Speaker
Description
Optical spectroscopy has become a powerful probe of accretion and ejection in outbursting low-mass X-ray binaries. Over the past decade, outflow signatures once confined to X-rays have been routinely identified at optical, near-infrared, and ultraviolet wavelengths, establishing a new framework for investigating accretion-disc winds and their connection to the accretion flow. Canonical optical wind tracers include P-Cygni profiles, broad emission wings, and flat-top line morphologies. However, interpreting these features remains challenging because they are superposed on complex line profiles produced within the accretion disc itself.
We focus on a prominent but long-neglected spectroscopic component: broad absorptions (BAs). These features can engulf the broad, sometimes double-peaked emission profiles arising from the outer regions of a Keplerian disc. Furthermore, they extend to velocities comparable to those of wind signatures, mimicking the blue-shifted absorption component of P-Cygni profiles or suppressing broad emission wings. A systematic characterisation of BAs is therefore essential for robust wind identification.
We present the first population study of BAs, complemented by a time-resolved spectroscopic database of six low-mass X-ray binaries in outburst. We show that BAs are ubiquitous, appearing in most black-hole systems despite limited outburst coverage. Their presence is independent of inclination and compact-object type, but favoured in short-period systems (P_orb < 11 h). They predominantly affect the Balmer series, strengthen toward shorter wavelengths, and persist across all X-ray states. BA profiles are well described by Gaussians with σ_abs = 1400 +- 500 km s⁻¹ and centroid velocities near the systemic velocity, pointing to an accretion-disc origin. Crucially, we also find that the absorption depth is anti-correlated with luminosity, while line ratios remain constant across the sample. Together, these properties support an origin in a stable, optically thick disc layer below a hotter, chromosphere-like emission region.
By establishing their observational fingerprints, we provide new diagnostics to distinguish intrinsic disc absorption from genuine outflows, improving studies of the accretion–ejection connection in accreting compact objects.