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

TL;DR

This study uses XRISM high-resolution X-ray spectroscopy to analyze plasma dynamics, velocity gradients, and accretion column geometry in the magnetic cataclysmic variable AM Herculis.

Contribution

First detailed measurement of velocity and temperature gradients in the accretion flow of AM Herculis using high-resolution X-ray spectroscopy.

Findings

Resolved satellite lines of highly ionized Fe.

Detected spin-phase modulations in Fe lines with specific velocities.

Derived shock temperature of 24 keV and density diagnostics for accretion columns.

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.