Cyclotron and Zeeman spectroscopy of MR Serpentis in low and high states of accretion.

Abstract

Phase-resolved low-resolution spectrophotometry of the polar MR Ser obtained during states of low and high accretion reveals cyclotron line emission as well as Balmer line Zeeman absorption. At both occasions the cyclotron lines move with phase. The whole system of cyclotron lines appears wavelength- and phase-shifted when the system switches from the high to the low state. These shifts indicate a migration of the emission/accretion region backwards in phase and towards the rotational pole at the state of reduced accretion rate. During the high accretion state stationary Zeeman features are seen against the bright cyclotron background. Both, cyclotron emission lines and Zeeman absorption lines yield the same field strength, B = 24 - 25 MG, indicating the coexistence of cool halo material and hot emitting plasma. The Zeeman aborption lines observed in the low state are of photospheric origin. At times one may recognize the complete set of Zeeman components of H-alpha. The effective photospheric field strength varies between 27.3 MG and 28.5 MG at spectroscopic phases 0.0 and 0.5, respectively. The photospheric Zeeman lines are much narrower than observed in other magnetic white dwarfs of comparable field strength and much narrower than expected for a centered dipolar field and a uniformly emitting photosphere. A spot model seems not be viable, hence, the field structure is clearly and not simply dipolar. Model calculations reveal a strongly decentered dipolar (d_off = 0.3 R_wd) or the equivalent. Our analysis is facilitated by the spectroscopic identification of the secondary star in MR Ser. The determination of its spectral type as late M-dwarf (M5 - M6) allows the distance to be estimated, d = 139 +/- 13 pc. Radial velocity variations of photospheric absorption lines as well as quasichromospheric emission lines are used to determine the system geometry and the white dwarf mass, M_wd approx. 0.5 solar mass.