Katja Pottschmidt

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.

Modeling the observed emission from highly magnetized accreting neutron stars is essential for interpreting their X-ray spectra and variability. This task is challenging due to the dynamic, multidimensional nature of the accretion columns and the strong gravitational field. Radiation propagation outside the column, within the neutron star's gravitational field, significantly shapes the observed emission and its variability with rotational phase. Geometry -- the location of the magnetic poles and the observer’s inclination -- is one of the key factors influencing the observables due to the anisotropy of the problem. In this work, we study how visibility effects in the gravitational field of the neutron star influence the direct emission from an accretion column, highlighting their importance when interpreting plasma parameters from observed spectra. Building on the physical model from the previous paper in this series, we investigate the directly observable X-ray flux from one and two columns with fan-beam wall emission, down-boosted by the bulk flow, focusing on the impact of geometry. We find that the observed flux strongly depends on the geometrical setup, which we illustrate across various observables. Particularly notable are special geometries in which a column on the far side of the neutron star aligns with the line of sight. In these cases, shadowing and strong light bending produce a narrow intense peak in pulse profiles at soft energies and a pronounced dip at higher energies (≳ 20, ). The combination of shadowing and height dependence of emission also leads to spectral softening. Flux-derived luminosities can be overestimated by up to a factor of 10. The fundamental cyclotron resonant scattering feature (CRSF) appears predominantly in emission across geometries, and we discuss possible causes. keV Our results demonstrate that geometry has a major impact on pulse profiles, phase-averaged spectra, phase–energy maps, and the anisotropy factor relevant for luminosity estimates. Future studies should include the reflected component in addition to the direct emission considered here, and account for a more accurate treatment of resonant redistribution in the CRSF-forming region.

Accretion-powered X-ray pulsars show complex spectral and timing behavior that reflects the extreme conditions near the neutron star. Modeling these observables remains challenging, particularly at high mass-accretion rates, dot M 17 , , ^ g s -1 , where the extended accretion column plays a role in shaping anisotropic emission seen by a remote observer. Here, we present a modular framework that links radiation processes inside the column to the emission observed at infinity. The framework combines existing models for continuum and cyclotron resonant scattering feature (CRSF) formation in sidewall column emission with ray tracing, thereby bridging the gap between the rest-frame emission and the observable flux. We model the radiation from the accretion column using continuum emission from the column walls, calculated in the regime of saturated Comptonization in the presence of a radiation-dominated shock. This emission then passes through a thin outer layer with a fast-moving bulk flow, where CRSFs, derived from Monte Carlo simulations, are imprinted. Finally, a geometry-independent ray-tracing code accounts for light bending in the Schwarzschild metric, incorporating the shape and location of the emitting regions and neutron star rotation to compute the observed phase- and energy-dependent flux. We present the height-dependent anisotropic emission in the column rest frame. For the chosen model, approximately 70% of the radiation is emitted within 1, of the base of the column, predominantly at large angles to the magnetic field. Relativistic boosting in the bulk flow affects the appearance of CRSFs in the spectra, leading to a strong dependence of the line locations on the angle relative to the magnetic field. The proposed theoretical framework allows a straightforward combination of internal emission models with ray tracing to study the observed flux. Here, we focus on the joint treatment of the two-dimensional column structure and CRSF formation in the neutron star rest frame. The observed phase and spectral variations of the flux are highly sensitive to the location of the emitting regions and the observer's viewing angle, which is explored in an accompanying paper.

The supermassive binary system, η Carinae, is experiencing enormous wind-driven mass loss at a rate unparalleled in the rest of the Galaxy. Their wind–wind collision (WWC) continuously produces shock heated, X-ray-emitting plasmas. The XRISM X-ray observatory observed the system in 2023 and 2024 when the X-ray emission began to increase toward periastron passage in 2025. This paper reports unprecedentedly high-resolution X-ray spectra in the Fe K α band between 6.2 and 7.1 keV, obtained with the Resolve X-ray microcalorimeter. The hydrogen-like (Ly α ) and helium-like (He α ) lines reveal three velocity components. Two of them are broadened with maximum velocities of 2000–3000 km s ⁻¹ , likely originating from the postshock companion wind. The other is relatively narrow, with a Gaussian broadening of only ∼290 km s ⁻¹ in 1 σ , which may originate from the postshock companion wind at the WWC stagnation point or penetrating the primary wind. The Fe fluorescent lines exhibit a moderate blueshift and broadening with velocities at 100–200 km s ⁻¹ , consistent with the primary wind’s velocity field. The spectra also confirm a Compton shoulder of the He α line complex for the first time. Both fluorescing and scattering spectral profiles indicate that the binary system is seen from the companion side during these observations. The flux ratio of the Compton-scattering emission to the fluorescent line suggests substantial hydrogen depletion of the primary wind, expected from CNO-cycled hydrogen nuclear fusion gas.

The 2023/2024 NICER monitoring campaign of the 7 Crab bright black hole X-ray binary Swift J1727.8–1613 covered the outburst in almost all accretion states. High-quality data are available in the high-Eddington-fraction hard-intermediate state, the hard-to-soft transition, the soft state, and the poorly studied back transition to the dim hard state, making it an ideal dataset for comparing the accretion flow at vastly different accretion rates. We apply disk continuum fitting techniques to investigate the evolution of the inner-disk radius throughout the outburst. Taking a temperature-dependent color-correction factor into account, we see an evolution of the disk inner radius by a factor of a few comparing the hard states to the thermal/soft state. We tentatively detect an onset of disk truncation in the soft-to-hard transition, right after the source leaves the soft state. After accounting for model systematics, we find the disk to be more truncated in the high-luminosity bright hard state compared to the low-luminosity dim hard state.

