# High-resolution spectroscopy of the intermediate polar EX Hydrae: II. The inner disk radius

Source: [High-resolution spectroscopy of the intermediate polar EX Hydrae: II. The inner disk radius](<https://www.sota2.com/research/paper/68fa46611e209131ce91fd01>)

Collection index: [llms.txt](<https://www.sota2.com/research/llms.txt>)

Data updated: 2025-10-25T05:46:55.643Z

Authors: K. Beuermann, K. Reinsch

Published: 2024-02-21T14:23:41.000Z

## Abstract

EX Hya is one of the best studied, but still enigmatic intermediate polars. We present phase-resolved blue VLT/UVES high-resolution ($\\lambda/\\Delta \\lambda\\simeq16.000$) spectra of EX Hya taken in January 2004. Our analysis involves a unique decomposition of the Balmer line profiles into the spin-modulated line wings that represent streaming motions in the magnetosphere and the orbital-phase modulated line core that represents the accretion disk. Spectral analysis and tomography show that the division line between the two is solidly located at $\\mid\\upsilon\_{rad}\\mid\\simeq1200$ km s$^{-1}$, defining the inner edge of the accretion disk at $r\_{in}\\simeq{7}\\times 10^{9}$ cm or $\\sim10 R\_1$ (WD radii). This large central hole allows an unimpeded view of the tall accretion curtain at the lower pole with a shock height up to $h\_{sh}\\sim1 R\_1$ that is required by X-ray and optical observations. Our results contradict models that advocate a small magnetosphere and a small inner disk hole. Equating $r\_{in}$ with the magnetospheric radius in the orbital plane allows us to derive a magnetic moment of the WD of $\\mu\_1\\simeq1.3\\times 10^{32}$ G cm$^{3}$ and a surface field strength $B\_1\\sim0.35$ MG. Given a polar field strength $B\_{p} \\lesssim 1.0$ MG, optical circular polarization is not expected. With an accretion rate $\\dot M = 3.9\\times10^{-11}$ $M\_{\\odot}$yr$^{-1}$, the accretion torque is $G\_{acc}\\simeq 2.2 \\times 10^{33}$ g cm$^{2}$s$^{-2}$. The magnetostatic torque is of similar magnitude, suggesting that EX Hya is not far from being synchronized. We measured the orbital radial-velocity amplitude of the WD, $K\_1=58.7\\pm3.9$ km s$^{-1}$, and found a spin-dependent velocity modulation as well. The former is in perfect agreement with the mean velocity amplitude obtained by other researchers, confirming the published component masses $M\_1\\simeq0.79 M\_\\odot$ and $M\_2\\simeq0.11 M\_\\odot$.

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## Original sources and implementation

[Original paper](<https://arxiv.org/abs/2402.13834>)

[PDF](<https://arxiv.org/pdf/2402.13834.pdf>)