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HOney-BeeS II. Be-X-ray binaries as testbeds for spectroscopic studies of Be stars

S. Janssens, H. Sana, T. Shenar, J. Bodensteiner, M. Abdul-Masih

TL;DR

Be-X-ray binaries provide fixed X-ray orbital benchmarks to test spectroscopic RV methods in Be stars, where disc variability and rapid rotation complicate traditional analyses. The study compares cross-correlation, bisector, and line-profile fitting on absorption and emission lines using seven BeXRBs with high-resolution multi-epoch spectra, finding cross-correlation to be the most robust and emission lines (preferably Hβ) to yield higher RV precision than absorption lines. Large-scale disc variability (e.g., V/R changes, blue broadening) can spuriously shift RVs and mask true orbital motion, making independent orbital solutions challenging in several systems. The work emphasizes the need to understand companion–disc interactions and to tailor analysis to each system, using emission lines for population statistics while cautiously interpreting individual orbital solutions. The results provide mass-function-based constraints on companions in some targets and highlight how disc dynamics can influence inferred binary properties, informing future spectroscopic campaigns and models of Be-star binaries, with practical implications for Be population studies and compact-object progenitor demographics. $f_m = rac{m_c^3 \, ext{sin}^3 i}{(m_{Be}+m_c)^2} = rac{P K_{Be}^3}{2\pi G} (1-e^2)^{3/2}$ for gravitationally bound Be-star binaries is used to discuss companion masses when orbital elements are available.$

Abstract

The majority of massive classical Be stars are thought to be binary interactions products. Their rapid rotation and often strong, variable, and emission-line dominated spectrum, make spectroscopic analysis challenging. Hence, robust binary properties and statistical constraints are still lacking for the Be population. In this study, we use seven Be-X-ray binaries, and their orbital periods derived from the X-rays, to investigate the reliability of different spectral lines and numerical methods for the measuring of radial-velocities and orbital period determination of Be stars. We use multi-epoch high-resolution HERMES spectra and compare absorption- and emission-line radial velocities obtained with cross-correlation, line-profile fitting, and the bisector method. Line-profile variability affects the bisector method and line-profile fitting requires model templates that do not encompass the complexity of Be-star line profiles. Therefore, we recommend using cross-correlation: it is independent of models and easily compatible with line blends seen in Be-star spectra. The obtained statistical uncertainty on the radial-velocities from cross-correlation is 0.2-0.3km/s for H$α$ (emission) and ~5km/s for absorption lines, excluding potential systematics. In general, whether the goal is to do binary statistics of a population or an in-depth study of a specific system, we suggest using emission lines, due to a higher precision and less scatter than absorption lines. Here, H$β$ is preferred over H$α$ because of its lesser variability. However, large-scale variability may cause large shifts in emission-line radial velocities, resulting in spurious eccentricities. In this case, orbital solutions should ideally be compared to lower-signal absorption lines (if present). Finally, we highlight the need for understanding how companion-disc interactions alter emission-line appearance.

HOney-BeeS II. Be-X-ray binaries as testbeds for spectroscopic studies of Be stars

TL;DR

Be-X-ray binaries provide fixed X-ray orbital benchmarks to test spectroscopic RV methods in Be stars, where disc variability and rapid rotation complicate traditional analyses. The study compares cross-correlation, bisector, and line-profile fitting on absorption and emission lines using seven BeXRBs with high-resolution multi-epoch spectra, finding cross-correlation to be the most robust and emission lines (preferably Hβ) to yield higher RV precision than absorption lines. Large-scale disc variability (e.g., V/R changes, blue broadening) can spuriously shift RVs and mask true orbital motion, making independent orbital solutions challenging in several systems. The work emphasizes the need to understand companion–disc interactions and to tailor analysis to each system, using emission lines for population statistics while cautiously interpreting individual orbital solutions. The results provide mass-function-based constraints on companions in some targets and highlight how disc dynamics can influence inferred binary properties, informing future spectroscopic campaigns and models of Be-star binaries, with practical implications for Be population studies and compact-object progenitor demographics. for gravitationally bound Be-star binaries is used to discuss companion masses when orbital elements are available.$