Context. The X-ray spectra of neutron stars with moderate magnetic fields ( B ∼ 10 ¹² G) in high-mass X-ray binaries (HMXBs) at low X-ray luminosities ( L _X ≲ 10 ³⁵ erg s ⁻¹ ) are characterized by a double humped shape. This shape has been explained either as the radiation from a two-temperature magnetized atmosphere, where thermal radiation dominates at soft X-rays below about 10 keV, and cyclotron radiation with an imprinted cyclotron line dominates at high energies, or by the complex redistribution of primary X-rays in a structured atmosphere. Aims. The theoretical explanations of the double humped structure predict the spectra to depend on the magnetic field. We aim to connect the model predictions with observations. Methods. We analyzed archival NuSTAR observations of four HMXBs consisting of a neutron star and a Be star (BeXRBs), with known magnetic fields at luminosities low enough to show the characteristic double-hump spectrum. We modeled these spectra empirically and derived a relation between the energy of the intersection of the two humps and the magnetic field strength. In a second step, we tested whether this correlation is supported by fitting synthetic spectra simulated with the physically self-consistent polcap model. Results. We find a linear correlation between the magnetic field strength and the intersection energy for the real BeXRB NuSTAR spectra and polcap -based simulated NuSTAR spectra alike. Conclusions. The effect of the magnetic field on spectral formation results in an observable correlation between the field strength and spectral shape. This derived positive correlation between intersection energy and magnetic field strength also allowed us to roughly estimate the magnetic field strength via our proposed 2-B-12 rule. Additional observations of XRBs and dedicated modeling efforts will be necessary to determine whether this approach is valid beyond the B -field range of a few 10 ¹² –10 ¹³ G that was tested in this work.

We report on a pair of X-ray Multi-Mirror Mission (XMM-Newton) observations of the Supergiant X-ray binary 4U 1909+07, which were performed on 2021 October 3 and 8, respectively. We measure the neutron star rotation period in each observation to be ∼602.62 s. This continues a long spin-up trend that has persisted since 2001 where the neutron star spin period was found to be ∼604.66 s. In our timing analysis, we observe strong variations in the amplitude of the 1–10 keV pulse profile as a function of time, and for the first time, we find a low flux interval extending for a single pulse period in which pulsations are no longer detected. We interpret this low flux interval as a pulse dropout similar to those observed in Vela X-1 and GX 301-2, which were each explained by a low-density cavity in the wind driving the propeller effect. In our time-resolved spectral analysis, we observed the spectral continuum, which can be described as an absorbed power law modified by a high-energy cutoff, to significantly soften during the pulse-dropout phase. No evidence of an increasing absorption column density was found. The observed softening in 4U 1909+07 also supports an interpretation that the observed pulse dropout may be driven by the propeller effect, but the quasi-spherical settling accretion regime cannot be ruled out.

The XRISM/Resolve microcalorimeter directly measured the gas velocities in the core of the Virgo Cluster, the closest example of active galactic nucleus (AGN) feedback in a cluster. This proximity allows us to resolve the kinematic impact of feedback on scales down to 5 kpc. Our spectral analysis reveals a high velocity dispersion of σ v = 26 2 − 38 + 45 km s ⁻¹ near the AGN, which steeply declines to ∼60 km s ⁻¹ between 5 and 25 kpc in the northwest direction. The observed line-of-sight bulk velocity in all regions is broadly consistent with the central galaxy, M87, with a mild trend toward blueshifted motions at larger radii. Systematic uncertainties have been carefully assessed and do not affect the measurements. The central velocities, if attributed entirely to isotropic turbulence, correspond to a transonic intracluster medium at sub-6 kpc scales with 3D Mach number 0 . 69 -0.11 +0.14 and a nonthermal pressure fraction of 21 -5 +7 % . Simple models of weak shocks and sound waves and calculations assuming isotropic turbulence both support the hypothesis that the velocity field reflects a mix of shock-driven expansion and turbulence. Compared to other clusters observed by XRISM to date, M87’s central region stands out as the most kinematically disturbed, exhibiting both the highest velocity dispersion and the largest 3D Mach number, concentrated at the smallest physical scales.

The timing (cross-)calibration of astronomical instruments is often done by comparing pulsar times-of-arrival (TOAs) to a reference timing model. In high-energy astronomy, the choice of solar system ephemerides and source positions used to barycenter the photon arrival times has a significant impact on the procedure, requiring a full reprocessing of the data each time a new convention is used. Our method, developed as part of the activities of the International Astronomical Consortium for High Energy Calibration, adapts an existing pulsar solution to arbitrary JPL ephemerides and source positions by simulating geocentric TOAs and refitting timing models (implemented with PINT). We validate the procedure and apply it to thousands of observations of the Crab pulsar from 15 missions spanning 1996–2025, demonstrating inter-ephemeris TOA consistency at the ≲5 μ s level, using the DE200/FK5-based Jodrell Bank Monthly Ephemeris as a common reference. We release the TOAExtractor open-source tool and a TOA database to support future calibration and scientific studies. Instrument timing performance is broadly consistent with mission specifications; the X-ray-to-radio phase offset varies with energy and time at a level that is marginally consistent with the uncertainties of the radio ephemeris, motivating coordinated multiwavelength follow-up.