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arXiv · 2604.06591

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

Yukikatsu Terada·Kaya Mori·Takayuki Hayashi·Gabriel L. Bridges·Manabu Ishida·Axel D. Schwope·Mariko Kimura·Masayoshi Nobukawa·David A. H. Buckley·Solen Balman·Taichi Ichikawa·Atsuto Matsumura·Mai Takeo·Charles J. Hailey·Gavin Ramsay·Antonio Rodriguez·Samantha Walker
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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 semi-amplitudes of $81.8\pm6$ km s$^{-1}$ and $132.5\pm9$ km s$^{-1}$, and mean velocities of $143.6\pm6$ km s$^{-1}$ and $225.6\pm8$ km s$^{-1}$, respectively. After removing these bulk Doppler shifts, we obtain intrinsic Doppler widths of $5.23_{-0.15}^{+0.16}$ eV for Fe XXV and $6.23_{-0.18}^{+0.19}$ 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. (1999, 2001): the equivalent widths 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 $24.0\pm0.1$ keV and shock velocity of $1,116\pm2$ km s$^{-1}$. Radiative transfer simulations of the resonance lines further constrain the shock density to about $(5 - 6)\times10^{15}$ 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.

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Yukikatsu Terada, Kaya Mori, Takayuki Hayashi, Gabriel L. Bridges, Manabu Ishida, Axel D. Schwope, Mariko Kimura, Masayoshi Nobukawa, David A. H. Buckley, Solen Balman, Taichi Ichikawa, Atsuto Matsumura, Mai Takeo, Charles J. Hailey, Gavin Ramsay, Antonio Rodriguez, Samantha Walker. 2026-04-08. Plasma Dynamics of Radiative Cooling Accretion Flow in AM Herculis with XRISM. https://doi.org/10.3847/1538-4357/ae5f97

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