Discovery of magnetic field variations with the 12.1-minute pulsation period of the roAp star γ Equulei

We have discovered the first magnetic field variations over the pulsation period in an roAp star. The amplitude of the magnetic variability we have found is significant at the 4.1- to 6.6 sigma level measured for four strong lines of NdIII for gamma Equ with the highest amplitude found being 240+/-37 G for the lambda5845.07 Angstrom line, with a period of 12.1 min. This magnetic field variation is in good agreement with theoretical expectations, and the period agrees well with the known photometric periods. We have also found that the time of most negative effective magnetic field occurs 0.15+/-0.05 cycles prior to maximum pulsation velocity of recession. There is a small but significant variation in the equivalent width of two of the NdIII lines, but no equivalent width variation is detectable for the other two lines. Measurements of four lines of CaI show no variations at all in equivalent width, radial velocity or effective magnetic field strength. We find a difference in the mean effective magnetic field strength of four NdIII lines and four CaI lines and speculate that this could be a real effect caused by the surface concentration of NdIII towards the magnetic pole. If true, this provides a new way to map the horizontal abundance distribution of elements in slowly rotating Ap stars for which Doppler imaging is not possible.