This paper presents a first observational investigation of the faint Of ? p star NGC 1624-2 , yielding important new constraints on its spectral and physical characteristics , rotation , magnetic field strength , X-ray emission and magnetospheric properties .
Modeling the spectrum and spectral energy distribution , we conclude that NGC 1624-2 is a main sequence star of mass M \simeq 30 ~ { } M _ { \odot } , and infer an effective temperature of 35 \pm 2 kK and \log g = 4.0 \pm 0.2 .
Based on an extensive time series of optical spectral observations we report significant variability of a large number of spectral lines , and infer a unique period of 157.99 \pm 0.94 d which we interpret as the rotational period of the star .
We report the detection of a very strong - 5.35 \pm 0.5 kG - longitudinal magnetic field \langle B _ { z } \rangle , coupled with probable Zeeman splitting of Stokes I profiles of metal lines confirming a surface field modulus \langle B \rangle of 14 \pm 1 kG , consistent with a surface dipole of polar strength \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 20 kG .
This is the largest magnetic field ever detected in an O-type star , and the first report of Zeeman splitting of Stokes I profiles in such an object .
We also report the detection of reversed Stokes V profiles associated with weak , high-excitation emission lines of O iii , which we propose may form in the close magnetosphere of the star .
We analyze archival Chandra ACIS-I X-ray data , inferring a very hard spectrum with an X-ray efficiency \log L _ { x } / L _ { bol } = -6.4 , a factor of 4 larger than the canonical value for O-type stars and comparable to that of the young magnetic O-type star \theta ^ { 1 } Ori C and other Of ? p stars .
Finally , we examine the probable magnetospheric properties of the star , reporting in particular very strong magnetic confinement of the stellar wind , with \eta _ { * } \simeq 1.5 \times 10 ^ { 4 } , and a very large Alfven radius , R _ { Alf } = 11.4 ~ { } R _ { * } .