Optical spectroscopy of 1A 0535+262 before, during, and after the 2020 giant X-ray outburst

TL;DR

This study investigates the optical spectral evolution of the Be/X-ray binary 1A 0535+262 around its 2020 giant X-ray outburst, revealing disc size changes and complex disc geometry dynamics.

Contribution

It provides the first detailed optical spectral analysis of the Be star's disc evolution during a major X-ray outburst in 1A 0535+262, highlighting disc misalignment and precession.

Findings

Hα line showed significant variability and profile changes.

Disc size decreased during the outburst.

Evidence of a misaligned, precessing, and warped Be disc.

Abstract

We present the findings from our study of the Be/X-ray binary 1A 0535+262/HD 245770 during the giant X-ray outburst in October 2020. We utilized the 1.2-m telescope at Mount Abu Infrared observatory for optical observations of the Be companion star. The outburst reached a peak X-ray flux of approximately 11 Crab in the 15-50 keV range, marking the highest ever recorded X-ray outburst from the pulsar. We conducted optical observations in the 6000-7200 angstrom range before, during, and after the X-ray outburst, aiming to examine the evolution of the circumstellar disc of the Be star from February 2020 to February 2022. Our optical spectra displayed prominent emission lines at 6563 angstrom (H I), 6678 angstrom (He I), and 7065 angstrom (He I). Notably, the H$\alpha$ line exhibited significant variability in the spectra. Prior to and during the outburst, the line profiles appeared single-peaked, and asymmetric with broad red and blue wings, respectively. However, post-outburst observations revealed a double-peaked profile with asymmetry in the blue wing. Our pre-outburst observations confirmed a larger Be circumstellar disc that diminished in size as the outburst progressed. Additionally, the observed variations in the H$\alpha$ line profile and parameters indicate the presence of a highly misaligned, precessing, and warped Be disc.