NGC 1624-2: a slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field

Abstract

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. Modelling the spectrum and spectral energy distribution, we conclude that NGC 1624-2 is a main-sequence star of mass M 30 M , and infer an effective temperature of 35 2 kK and log g = 4.0 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 0.94 d which we interpret as the rotational period of the star. We report the detection of a very strong (5.35 0.5 kG) longitudinal magnetic field B z , coupled with probable Zeeman splitting of the Stokes I profiles of metal lines confirming a surface field modulus B of 14 1 kG, consistent with a surface dipole of polar strength 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 analyse archival Chandra ACIS-I X-ray data, inferring a very hard spectrum with an X-ray efficiency of log L x /L bol = -6.4, a factor of 4 larger than the canonical value for O-type stars Based on spectropolarimetric observations