Magnetic field geometry and magnetospheric environment of the strongly magnetic Of?p star NGC 1624-2

NGC 1624-2 is an O7f?p star with a reported probable polar magnetic field strength $\ge$ 20 kG, which is the strongest magnetic field ever measured in an O-type star. We study the variability of the mean longitudinal magnetic field $⟨B_z⟩$ and the mean field modulus $⟨B⟩$ to obtain constraints on its field geometry. Only one magnetic pole is observable over the rotation cycle. The approximately sinusoidal variation of $\langle B_z \rangle$andtheratioofthevaluesoftheextremaof$\langle B \rangle$indicatethatthereisanimportantcomponentofthefieldthatisdipolar\mathrm{.}The$\langle B_z \rangle$ values measured over the rotation cycle are in the range from $-0.2$ to 4.5 kG, whereas the values for $⟨B⟩$ vary between 9 and 12 kG. The $⟨B_z⟩$ values obtained using the O III $\lambda$7455 emission line are in the range from 0.4 to 2.3 kG and show a variability pattern similar to that detected for the absorption lines. The fact that the phase of the $⟨B_z⟩$ minimum coincides with the phase of the $⟨B⟩$ maximum, indicates that the field structure must significantly depart from a centred dipole. Further, we discuss the nature of the observed variable Stokes $V$ profiles corresponding to a longitudinal field of negative polarity detected in the emission He I lines and present the first MHD numerical simulations of the gas flow in the magnetosphere of this star.