David P. Huenemoerder

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.

The Chandra Interactive Analysis of Observations (CIAO) software, developed by the Chandra X-ray Center, has been the data analysis package for the Chandra X-ray Observatory since its launch in 1999. Over nearly three decades, CIAO has grown from a small software suite into a widely used system for X-ray data analysis and beyond. CIAO provides tools for calibration, spectral, imaging, and timing analysis, together with high-level scripts and the Sherpa modeling and fitting application. Its modular design and unified data model allow users to build flexible analysis workflows while maintaining consistency with the Chandra data processing pipeline. Visualization capabilities are provided through integration with SAOImageDS9 and Python-based tools, and simulation components such as ChaRT and MARX extend the analysis environment to include detailed modeling of instrumental effects. In this paper, we describe CIAO’s design, evolution, and capabilities after 25 yr of Chandra operations. We also describe its core architecture, scripting environment, modeling, visualization tools, simulation components, and testing infrastructure, as well as the documentation and user support system that have contributed to its widespread use. CIAO’s continued development and broad adoption highlight its important role in X-ray astronomy and its usefulness in multiwavelength astrophysical research.

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.

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 investigated the temporal and spectral features of γ Cassiopeiae’s X-ray emission within the context of the white dwarf (WD) accretion hypothesis. We find that the variabilities present in the X-ray data show two different signals, one primarily due to absorption and the other due to flickering like in nonmagnetic cataclysmic variables. We then use this two-component insight to investigate previously unreported simultaneous XMM and NuSTAR data. The results of model fitting find WD properties consistent with optical studies alongside a significant secondary, thermal source. We propose a secondary shock between the Be decretion disk and WD accretion disk as the source. Finally, we analyzed a unique, low-count-rate event of the XMM light curve as potential evidence for the WD encountering Be decretion disk structures.

We present conditions for which X-ray spectra can be “unfolded” to present an accurate representation of the true source spectra. The method we use to unfold the data is implemented in the Interactive Spectral Interpretation Software and distinguishes itself as being model-independent. We find that this method of unfolding makes accurate representations of the true source spectra: (1) The detector is high-resolution, and (2) The spectrum is not steeply sloped. These criteria are not simple conditions that give concrete determinations; each detector and spectrum must be judged individually. We find that both grating and imaging detectors can be unfolded with minimal distortions as compared to both continuum and local spectral features, the latter CCD detectors being much more energy dependent. We also provide example use cases for unfolding in the context of current generation X-ray observatories and important caveats.

The Orion Nebula Cluster (ONC) is the closest site of very young (∼1 Myr) massive star formation The ONC hosts more than 1600 young and X-ray bright stars with masses ranging from ∼0.1–35 M _⊙. The Chandra HETGS Orion Legacy Project observed the ONC with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 2.1 Ms. We describe the spectral extraction and cleaning processes necessary to separate overlapping spectra. We obtained 36 high-resolution spectra, which include a high-brilliance X-ray spectrum of θ ¹ Ori C with over 100 highly significant X-ray lines. The lines show Doppler broadening between 300 and 400 km s⁻¹. Higher spectral diffraction orders allow us to resolve line components of high Z He-like triplets in θ ¹ Ori C with unprecedented spectral resolution. Long-term light curves spanning ∼20 yr show all stars to be highly variable, including the massive stars. Spectral fitting with thermal coronal emission line models reveals that most sources show column densities of up to a few times 10²² cm⁻² and high coronal temperatures of 10–90 MK. We observe a bifurcation of the high-temperature component where some stars show a high component of 40 MK, while others show above 60 MK, indicating heavy flaring activity. Some lines are resolved with Doppler broadening above our threshold of ∼200 km s⁻¹, up to 500 km s⁻¹. This data set represents the largest collection of HETGS high-resolution X-ray spectra from young pre-main-sequence stars in a single star-forming region to date.