Claude R. Canizares

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.

High-resolution X-ray spectroscopy provides a powerful probe of the interstellar medium (ISM), giving direct access to the composition and physical state of dust grains and atomic species in dense environments. We present a study of the gas and dust along the line of sight to the bright low-mass X-ray binary GX 340+00, which samples higher-density gases in the ISM. Using deep Chandra/HETG spectra, we characterize X-ray absorption fine structure from dust, gas absorption lines, and the optical depths of the Si, S, and Fe K edges. By fitting these three edges simultaneously, we reduce degeneracies in the dust composition and find that amorphous olivine dominates the fractional contribution among the dust columns (∼65%), followed by metallic iron (∼19%), iron sulfides (pyrrhotite and troilite; ∼10%), and fayalite (∼5%), with the remaining species contributing only a few percent in total. From the inferred stoichiometry, we estimate that ∼74% of Fe is associated with silicates, ∼8% with sulfides, and ∼18% with metallic iron, suggesting that Fe is predominantly incorporated in iron-rich silicate grains along this sight line. We detect S ii absorption and infer a sulfur dust fraction of ∼35%. We also detect absorption structure near the Ca and Ar K edges, highlighting the need for improved atomic photoabsorption data. The Chandra/HETG spectral resolution remains essential to disentangle dust and gas contributions at the Si and S K edges, providing a benchmark for dust characterization in high-density regions in the ISM.

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.

X-ray binaries are known to launch powerful accretion disk winds that can have a significant impact on the binary systems and their surroundings. To quantify the impact and determine the launching mechanisms of these outflows, we need to measure the wind plasma number density, an important ingredient in the theoretical disk wind models. While X-ray spectroscopy is a crucial tool for understanding the wind properties, such as their velocity and ionization, in nearly all cases, we lack the signal-to-noise ratio to constrain the plasma number density, weakening the constraints on the outflow location and mass outflow rate. We present a new approach to determining this number density in the X-ray binary Hercules X-1, by measuring the speed of the wind ionization response to the time-variable illuminating continuum. Hercules X-1 is powered by a highly magnetized neutron star, pulsating with a period of 1.24 s. We show that the wind number density in Hercules X-1 is sufficiently high to respond to these pulsations by modeling the ionization response with the time-dependent photoionization model tpho. We then perform a pulse-resolved analysis of the best-quality XMM-Newton observation of Hercules X-1 and directly detect the wind response, confirming that the wind density is at least 10¹² cm⁻³. Finally, we simulate XRISM observations of Hercules X-1 and show that they will allow us to accurately measure the number density at different locations within the outflow. With XRISM, we will rule out ∼3 orders of magnitude in density parameter space, constraining the wind mass outflow rate, energetics, and its launching mechanism.

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.

High-resolution X-ray spectra of π Aqr, a γ Cas-type star, obtained with the Chandra/HETG spectrometer, revealed emission lines of H-like ions of Mg, Si, S, and Fe; a strong, hard continuum; and a lack of He-like ions, indicating the presence of very hot thermal plasma. The X-ray light curve showed significant fluctuations, with coherent variability at a period of about 3400 s in one observation. The hardness ratio was relatively constant except for one observation in which the spectrum was much harder and more absorbed. We interpret the X-ray emission as arising from accretion onto the secondary, which is likely a magnetic white dwarf, an intermediate polar system.

Knowledge of the dust content in interstellar matter is important to our understanding of the composition and evolution of the interstellar medium. The Chandra High Energy Transmission Grating (HETG) Spectrometer provides a unique opportunity to measure X-ray absorption of interstellar dust and its compositions through the X-ray edge absorption structure. We measure gas and dust optical depths at Si K toward nine bright low-mass x-ray binaries in the Galactic Bulge with very high-precision and pileup-free spectra. We include a likely instrumental feature affecting the Si K edge structure in our analysis. While gas optical depths grow consistently with broadband hydrogen-equivalent columns, the dust optical depths do not. Calculations including dust self-shielding show that the observed dust optical depths can be explained by variations in dust grain columns between various lines of sight. At least three grain column regimes can be identified toward the Galactic Bulge. While grain sizes define the self-shielding effect, variations in grain size distributions do not seem relevant. This shows that the gas-to-dust optical depth ratio toward sources in the Galactic Bulge is not homogeneous. The dust optical depths also roughly correlate with molecular hydrogen columns. Lowly ionized Si K contributions toward the Galactic Bulge were detected but are very small. We also find Si xiii absorption with velocity widths of 800–1100 km s⁻¹, which we attribute to the circumbinary medium.

Hercules X-1 is a nearly edge-on accreting X-ray pulsar with a warped accretion disk, precessing with a period of about 35 days. The disk precession allows for unique and changing sightlines toward the X-ray source. To investigate the accretion flow at a variety of sightlines, we obtained a large observational campaign on Her X-1 with XMM-Newton (380 ks exposure) and Chandra (50 ks exposure) for a significant fraction of a single disk precession cycle, resulting in one of the best data sets taken to date on a neutron star X-ray binary. Here we present the spectral analysis of the high state high-resolution grating and CCD data sets, including the extensive archival data available for this famous system. The observations reveal a complex Fe K region structure, with three emission line components of different velocity widths. Similarly, the high-resolution soft X-ray spectra reveal a number of emission lines of various widths. We correct for the uncertain gain of the European Photon Imaging Camera pn Timing mode spectra, and track the evolution of these spectral components with Her X-1 precession phase and observed luminosity. We find evidence for three groups of emission lines, the first of which originates in the outer accretion disk (10⁵ R _G from the neutron star). The second line group plausibly originates at the boundary between the inner disk and the pulsar magnetosphere (10³ R _G). The last group is too broad to arise in the magnetically truncated disk and instead must originate very close to the neutron star surface, likely from X-ray reflection from the accretion curtain (∼10² R _G).

Supermassive black holes (SMBH) interact with gas in the interstellar and intergalactic media (ISM/IGM) in a process termed “feedback” that is key to the formation and evolution of galaxies and clusters. Characterizing the origins and physical mechanisms governing this feedback requires tracing the propagation of outflowing mass, energy and momentum from the vicinity of the SMBH out to megaparsec scales. Our ability to understand the interplay between feedback and structure evolution across multiple scales, as well as a wide range of other important astrophysical phenomena, depends on diagnostics only available in soft X-ray spectra (10-50 Å). Arcus combines high-resolution, efficient, lightweight X-ray gratings with silicon pore optics to provide R~2500 with an average effective area of ~200 cm², an order of magnitude larger than the Chandra gratings. Flight-proven CCDs and instrument electronics are strong heritage components, while spacecraft and mission operations also reuse highly successful designs.