Low-mass X-ray binaries are systems in which a star transfers matter onto a compact object—either a black hole or a neutron star—producing energetic outbursts. During these events, their optical spectra provide a way to study extreme processes of accretion and matter ejection. While some spectroscopic features have been analysed in detail (e.g., revealing disc expansion and the presence of optical winds), the appearance of broad absorptions in the optical regime has traditionally been neglected.

In this work, we present the first systematic study of these broad absorptions. We carry out the first population study dedicated to these features, with the aim of understanding their origin and fundamental properties. We complement this with a spectroscopic database of six low-mass X-ray binaries during outburst, allowing us to track their evolution. We find that these features are very common: 78% of black hole systems and 29% of neutron star systems with at least three spectroscopic epochs exhibit them. These values represent firm lower limits, especially given the transient visibility of broad absorptions. Their presence does not depend on the system’s orientation or on the nature of the compact object, and they are mainly observed in hydrogen lines.

Based on their properties, we propose that they form in a relatively cool, optically thick layer of the disc, located below a hotter, optically thin region that produces the traditional emission lines. Our results suggest that these broad absorptions are present throughout the entire outburst, although their visibility is affected by the bright X-ray reprocessed continuum (which veils these features) and by the relative strength of the emission lines that partially fill their cores.