Using Optical Spectroscopy to Map the Geometry and Structure of the Irradiated Accretion Discs in Low-mass X-ray Binaries: The Pilot-Study of MAXI J0637$-$430

TL;DR

This study uses multi-wavelength spectroscopy over 8 months to map the structure of accretion discs in a low-mass X-ray binary during outburst, revealing how X-ray irradiation affects emission line profiles and disc geometry.

Contribution

It demonstrates how optical spectroscopy can diagnose the geometry and physical properties of accretion discs influenced by X-ray irradiation in LMXBs.

Findings

Correlation between X-ray irradiation and He II emission line profiles.

Changes in irradiation properties are reflected in H/He emission lines.

Evidence of disc structure evolution during outburst cycles.

Abstract

The recurring transient outbursts in low-mass X-ray binaries (LMXBs) provide us with strong test-beds for constraining the poorly understood accretion process. While impossible to image directly, phase-resolved spectroscopy can provide a powerful diagnostic to study their highly complex, time-dependent accretion discs. We present an 8-month long multi-wavelength (UV, optical, X-ray) monitoring campaign of the new candidate black hole LMXB MAXI J0637$-$430 throughout its 2019/2020 outburst, using the {\em Neil Gehrels Swift Observatory}, as well as three quasi-simultaneous epochs of Gemini/GMOS optical spectroscopy. We find evidence for the existence of a correlation between the X-ray irradiation heating the accretion disc and the evolution of the He {\sc ii} 4686 \AA emission line profiles detected in the optical spectra. Our results demonstrate a connection between the line emitting regions and physical properties of the X-ray irradiation heating the discs during outburst cycles of LMXBs. Further, we are able to show that changes in the physical properties of the irradiation heating the disc in outburst can be imprinted within the H/He emission line profiles themselves in these systems.