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

EX Hya is one of the best studied, but still enigmatic intermediate polars. We present phase-resolved blue VLT/UVES high-resolution (λ/Δ λ16.000) 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 υrad1200 km s⁻¹, defining the inner edge of the accretion disk at rᵢₙ7× 10⁹ cm or ~10 R₁ (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 hₛₕ~1 R₁ 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 rᵢₙ with the magnetospheric radius in the orbital plane allows us to derive a magnetic moment of the WD of μ₁1.3× 10³² G cm³ and a surface field strength B₁~0.35 MG. Given a polar field strength Bₚ 1.0 MG, optical circular polarization is not expected. With an accretion rate M = 3.9×10⁻¹¹ M_yr⁻¹, the accretion torque is Gacc 2.2 × 10³³ g cm²s⁻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, K₁=58.7±3.9 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 M₁0.79 M_ and M₂0.11 M_.