Normalization of polarized spectral contours for high-accuracy magnetic field measurements

TL;DR

This paper introduces a normalization method for polarized spectral profiles, improving high-accuracy magnetic field measurements by addressing signal-to-noise and spectral line variability issues.

Contribution

The paper presents the COALA normalization method, enhancing the SL magnetic field measurement technique for polarized spectra, applicable to synthetic and real stellar data.

Findings

Effective normalization of polarized profiles demonstrated on synthetic spectra.

Application to real stellar spectra of β Aql shows improved measurement consistency.

Method reduces dependence on spectral line selection and noise levels.

Abstract

Designed at the Crimean Astrophysical Observatory the SL-method of stellar magnetic field measurement is based on the calculation of the distance between the centers of gravity of the left and right circularly polarized components of the spectral line. The SL-method is free from the limitations of the LSD-method, namely its requirement for weak magnetic fields, weak spectral lines, the same shape of polarized profiles. Also, this method is free from model limitations of the PCA, NDD and ZCD methods also. The SL-method involves the calculation of the magnetic field using individual spectral lines and the subsequent formation of uniform arrays of the magnetic field measurements. The formation of the arrays is based on different criteria, e.g. using spectral lines of a chemical element, assumption of the same physical conditions in the areas of the spectral line formation, a statistically significant difference between the values of the magnetic field calculated from different arrays of spectral lines etc. If there are several arrays of spectral lines, it is necessary to obtain normalized polarized profiles using predetermined parameters for each array: the magnitude of the field, wavelength for normalization, Lande factor and the central residual intensity of the line. In this paper, we present a method of normalization of polarized profiles based on the dependence of the residual intensity of the observed polarized profiles on their central residual intensity (COntour Algorithm of the Line Approximation: COALA). The SL and COALA methods strong depend on the signal-to-noise ratio and a number of used unblended spectral lines. In this work the application of the proposed method is demonstrated for synthetic polarized spectra and for circularly polarized spectra of the yellow subgiant $\beta$ Aql (Sp G8 IV) obtained on 2014 October 06.