We observed NGC 1624-2 , the O-type star with the largest known magnetic field ( B _ { p } \sim { 20 } kG ) , in X-rays with the ACIS-S camera onboard the Chandra X-ray Observatory .
Our two observations were obtained at the minimum and maximum of the periodic H \alpha emission cycle , corresponding to the rotational phases where the magnetic field is the closest to equator-on and pole-on , respectively .
With these observations , we aim to characterise the star ’ s magnetosphere via the X-ray emission produced by magnetically confined wind shocks .
Our main findings are : ( i ) The observed spectrum of NGC 1624-2 is hard , similar to the magnetic O-type star \theta ^ { 1 } Ori C , with only a few photons detected below 0.8 keV .
The emergent X-ray flux is 30 % lower at the H \alpha minimum phase .
( ii ) Our modelling indicated that this seemingly hard spectrum is in fact a consequence of relatively soft intrinsic emission , similar to other magnetic Of ? p stars , combined with a large amount of local absorption ( \sim 1-3 \times 10 ^ { 22 } cm ^ { -2 } ) .
This combination is necessary to reproduce both the prominent Mg and Si spectral features , and the lack of flux at low energies .
NGC 1624-2 is intrinsically luminous in X-rays ( \log L ^ { \mathrm { em } } _ { \mathrm { X } } \sim 33.4 ) but 70-95 % of the X-ray emission produced by magnetically confined wind shocks is absorbed before it escapes the magnetosphere ( \log L ^ { \mathrm { ISMcor } } _ { \mathrm { X } } \sim 32.5 ) .
( iii ) The high X-ray luminosity , its variation with stellar rotation , and its large attenuation are all consistent with a large dynamical magnetosphere with magnetically confined wind shocks .