Abstract

The majority of massive classical Be stars are thought to be binary interactions products. Their rapid rotation and often strong, variable, and emission-line dominated spectrum, make spectroscopic analysis challenging. Hence, robust binary properties and statistical constraints are still lacking for the Be population. In this study, we use seven Be-X-ray binaries, and their orbital periods derived from the X-rays, to investigate the reliability of different spectral lines and numerical methods for the measuring of radial-velocities and orbital period determination of Be stars. We use multi-epoch high-resolution HERMES spectra and compare absorption- and emission-line radial velocities obtained with cross-correlation, line-profile fitting, and the bisector method. Line-profile variability affects the bisector method and line-profile fitting requires model templates that do not encompass the complexity of Be-star line profiles. Therefore, we recommend using cross-correlation: it is independent of models and easily compatible with line blends seen in Be-star spectra. The obtained statistical uncertainty on the radial-velocities from cross-correlation is 0.2-0.3km/s for H (emission) and ~5km/s for absorption lines, excluding potential systematics. In general, whether the goal is to do binary statistics of a population or an in-depth study of a specific system, we suggest using emission lines, due to a higher precision and less scatter than absorption lines. Here, H is preferred over H because of its lesser variability. However, large-scale variability may cause large shifts in emission-line radial velocities, resulting in spurious eccentricities. In this case, orbital solutions should ideally be compared to lower-signal absorption lines (if present). Finally, we highlight the need for understanding how companion-disc interactions alter emission-line appearance.

Paper Structure

This paper contains 38 sections, 4 equations, 19 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Sample selection and observations
  3. Sample selection
  4. Observations
  5. Methods for radial velocity measurements
  6. Cross correlation
  7. Bisector
  8. Line-profile fitting
  9. Uncertainties and intrinsic variability
  10. Benchmarking methods and diagnostic lines
  11. Used methods and diagnostic lines
  12. The stability of the methods and diagnostic lines
  13. Searching for periodicities and orbital solutions
  14. Group A: strong absorption lines
  15. Group S: stable emission lines with weaker absorption
  16. ...and 23 more sections

Figures (19)

  • Figure 1: Radial velocities obtained with different methods on different spectral lines for V420 Aur (top row) for one night, HD 259440 (middle row) for all nights with three consecutive spectra, and V615 Cas (bottom row) for all nights with three consecutive spectra. The x-axis shows the time spectra were taken since the first spectra shown. Left: hydrogen lines and CC. Middle: non-hydrogen lines and CC. Right: hydrogen lines and lpf and bisector. In the legend, 'Fe' refers to the Fe ii emission lines in the region of 5200$\AA$.
  • Figure 2: Ratio of $\mathrm{rms}_\mathrm{short}$ and $\bar{\sigma}$ for different spectral lines (indicated on the x-axis) for V420 Aur (circles), HD 259440 (squares), and V615 Cas (triangles). Different methods are indicated with different edge colors. The color map in the symbols indicates the value of $\mathrm{rms}_\mathrm{short}$. A value of 1 for $\mathrm{rms}_\mathrm{short}/\bar{\sigma}$ is shown with a solid line.
  • Figure 3: Phase-folded RVs and fits of V615 Cas with an orbital period of 26.506 d and T0 = 10494.39. Left panel: different spectral lines, excluding H$\alpha$ emission, and the derived H$\beta$ RV curve. Right panel: H$\beta$ and the H$\alpha$ emission component with both RV curves. Darker-blue triangles show H$\alpha$ with a 5 d difference in T0 (as listed in Table \ref{['table_orbital_V615_Cas']}). Grey stars show the RVs obtained from H$\beta$ emission, also with a 5 d shift in T0.
  • Figure 4: Radial velocities for EM* VES 826. Vertical dashed lines indicate a 155 d period starting from the first data point.
  • Figure 5: Phase-folded RVs and fits for HD 259440 on a 317.3 d period. Left panel: RVs from non-He lines and the derived H$\beta$ RV curve. Right panel: original (not fixed to the T0 of H$\beta$) RV fit for He i em and He i abs, with the fit of H$\beta$ as a reference. The grey arrow indicates a data point with RV $>$ 100 km s$^{-1}$ and falls outside the range of the plot (25 km s$^{-1}$).
  • ...and 14 more figures