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 λ ≃ 16.000) 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 ∣ υ rad ∣ ≃ 1200 km s −1 , defining the inner edge of the accretion disk at r in ≃ 7 × 10 9 cm or ∼10 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 ∼ 1 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 μ 1 ≃ 1.3 × 10 32 G cm 3 and a surface field strength B 1 ∼ 0.35 MG. Given a polar field strength B p ≲ 1.0 MG, optical circular polarization is not expected. With an accretion rate Ṁ = 3.9 × 10 −11 M ⊙ yr −1 , the accretion torque is G acc ≃ 2.2 × 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 ± 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 M 1 ≃ 0.79 M ⊙ and M 2 ≃ 0.11 M ⊙ .