H2215-086/FO AQR: The "king" of intermediate polars spins down

Abstract

Our 5-color photometric observations of the intermediate polar H2215-086/FO Aqr which were collected between 1982 and the end of 1985 reveal that the 20.9 min rotational periodincreases with a spin-down rate of (1.0±0.2) 10−10 ss−1. This is in marked contrast to the spin-up behaviour of other well studied DQ Her systems. The pulse profile of the optical modulation as a function of wavelength changes drastically across the Balmer jump suggesting the presence of two distinct components of pulsed optical emission which furthermore are shifted in phase with respect to the X-ray maxima observed with EXOSAT. The UV, optical and IR flux distribution can adequately be accounted for by an accretion disc model with a mass-flow rate $$\dot M$$ = 1017 g s−1 and an inner boundary which is ≈ 10 times larger than the radius of the white dwarf. Our results support the magnetic accretion torque theory worked out by Ghosh and Lamb which can account for both spin-up and spin-down of the accreting star depending on the value of the fastness parameters ωs. We derive a magnetic moment μ ≃ 1.0 1033 G cm3 which will allow H2216-086 to evolve into an AM Her system once it has crossed the 2–3 h period gap.