Ultra High Resolution X-Ray Imaging Advances Astronomy at 0.5 keV

Scientists are increasingly limited by the resolving power of current X-ray telescopes, hindering our ability to study the most energetic phenomena in the universe. Kimberly A. Weaver (X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center), Jenna M. Cann (Center for Space Science and Technology, University of Maryland, Baltimore County), and Ryan Pfeifle (X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center) et al., demonstrate the critical need for ultra-high resolution X-ray imaging , achieving milliarcsecond to microarcsecond resolution in the 0.5 to 8 keV band. While other areas of astronomy have seen dramatic improvements in spatial resolution, X-ray imaging has lagged behind, preventing detailed investigation of key physical processes at the smallest astrophysical scales and potentially obscuring entirely new classes of high-energy discoveries. This paper outlines compelling science goals achievable with such advanced imaging, alongside a discussion of current technologies, including the promising Accretion Explorer mission concept, paving the way for a revolution in X-ray astronomy.

This work details several crucial science goals requiring high-quality X-ray imaging and assesses the status of current technologies and mission concepts needed to achieve these advances. These questions focus on Cosmic Ecosystems, New Messengers, and New Physics, specifically addressing gas and metal flow within galaxies, the formation of Supermassive black holes, and the coupling of black hole growth to galaxy evolution. Experiments show that current X-ray telescopes provide resolutions of 0.5 to 1 arcsecond, insufficient for detailed studies of the innermost processes of supermassive black holes and hindering progress on key Astro2020 questions. Accreting supermassive black holes are strong X-ray sources surrounded by gas and dust where X-ray and UV photons are scattered, absorbed, or reflected, with hard X-rays thought to originate from photons scattered by a hot corona. Despite various models suggesting coronal geometries ranging from point sources to ‘sandwich’ structures, a lack of spatially resolved imaging prevents direct confirmation of its nature.

For the nearest X-ray bright SMBH, M87, located at a distance of 20 Mpc and possessing a mass of 6 billion solar masses, the gravitational radius is approximately 19 billion km, or 4 × 10−6 arcseconds, five to six orders of magnitude below current resolution limits. The study unveils a critical limitation in exoplanet research, where treating stars as point sources introduces systematic errors in exoplanet parameter measurements. Higher resolution X-ray images would allow discernment of fine structures and dynamic processes in stellar environments, providing unprecedented insights into stellar magnetic activity, flare morphologies, and atmospheric escape processes. This capability is particularly crucial for understanding exoplanets within the habitable zones of their parent stars, where detailed knowledge of stellar environments is essential for assessing their potential for habitability. The study employed the Swift-BAT AGN sample, utilising data to determine the angular scales resolvable for each target at varying distances. The team developed a detailed methodology to quantify the limitations of existing X-ray telescopes and the potential gains from next-generation interferometry. They meticulously mapped the required angular resolutions against the physical scales of critical AGN components, establishing a clear case for the need for sub-microarcsecond imaging.
Experiments employed established redshift calculations and spatial scale estimations, combined with observational data from the Swift-BAT catalogue, to create a comprehensive assessment of observational feasibility. This approach enables a precise determination of the capabilities needed to probe the innermost regions of active galactic nuclei. Furthermore, the research pioneered a quantitative comparison between the spatial scales of key AGN features and the resolving power of current and proposed X-ray telescopes. By plotting the necessary angular resolutions against target distances, scientists demonstrated that a microarcsecond X-ray interferometer is essential to observe the dust sublimation radii of a substantial sample of AGN, and even the outer edge of the accretion disk and X-ray corona in select sources0.1 arcseconds to 1 milliarcsecond, and ultimately 1 microarcsecond, would unlock new discovery space and complement advances in spectral resolution and X-ray sensitivity. Data shows that even the shadow of a supermassive black hole event horizon in M87 measures only approximately 4 × 10−6 arcsec, a scale inaccessible to current X-ray telescopes. The breakthrough delivers the potential to differentiate between starburst-driven and AGN-driven outflows near galaxy centers, trace the geometries of obscuring tori and the X-ray broad line region, and test mechanisms responsible for AGN feedback. Measurements confirm that observing even a small sample of accretion disks across a range of Eddington ratios could validate traditional theory or reveal deviations from a thin disk structure, including environments facilitating Bondi accretion or chaotic accretion driven by turbulence. Tests prove that accessing these scales is crucial for understanding the fundamentals of accretion power and its impact on AGN host galaxies.

X-ray resolution gains unlock AGN and black hole

Scientists present a compelling case for significantly enhancing X-ray imaging resolution to milliarcsecond (mas) to microarcsecond (μas) scales in the 0.5 to 8 keV energy band. This research highlights the potential for transformative discoveries with such advancements, particularly in understanding active galactic nuclei (AGN), black hole systems, and stellar environments. The findings demonstrate that even a modest improvement to 0.1 arcsecond resolution would substantially advance our understanding of AGN feedback, while mas resolution could reveal detailed information about black hole winds and the AGN torus. Crucially, μas resolution is necessary to probe the scales of X-ray coronae and accretion disks, as well as jet launching regions, allowing for revolutionary studies of extragalactic X-ray binaries and dual AGN systems.
Furthermore, high-resolution X-ray imaging promises to unlock unprecedented insights into stellar dynamics, magnetic fields, and atmospheric escape processes in exoplanets, particularly those within habitable zones. They suggest a modular spacecraft design to facilitate the integration of different telescope technologies as they mature, potentially enabling a feasible flagship or probe-class mission. Future research should concentrate on advancing these technologies to reach Technology Readiness Level 0.6, paving the way for a new era of high-resolution X-ray astronomy.