The rapidly oscillating Ap star γ Equ: linear polarization as an enhanced pulsation diagnostic?

We present the first short time-scale observations of the rapidly oscillating Ap (roAp) star γ Equ in linear polarized light obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument installed at the Large Binocular Telescope. These observations are used to search for pulsation variability in Stokes Q and U line profiles belonging to different elements. The atmospheres of roAp stars are significantly stratified with spectral lines of different elements probing different atmospheric depths. roAp stars with strong magnetic fields, such as γ Equ with a magnetic field modulus of 4 kG and a pulsation period of 12.21 min, are of special interest because the effect of the magnetic field on the structure of their atmospheres can be studied with greatest detail and accuracy. Our results show that we may detect changes in the transversal field component in Fe I and rare earth element lines possessing large second-order Landé factors. Such variability can be due to the impact of pulsation on the transverse magnetic field, causing changes in the obliquity angles of the magnetic force lines. Further studies of roAp stars in linear polarized light and subsequent detailed modelling are necessary to improve our understanding of the involved physics.