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 >= 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 < B-z > and the ratio of the values of the extrema of < B > indicate that there is an important component of the field that is dipolar. The < B-z > 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 magnetohydrodynamical numerical simulations of the gas flow in the magnetosphere of this star.