2014MNRAS.440.1779H

Our recent search for the presence of a magnetic field in the bright early A-type supergiant HD 92207 using FOcal reducer low dispersion spectrograph (FORS) 2 in spectropolarimetric mode indicated the presence of a longitudinal magnetic field of the order of a few hundred gauss. Assuming the ideal case of a non-variable star, this discovery has recently been questioned in one work trying to demonstrate the importance of non-photon noise in FORS 2 observations. The assumption of non-variability of HD 92207 can, however, not be held since substantial profile variations of diverse lines on a time-scale of minutes or maybe even a fraction of a minute are detected in FORS 2 spectra. The presence of short-term spectral variability in blue supergiants, which are considered as Type II supernova progenitors, has not been a subject of systematic studies before and is critical for the current theoretical understanding of their physics. Given the detected short-term variability, the question of the presence of a magnetic field cannot be answered without proper modelling of the impact of such a variability on the measurements of the magnetic field. Since the short-term periodicity does not fit into the currently known domain of non-radially pulsating supergiants, its confirmation is of great importance for models of stellar evolution.