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

TL;DR

This paper addresses broad absorptions (BAs) observed in the optical Balmer lines of low-mass X-ray binaries (LMXBs) during outbursts, revealing their ubiquity and characteristic properties. Using a population database split into BH and NS systems and a dedicated six-source spectroscopic dataset, the authors show BAs are common across BHs and NSs, favoring short orbital periods with a threshold near $P_{ m orb} \sim 11\,\mathrm{h}$, and appear independent of orbital inclination. BA depths anti-correlate with optical luminosity, while line ratios remain constant, suggesting a stable, optically thick layer in the outer accretion disc that is veiled by the X-ray reprocessed continuum and emission-line filling. The proposed disc-origin scenario aligns with several CV-derived models but also accommodates the lack of strong inclination dependence and possible contributions from outflows, highlighting the BA formation region as a persistent, geometry-sensitive facet of disc physics in LMXBs.

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.

Figures (16)

• Figure 1: Optical spectra from our database, zoomed into the Balmer $\rm H\delta$, $\rm H\gamma$ and $\rm H\beta$ transitions. They all show BAs with cores partially filled by the emission line. From bottom to top: J1807 (epoch 3 of dataset A, blue), J1727 (epoch 25, red), J1118 (epoch 1, green) and J1753 (epoch 2, magenta). Telluric bands and DIBs are shown as grey and yellow-shaded regions, respectively. The rest wavelength of the transition is marked with a black-dashed line, while blue-dotted lines mark velocity shifts of $\pm 2500\, \rm{km\,s^{-1}}$ as a visual reference.
• Figure 2: $b$ against $N_{\rm epoch}$ for the BH (left) and NS (right) population. Cyan symbols refer to systems with detected BA features (crosses for candidates, filled circles for confirmed systems), black symbols to non-BA systems (crosses for candidates, filled triangles for confirmed systems) and red dots to systems without available spectroscopic data (aka uncharted).
• Figure 3: $P_{\rm orb}$ against $b$ for the BH (left) and NS (right) population. The colour and symbol description matches that of Fig. \ref{['fig:popepoch']}. The black-dashed line marks the $11\,{\rm h}$ threshold described in the text.
• Figure 4: $P_{\rm orb}$ against $i$ for the BH population. The colour and symbol description matches that of Fig. \ref{['fig:popepoch']}. Dashed lines mark the thresholds at $30^ {\circ}$ and $60^ {\circ}$ defining the low, mid and high inclination regions. If only constraints to the $i$ parameter are available, we plot the central value and use the range as the associated uncertainty.
• Figure 5: Left panel: $EW_{\rm abs,H\delta}$ against r-band brightness (left axis for flux density units, $f_{\rm \nu}$; right axis for absolute magnitudes, $M_{\rm r}$) from the best fitting results of the subsample with flux-calibrated spectra. The colour code mark the systems associated with each data point. A black, dashed line depicts the best linear fit to the correlation, being cyan-dashed, red-dashed and black-dashed lines the equivalent for individual datasets. Right panel: $EW_{\rm abs,H\beta}$ against $EW_{\rm abs,H\gamma}$ from the best fitting results of the whole spectroscopic sample. The colour code marks the systems associated with each data point, while symbols differentiate between subsets as described in the legend. A black, dashed line depicts the best linear fit to the correlation, consistent with a 1:1 ratio. Transparent symbols correspond to fitting parameters consistent with null BA within $3\sigma$.