The long-term polarimetric variability of the strongly magnetic white dwarf Grw+70° 8247

Abstract

Some of the white dwarfs (WDs) exhibit among the strongest magnetic fields in the universe. Many of these degenerate magnetic stars are also rotating very slowly. Among these objects, Grw+70° 8247, with its century-long suspected rotation period and its 400 MG magnetic field, stands as a particularly interesting object. Surprisingly, for this star, the first WD in which a magnetic field was discovered, no spectropolarimetric observations have been discussed in the literature in the last 40 yr. Here, we present two sets of linear and circular polarization spectra taken in 2015 and 2018, and we compare them with spectropolarimetric data obtained in the 1970s. Polarization shows variability over a time interval of four decades, but some subtle changes may have been detected even over a three-year time interval. Using the variation of the polarization position angle as a proxy for the rotation of the magnetic axis in the plane of the sky, we conclude that the star's rotation period probably lies in the range of 10<SUP>2</SUP>-10<SUP>3</SUP> yr. Our data analysis is accompanied by a description of our various calibrations and tests of the ISIS instrument at the William Herschel Telescope that may be of general interests for linear spectropolarimetric measurements. We also found discrepancies in the sign of circular polarization as reported in the literature, and made explicit the definitions that we have adopted.