Using optical spectroscopy to map low-mass X-ray binary accretion discs

Accretion is of fundamental importance on all scales in our Universe, driving the birth of newborn stars and planets, growth of supermassive black holes at the center of galaxies, and even death of white dwarfs in supernovae. However, despite its importance, our understanding of this process is fragmented and incomplete. The recurring transient outbursts in low-mass X-ray binaries (LMXBs) are ideal laboratories to remedy this, providing a rare opportunity to study an evolving accretion disc in real-time. Unlike their supermassive relatives, LMXBs are far too small and distant to be imaged directly. Fortunately, phase-resolved spectroscopy can provide an alternative diagnostic to study their highly complex, time-dependent accretion discs. The primary spectral signature of LMXBs are strong, disc-formed emission lines detected at optical wavelengths. The shape, profile, and appearance/disappearance of these lines change throughout a binary orbit, and thus, can be used to trace how matter in these discs behaves and evolves over time. By combining multi-wavelength monitoring campaigns via the Neil Gehrels Swift Observatory, phase-resolved spectroscopy from the GTC and Liverpool telescopes, and modern astrotomography techniques, we have been able to track and quantify how variations in the thermodynamic heating process during an outburst cycle, affect physical properties of the gas in the accretion disc of BH-LMXB MAXIJ1820+070, through detected H/He emission lines. Here, we present evidence for a clear empirical connection existing between the line emitting regions and physical properties of the X-rays heating the disc in J1820, and show how changes in the physical properties of the X-ray irradiation heating mechanism itself is actually imprinted within the H/He emission line profiles themselves.