Sean J. Gunderson

We present a multiwavelength analysis of the star ρ Oph C, covering X-ray, optical, near-infrared (NIR), and radio observations to test for the presence of interferometrically detected cool star companions. The X-ray observations from Chandra, XMM-Newton, and NuSTAR show flare-like events lasting minutes or days in time and have spectral properties consistent with those of cooler stars instead of the primary magnetic B star. The NIR data are also consistent with the proposed trinary system properties due to the presence of CO bandheads. After removing the primary B star’s rotation, the optical data from TESS show a residual signal at the level of 0.8 mmag, matching the variations expected from cool stars in orbit around a bright B star. Finally, multiepoch radio data reveal that ρ Oph C exhibits significant large-scale flux variations that are atypical for a magnetic massive star. Taken as a whole, the multiepoch and -wavelength data present a consistent picture of ρ Oph C as a rare trinary system composed of a hot star and two cool stars.

Context. Westerlund 1 (Wd 1) is the closest supermassive star cluster to the Sun, containing more than 100 000 stars of all spectral types and masses down to brown dwarfs. This population considerably heats the surrounding interstellar medium (ISM), making Wd 1 a relevant place to investigate stellar feedback on intracluster gas. Aims. We present the most detailed X-ray study to date of the diffuse emission in this region, aimed at providing the most favourable observational configuration for distinguishing and quantifying the point-source contribution to the true diffuse emission. Methods. We analysed 36 Chandra ACIS-I observations of Wd 1 within an 8 × 8 arcmin window. After removing 4922 point sources with ACIS Extract, using energy-dependent point spread function (PSF) models and adaptive 99% enclosed-energy masks, we subtracted PSF-wing contamination from overlapping neighbours. It is critical in this crowded field to correct for background. We applied adaptive smoothing to reveal hot gas between stars in the soft (0.5–1.2 keV), medium (1.2–1.9 keV), and hard (1.9–7.0 keV) bands. Results. The spectral fitting suggests a shocked thermal plasma, with soft emission extending beyond the stellar core and diffuse X-rays correlating spatially with the massive stars. The cluster core (region #2) is best described by an APEC+PSHOCK model, capturing the continuous range of post-shock ionisation timescales produced by the many coexisting stellar wind shocks; it is characterised by collisionally excited plasma temperatures of 1.81 ± 0.24 and 4.11 ± 1.98 keV and the 6.7 keV FeK _α emission line. The outer region (Region #1) exhibits charge exchange emission (CXE) and is best described by a CXE+NEI model, with softer temperatures of 0.27 ± 0.04 and 1.09 ± 0.05 keV. Absorption column densities ( N _H ) range between 1.84 and 2.24 ×10 ²² cm ⁻² and the total (0.5–8.0 keV) diffuse luminosity of both regions is ≃9.6 (±0.3)×10 ³³ erg s ⁻¹ . Conclusions. Near the core, the hard emission likely originates from thermalised Wolf-Rayet (WR) stellar winds and intense wind–wind collisions in regions of high massive-star density, while the adiabatic expansion of the hot plasma and turbulent mixing with the denser, cooler ISM ends up softening it. The soft diffuse component extends farther out and may be associated with CXE processes, spatially correlated with regions of lower extinction. No hint of a non-thermal contribution is revealed. The analytical cluster wind model, using the observed WR population, predicts L _X ^CWM ≃ 2.1 (±0.10 × 10 ³⁴ erg s ⁻¹ , a factor of ∼2.2 above the observed luminosity, consistent with the low-density, partially evacuated intracluster medium (ICM) inferred from the X-ray and infrared morphology. This low-density morphology at the cluster centre suggests that ultraviolet (UV) radiation can clear the ambient medium, producing an apparent outward displacement that likely reduces the L _w -to- L _X conversion efficiency, measured as η = L _X / L _w ≃ 4.3 × 10 ⁻⁶ , approximately two orders of magnitude below that of the Cygnus OB2 association and systematically lower than every other massive star-forming region compiled here, resulting in comparatively subluminous diffuse X-ray emission.

We present a comprehensive analysis of the X-ray spectrum of the highly obscured high-mass X-ray binary IGR J16318−4848 based on ∼250 ks Chandra/HETGS observations covering different orbital phases. We detected a strong narrow Fe K α emission line and interpret our finding as the iron line originating in the stellar wind close to the compact object. We tentatively identify a Compton shoulder associated with the Fe K α line. The ratio of the Compton-shoulder flux to that of the Fe K α core exhibits orbital-phase variability, reaching a maximum of ∼10% around orbital phase ϕ _orb ≃ 0.33. In addition, we do not detect ionized lines, implying that the plasma in the system is predominantly cold. We preliminarily report that the Fe K α line centroid exhibits an orbital modulation with an amplitude of 167 ± 32 km s ⁻¹ , adopting the best-known circular orbit in the source. This study presents the first detailed view of the high-resolution X-ray spectra of IGR J16318−4848 across different orbital phases, providing timely context for the newly taken XRISM observations of the source.

