Periods of magnetic field variations in the Ap star $\gamma$ Equulei (HD 201601)

TL;DR

This study compiles over 67 years of magnetic field measurements of the Ap star γ Equ, determining its long-term magnetic variation period as approximately 97 years through sine wave fitting.

Contribution

It provides the longest magnetic field variation record for γ Equ and establishes a reliable long-term period using sine wave fitting methods.

Findings

The long-term magnetic variation period of γ Equ is approximately 97 years.

A comprehensive dataset of 395 magnetic field measurements over 67 years was compiled.

Sine wave fitting was identified as the most reliable method for period determination in this case.

Abstract

This paper presents a series of 95 new measurements of the longitudinal (effective) magnetic field $B_e$ of the Ap star $\gamma$ Equ (HD 201601). Observations were obtained at the coud\'e focus of the 1-m reflector at the Special Astrophysical Observatory (SAO RAS) in Russia over a time period of 4190 days (more than 11 years). We compiled a long record of $B_e$ points, adding our measurements to all published data. The time series of magnetic data consists of 395 $B_e$ points extending for 24488 days, or over 67 years. Various methods of period determination were examined for the case in which the length of the observed time series is rather short and amounts only to ~69 percent of the period. We argue that the fitting of a sine wave to the observed $B_e$ points by least squares yields the most reliable period in the case of $\gamma$ Equ. Therefore, the best period for long-term magnetic variations of $\gamma$ Equ, and hence the rotational period, is $P_{\rm rot}=35462.5 \pm 1149$ days $= 97.16 \pm 3.15$ years.