widths (EWs) of the Fe XXV and Fe XXVI resonance lines increase at the pole-on phase by factors of 1.30–1.35, in positive correlation with their oscillator strengths. Combining XRISM with simultaneous NuSTAR data and PSAC/MCVSPEC plasma models, we derive a self-consistent shock temperature of 24.0 ± 0.1 keV and shock velocity of 1116 ± 2 km s −1 . Radiative transfer simulations of the resonance lines further constrain the shock density to ≈(5–6) × 10 15 cm −3 , providing a new density diagnostic for accretion columns. The resulting accretion-column geometry has a height of 200–300 km and a radius of 200–400 km.