Detection of the white-dwarf spin of V1082 Sgr

Abstract: We report on the discovery of circular polarization modulated with a period of 1.943 +- 0.002 h in the cataclysmic variable V1082 Sgr. These findings unambiguously reveal the rotation of a magnetic white dwarf and establish its intermediate polar (IP) nature. Along with its extraordinary long orbital period, Porb, of 20.8 h, the spin period (Pspin) places this system in an extreme position of the Pspin versus Porb distribution. The circular polarization phase diagram has a single peak and an amplitude smaller than 1%. These data were used to model the post-shock region of the accretion flow on the white-dwarf surface using the CYCLOPS code. We obtained a magnetic field in the white-dwarf pole of 11 MG and a magnetospheric radius consistent with the coupling region at around 2 - 3 white-dwarf radii. The Pspin/Porb value and the estimated magnetic field momentum suggest that V1082 Sgr could be out of spin equilibrium, in a spin-up state, possibly in a stream accretion mode.