Probing the nature of the TeV gamma-ray binary HESS J0632+057 by monitoring Be disk variability

TL;DR

This study monitors the Be star disk in the gamma-ray binary HESS J0632+057, revealing short-term variability and large-scale disturbances likely caused by pulsar wind interactions, providing new insights into disk-compact object interactions.

Contribution

First high-dispersion optical spectra covering the second half of the orbital cycle, revealing detailed variability patterns and estimating emission region sizes in the Be disk.

Findings

Short-term variability observed in Balmer lines after apastron.

Large-scale disk disturbances likely caused by pulsar wind, not tidal forces.

Emission regions estimated at specific stellar radii for different lines.

Abstract

We report on monitoring observations of the TeV gamma-ray binary HESS J0632+057, which were carried out to constrain the interaction between the Be circumstellar disk and the compact object of unknown nature, and provide for the first time high-dispersion (R > 50000) optical spectra in the second half of the orbital cycle, from apastron through periastron. The Halpha, Hbeta, and Hgamma line profiles are found to exhibit remarkable short-term variability for ~1 month after the apastron (phase 0.6--0.7), whereas they show little variation near the periastron. These emission lines show "S-shaped" variations with timescale of ~150 days, which is about twice that reported previously. In contrast to the Balmer lines, no profile variability is seen in any FeII emission line. We estimate the radii of emitting regions of the Halpha, Hbeta, Hgamma, and FeII emission lines to be ~30, 11, 7, and 2 stellar radii (R_*), respectively. The amplitudes of the line profile variations in different lines indicate that the interaction with the compact object affects the Be disk down to, at least, the radius of 7 R_* after the apastron. This fact, together with little profile variability near the periastron, rules out the tidal force as the major cause of disk variability. Although this leaves the pulsar wind as the most likely candidate mechanism for disk variations, understanding the details of the interaction, particularly the mechanism for causing a large-scale disk disturbance after the apastron, remains an open question.