A rather strong mean longitudinal magnetic field of the order of a few hundred Gauss was detected a few years ago in the Of ? p star CPD -28 ^ { \circ } 2561 using FORS2 low-resolution spectropolarimetric observations .
In this work we present additional low-resolution spectropolarimetric observations obtained during several weeks in 2013 December using FORS 2 ( FOcal Reducer low dispersion Spectrograph ) mounted at the 8-m Antu telescope of the VLT .
These observations cover a little less than half of the stellar rotation period of 73.41 d mentioned in the literature .
The behaviour of the mean longitudinal magnetic field is consistent with the assumption of a single-wave variation during the stellar rotation cycle , indicating a dominant dipolar contribution to the magnetic field topology .
The estimated polar strength of the surface dipole B _ { d } is larger than 1.15 kG .
Further , we compared the behaviour of the line profiles of various elements at different rotation phases associated with different magnetic field strengths .
The strongest contribution of the emission component is observed at the phases when the magnetic field shows a negative or positive extremum .
The comparison of the spectral behaviour of CPD -28 ^ { \circ } 2561 with that of another Of ? p star , HD 148937 of similar spectral type , reveals remarkable differences in the degree of variability between both stars .
Finally , we present new X-ray observations obtained with the Suzaku X-ray Observatory .
We report that the star is X-ray bright with \log { L _ { X } / L _ { bol } } \approx - 5.7 .
The low resolution X-ray spectra reveal the presence of a plasma heated up to 24 MK .
We associate the 24 MK plasma in CPD -28 ^ { \circ } 2561 with the presence of a kG strong magnetic field capable to confine stellar wind .