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 period increases 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 Ṁ = 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 parameter ω_s.

We derive a magnetic moment μ ≈ 1.0 10³³ G cm³ which will allow H2215−086 to evolve into an AM Her system once it has crossed the 2–3 h period gap.