Fluorescent Fe K line emission of gamma Cas stars II. Predictions for magnetic interactions and white-dwarf accretion scenarios

TL;DR

The paper investigates the origin of the hard X-ray emission in gamma Cas stars by predicting high-resolution Fe Kα fluorescence line profiles for two leading scenarios: magnetic star-disc interactions and accretion onto a white dwarf (WD), with separate considerations for non-magnetic and magnetic WD cases. The authors develop a fluorescence modeling framework incorporating reprocessing in Be disc structures, Be photospheres, WD discs, and magnetically fueled accretion flows, using TLUSTY NLTE atmospheres and Feautrier radiative transfer to compute line emissivities. They find distinct observational signatures: a non-magnetic WD yields a very broad Fe Kα complex with FWHM around $140$ eV, while magnetic WD and star-disc interactions produce narrower profiles near $40$ eV; centroids respond differently to orbital motion, enabling differentiation with high-resolution spectroscopy. The study demonstrates that upcoming XRISM Resolve and Athena X-IFU observations, at selected orbital phases, can constrain the primary X-ray source and illuminated material, shaping our understanding of gamma Cas stars and Be binary evolution.

Abstract

About 12 percent of the early-type Be stars, so-called gamma Cas stars, exhibit an unusually hard and bright thermal X-ray emission that could result from accretion onto a white dwarf companion or from magnetic interactions between the Be star and its decretion disk. Exploring the full power of high-resolution X-ray spectroscopy of gamma Cas stars requires comparison of observations of the fluorescent Fe Kalpha emission lines near 6.4 keV with synthetic profiles of this line complex computed in the framework of the magnetic interaction and the accreting WD scenarios. For the latter, we further distinguish between accretion onto a non-magnetic and a magnetic WD. Our models account for different reservoirs of reprocessing material: the Be circumstellar decretion disk, the Be photosphere, an accretion disk around the WD companion, a magnetically channelled accretion flow and the WD photosphere. We find considerably different line properties for the different scenarios. For a non-magnetic accreting WD, the global Fe Kalpha complex is extremely broad, reaching a full width of 140 eV, whilst it is ~ 40 eV for the magnetic star-disk interaction and the magnetic accreting WD cases. In the magnetic star-disk interaction, the line centroid follows the orbital motion of the Be star, whereas it moves along with the WD in the case of an accreting WD. For gamma Cas, given the 15 times larger amplitude of the WD orbital motion, the shift in position for an accreting WD should be easily detectable with high-resolution spectrographs such as Resolve on XRISM, but remains essentially undetectable for the magnetic star-disk interaction. Upcoming high-resolution spectroscopy of the fluorescent Fe Kalpha emission lines in the X-ray spectra of gamma Cas stars will thus allow to distinguish between the competing scenarios.

Figures (12)

• Figure 1: Upper box: Schematic pole-on view of the Be star (blue dot), its decretion disc and spiral extensions (sky blue), the WD companion, and its truncated accretion disc (magenta). The dark blue contour shows the Roche lobe of the system. The boxes at the bottom provide zooms on vertical cuts through the inner part of the Be decretion disc (left) and the surroundings of the WD (shown by the violet dot) with its magnetically truncated accretion disc (right). The red crosses correspond to the emitting regions of the primary X-rays. The left box illustrates the case of the magnetic star-disc interaction with magnetic field lines that connect the star and the disc. The right box corresponds to an accreting magnetic WD scenario where a magnetically channelled flow connects the truncated accretion disc to the magnetic poles of the WD.
• Figure 2: Normalised individual Fe K$\alpha$ line profile contributed by the Be decretion disc assuming an extended source represented by two rings of hot plasma of radius $1.5\,R_*$ and located $0.5\,R_*$ above or below the Be decretion disc. The line profile is binned into velocity bins of 50 km s$^{-1}$ and is normalised by dividing by the total flux over the entire line profile. The spectral energy distribution of the illuminating source is an optically thin thermal plasma model with $kT = 12.5$ keV. The red histogram corresponds to an axisymmetric disc truncated at $r = 176$ R$_{\odot}$. The blue histogram yields the results for a disc with spiral extensions as described in the text and computed out to $a = 375$ R$_{\odot}$. The normalised profiles are displayed in velocity bins of 50 km s$^{-1}$.
• Figure 3: Top : EW of the fluorescent Fe K$\alpha$ line contributed by the Be decretion disc in the star-disc interaction scenario as a function of $R_s$ for different values of $Z_s$. Bottom : EW contours for the same scenario as a function of $R_s$ and $Z_s$. The primary source is modelled as a uniform axisymmetric ring of radius $R_s$ from the polar axis of the Be star and of altitude $Z_s$ above the equatorial plane of the Be star. The hatched area in the bottom part corresponds to the scale height of the Be decretion disc.
• Figure 4: Rosseland optical depth inside the photosphere where the optical depth for photoelectric absorption of X-rays reaches unity. The continuous red line illustrates the results for the $T_{\rm eff} = 25\,000$ K, $\log{g} = 3.75$ TLUSTY model adopted for the Be star. The short-dashed cyan line and the long-dashed blue line correspond respectively to DA and DO WD models with $T_{\rm eff} = 60\,000$ K and $\log{g} = 8.5$.
• Figure 5: Individual Fe K$\alpha$ line profile contributed by the Be photosphere. The illuminating source is an extended ring-like source located at $R_s = 1.5\,R_*$ and $Z_s = 0.5\,R_*$. The spectral energy distribution of the illuminating source is an optically thin thermal plasma model with $kT = 12.5$ keV.