[Paper Review] Ubiquitous yet forgotten: broad absorptions in the optical spectra of low-mass X-ray binaries

D. Mata Sanchez, T. Munoz-Darias+3 more|arXiv (Cornell University)|Feb 12, 2026

Astrophysical Phenomena and Observations0 citations

TL;DR

The paper presents a population study and a dedicated spectroscopic analysis showing broad absorptions in optical Balmer lines are common in LMXBs, linked to the accretion disc, and evolve with outbursts.

ABSTRACT

Optical outburst spectra of low-mass X-ray binaries enable studies of extreme accretion and ejection phenomena. While some of their spectroscopic features have been analysed in detail, the appearance of broad absorptions in the optical regime has been traditionally neglected. In this work, we introduce the first population study dedicated to these features with the aim to understand their fundamental properties and discuss them in the context of their origin. We complement the study with a spectroscopic database of six low-mass X-ray binaries during outburst, in order to assess their evolution. We find that broad absorptions are ubiquitous, with the majority of black hole low-mass X-ray binaries exhibiting them in spite of a typically scarce outburst coverage. Their detection does not depend on the orbital inclination or the compact object nature, but they seem favoured in systems with orbital periods shorter than < 11 h. They predominantly occur in the hydrogen Balmer series, being stronger at shorter wavelengths, and they are detected across all X-ray states. We find that the normalised depth of these broad absorptions is anti-correlated with the system luminosity, and that they show constant line ratios over the whole sample. Based on these properties, we favour a scenario where BAs arise from a stable, optically thick layer of the accretion disc, below the hotter chromosphere-like region producing the emission line components. Our study is consistent with the continuous presence of broad absorptions during the whole outburst, with their visibility being conditioned by the emission lines filling the broad absorption profile and veiling by the X-ray reprocessed continuum.

Motivation & Objective

• Determine how common broad absorptions (BAs) are in the optical spectra of LMXBs during outbursts.
• Characterize BA properties (profiles, centroids, widths, equivalent widths) across multiple systems.
• Assess correlations with system parameters (orbital period, inclination, compact object type) and outburst state.
• Infer the origin of BAs and their evolution over weeks to months.

Proposed method

• Compile population databases of BH and NS LMXBs with published optical spectra during outburst.
• Classify BA presence as confirmed, candidate, non-BA, or uncharted.
• Construct a six-source spectroscopic database with multi-epoch Balmer-line spectra.
• Fit BA and emission components with a two-component model (double Gaussian emission plus Gaussian absorption).
• Use MCMC (emcee) to estimate parameter posteriors for line fits.
• Analyze BA evolution and search for correlations with outburst state and system properties.

Experimental results

Research questions

• RQ1 What fraction of BH and NS LMXBs show broad absorptions in optical Balmer lines during outburst?
• RQ2 How do BA properties (width, depth, centroid) evolve over outburst, and what do they imply about their origin?
• RQ3 Are BA detections preferentially associated with certain orbital periods, inclinations, or other system parameters?
• RQ4 Do BA features persist across X-ray states, and how might emission-line filling or continuum veiling affect their visibility?
• RQ5 What physical scenario best explains BA formation in the accretion disc context?

Key findings

• Broad absorptions are ubiquitous among studied LMXBs, with the majority of BHs showing BAs and many NSs exhibiting them in at least one epoch.
• BA profiles are best described by Gaussian absorption with a mean width σ_abs = 1400 ± 500 km/s and typical EWs of EW_abs = 4 ± 2 Å, reaching up to ~17 Å in extreme cases.
• Centroids of BA components are close to the systemic velocity, linking them to the accretion disc rather than outflows.
• Normalised BA depth anti-correlates with system luminosity, and line ratios remain roughly constant across the sample.
• BA occurrences are more common in systems with orbital periods <11 h, and detections do not strongly depend on compact object type or inclination within the limitations of the sample.
• A scenario is favored in which BAs arise from a stable, optically thick layer of the disc beneath a hotter chromosphere-like region producing the emission lines.