A 110 MG cyclotron harmonic in the optical spectrum of RX J1554.2+2721

Abstract

We report the detection of a 110 MG cyclotron harmonic in the SDSS-spectrum of the magnetic cataclysmic variable (MCV) RX J1554.2+2721. This feature was noted earlier by others but remained unexplained. The wide spectral coverage of the new spectrum together with the earlier detection of a Zeeman split Lyα line in a field of 144 MG makes the identification almost unambiguous. We propose to explain the non-conforming UV-optical photospheric temperature of the white dwarf by an as yet unobserved cyclotron component in the ultraviolet which also could significantly contribute to the overall energy balance of the accretion process.