A subset of Be stars, typified by the naked-eye star γ Cas, exhibits unusually bright and hard X-ray emission, the origin of which has remained debated for five decades. We performed high-resolution X-ray spectroscopic monitoring of γ Cas with the Resolve instrument aboard the X-Ray Imaging and Spectroscopy Mission (XRISM). X-ray lines from the ultra-hot plasma and fluorescence from cooler material exhibit Doppler shifts consistent with orbital motion, not of the Be star itself, but of its low-mass companion (previously shown to be a white dwarf). This first evidence of orbital motion for the hard X-ray emitting plasma uniquely links it to the scenario of accretion onto the white dwarf companion. The modest line broadening further indicates that fluorescence occurs on the white dwarf surface and excludes X-ray generation in the inner parts of an accretion disc. Our findings identify γ Cas and its analogues as the previously elusive, but long predicted class of binaries composed of a Be and a white dwarf. Identifying the origin of the hard X-rays from γ Cas and its analogues, which represent about 10% of early-type Be stars, provides a key input for population synthesis models of massive binary evolution.

We present a multiwavelength analysis of the central stellar pair of ρ Oph components A and B. Using recent high-resolution Chandra X-ray Observatory observations, we demonstrate with high confidence that the dominant X-ray source is ρ Oph B, while ρ Oph A is comparatively X-ray faint. This result contrasts with earlier XMM-Newton observations, which, due to limited spatial resolutions, attributed the X-ray emission to ρ Oph A. An analysis of ρ Oph B’s X-ray light curves and spectra reveals properties more consistent with a cool star than a hot star. We therefore propose that ρ Oph B is an Algol-like binary system, consisting of a B-type primary and an active, X-ray-emitting GK-type companion.

We present the first first X-ray polarization measurements of a white dwarf (WD), the intermediate polar EX Hya. We measured significant polarization only in the 2–3 keV energy band with a polarization degree of 8% at a 3 σ significance. No significant polarization was detected above 3 keV, which we attribute to the higher energy bands having lower signal to noise. We found that the scattering surface detected by the Imaging X-ray Polarimetry Explorer appears to be nearly perpendicular to the optical scattering plane, showing that the X-ray scattering surface is the WD and close to the base of the accretion column. Finally, we show how the polarization can be used to estimate the height of the accretion shock above the WD’s surface.

We present a 197 ks HETG and 95 ks NuSTAR spectra of the γ Cas-type object V750 Ara. The high-resolution X-ray spectra show that the target is similar to other objects of this class. Data are interpreted under the assumption that the X-rays come from an accreting white dwarf, and our analysis implies an accretion rate of about 3 × 10 ⁻¹¹ M _⊙ yr ⁻¹ . Emission lines are weak and predominantly from hydrogen-like ions: Mg XII , Si XIV , and S XVI . H-like and He-like Fe are both present, but Fe K fluorescence is weak, being significantly detected only in the NuSTAR spectrum, but it was not obviously detected in the HETG dispersed or zeroth-order spectra. The flux was variable above a level expected by Poisson statistics. There were no significant changes in the spectral hardness, though we are limited by lack of soft signal below 1 keV. Emission lines of Mg and Si were strong enough to measure velocity offsets and widths that were found to be marginally inconsistent. The H-like Mg line is consistent with instrumental broadening only but shows a 300 km s ⁻¹ blueshift. He-like Mg and H-like Si lines have no significant shift in velocity but are broadened by about 1000 km s ⁻¹ . This suggests either different physical origins or velocity structure differing with plasma temperature.

We present theoretical arguments toward the plausibility of a stellar wind to explain the 16,000 km s⁻¹ line broadening in the optical spectra of WS 35, the central star in the Pa 30 nebula. The wind model is discussed in the context of super-Eddington flows. We argue that WS 35 potentially occupies a new regime of wind driving theory as the first metal-only wind. This framework provides a promising avenue for explaining the high-speed flow. We further describe how future radio observations can provide an independent test of the spherical wind scenario. A magnetically channeled wind would likely produce a relatively flat and bright radio spectral energy distribution. By contrast, a spherical wind should result in a thermal radio spectrum with a canonical continuum slope of ν ^0.6, and a brightness level consistent with the currently predicted mass-loss rate.