# A method to measure the transverse magnetic field and orient the rotational axis of stars

## TL;DR

This paper introduces a novel high-resolution spectropolarimetric method to directly measure the transverse magnetic field component of stars and determine their rotational axis orientation, enhancing magnetic field analysis capabilities.

## Contribution

It extends existing techniques by enabling direct measurement of the transverse magnetic field and star axis orientation using high-resolution Stokes parameters.

## Key findings

- Successfully applied to rB data, demonstrating the method's effectiveness.
- Able to determine the star's rotational axis orientation from magnetic field variability.
- Provides a new tool for detailed stellar magnetic field studies.

## Abstract

Direct measurements of the stellar magnetic fields are based on the splitting of spectral lines into polarized Zeeman components. With few exceptions, Zeeman signatures are hidden in data noise and a number of methods have been developed to measure the average, over the visible stellar disk, of longitudinal components of the magnetic field. As to faint stars, at present observable only with low resolution spectropolarimetry, a method is based on the regression of the Stokes V signal against the first derivative of Stokes I. Here we present an extension of this method to obtain a direct measurement of the transverse component of stellar magnetic fields by the regression of high resolution Stokes Q and U as a function of the second derivative of Stokes I. We also show that it is possible to determine the orientation in the sky of the rotation axis of a star on the basis of the periodic variability of the transverse component due to its rotation. The method is applied to data, obtained with the Catania Astrophysical Observatory Spectropolarimeter, along the rotational period of the well known magnetic star \b{eta} CrB.