# The long term polarimetric variability of the strongly magnetic white dwarf Grw+70$^\circ$8247

## TL;DR

This study investigates the long-term polarimetric variability of the magnetic white dwarf Grw+70°8247 over four decades, revealing potential rotation periods of hundreds to thousands of years and providing new spectropolarimetric data and calibration insights.

## Contribution

First spectropolarimetric comparison of Grw+70°8247 over 40 years, proposing a long rotation period and detailing calibration procedures for linear spectropolarimetry.

## Key findings

- Polarisation varies over four decades.
- Possible rotation period between 100 and 1000 years.
- Discrepancies found in literature circular polarisation signs.

## Abstract

Some of the white dwarfs 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$^\circ$\,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 white dwarf in which a magnetic field was discovered, no spectropolarimetric observations have been discussed in the literature in the last 40 years. Here we present two sets of linear and circular polarisation spectra taken in 2015 and 2018, and we compare them with spectropolarimetric data obtained in the 1970s. Polarisation 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 polarisation 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 $10^2$ to $10^3$ years. 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 interest for linear spectropolarimetric measurements. We also found discrepancies in the sign of circular polarisation as reported in the literature, and made explicit the definitions that we have adopted.