Warped disks during type II outbursts in Be/X-ray binaries: evidence from optical polarimetry

TL;DR

This study presents optical polarimetric observations of Be/X-ray binary 4U 0115+63, revealing evidence of warped disks during type II outbursts through significant changes in polarization and spectral line profiles.

Contribution

First optical polarimetric and spectroscopic analysis of 4U 0115+63 during outbursts, providing direct evidence of disk warping in Be/X-ray binaries.

Findings

Significant polarization degree and angle changes during outbursts

Distorted Hα line profiles indicating disk distortion

Correlation between polarization variability and X-ray activity

Abstract

Current models that explain giant (type II) X-ray outbursts in Be/X-ray binaries (BeXB), are based on the idea of highly distorted disks. They are believed to occur when a misaligned and warped disk becomes eccentric, allowing the neutron star to capture a large amount of material. The BeXB 4U 0115+63 underwent two major outbursts in 2015 and 2017. Our aim is to investigate whether the structural changes in the disk expected during type II outbursts can be detected through optical polarimetry. We present the first optical polarimetric observations and new optical spectra of the BeXB 4U 0115+63 covering the period 2013-2017. We study in detail the shape of the H$\alpha$ line profile and the polarization parameters before, during, and after the occurrence of a type II X-ray outburst. We find significant changes in polarization degree and polarization angle and highly distorted line profiles during the 2017 X-ray outburst. The degree of polarization decreased by $\sim$ 1%, while the polarization angle, which is supposed to be related with the disk orientation, first increased by $\sim 10^{\circ}$ in about two months and then decreased by a similar amount and on a similar timescale once the X-ray activity ceased.We interpret the polarimetric and spectroscopic variability as evidence for the presence of a warped disk.