The Need for Ultra High Resolution X-ray Imaging

TL;DR

This paper argues that X-ray imaging has fallen behind other bands in angular resolution, limiting insights into SMBH accretion, AGN feedback, jets, XRB populations, and stellar phenomena. It advocates pursuing ultra-high-resolution X-ray imaging in the soft–medium energy range ($0.5$–$8$ keV) via a dispersed-aperture X-ray interferometer, exemplified by the Accretion Explorer concept, to reach from mas to $\mu$as scales. The authors review existing capabilities, technology pathways (polished silicon mirrors, diffractive lenses, phase Fresnel lenses, and interferometric approaches), and present a NIAC study outlining a modular, multi-spacecraft architecture with baselines of ~$20$–$100$ m and multiple energy channels. They argue that $\mu$as imaging would enable direct mapping of AGN coronae, inner accretion disks, torus structures, jet-launch regions, and stellar coronae, yielding transformative constraints on accretion physics, feedback, XRB evolution, and exoplanet habitability. The work also discusses mission design tradeoffs and a practical path toward a flagship or probe-class mission, emphasizing formation flying and precision pointing as central technological challenges.

Abstract

This paper discusses the broad science case for obtaining milliarcsecond to microarcsecond astronomical imaging resolution in the soft to medium-energy X-ray band (~0.5 to ~8 keV). Astronomy across much of the electromagnetic spectrum has been fundamentally transformed with a rapid increase in ground-based and space-based capabilities to examine celestial objects on small scales that relate directly to their relevant physical processes. X-ray imaging capabilities, however, have fallen far behind observations at longer wavelengths. As such, without decisive advances in X-ray imaging, we will be unable to uncover key phenomena on the smallest astrophysical scales, leaving entire classes of high-energy discoveries beyond our reach. Here we describe several science goals for which high quality X-ray imaging is crucial and the status of some current technologies or mission concepts that would be required for these advances. In particular, we discuss the Accretion Explorer, a mission architecture under current study for a dispersed aperture X-ray interferometer.

Figures (13)

• Figure 1: Multi-wavelength mismatch. Current X-ray imaging angular resolution falls behind the radio, IR and optical bands, across the range of astronomical objects in distance-size parameter space, preventing synergistic multi-wavelength studies of a wide range of astronomical objects at comparable spatial scales. Ground-based telescopes now routinely operate in the mas regime (red dot-dashed line here). Color coding for boxes is based on the red to violet optical spectrum. Objects dominated by longer wavelength radiation are red; X-ray dominated objects are violet. All of these physical scales are inaccessible to current X-ray capabilities (purple dot-dash line). Figure adapted from Rinehart2019
• Figure 2: Science discussed in this paper.
• Figure 3: Spatially resolved studies of AGN accretion are inaccessible for a wide range of spatial scales with current instrumentation. Left: Here we plot angular resolutions needed to probe spatial scales of 100 pc down to $10^{-3}$ pc in the nearby and distant Universe. Right: The angular scales that could be probed for targets in the Swift-BAT AGN sample bassDR2 as a function of distance. The blue, orange, green and red points correspond to the resolutions needed to resolve the X-ray corona, outer accretion disk, the dust sublimation radius, and the outer torus, respectively, for each target. The mas through $\mu$as angular resolutions discussed by this white paper reside under the horizontal grey line indicated by the downward arrow. As can be seen, mas resolution imaging may be sufficient to probe the dust sublimation radii of a few targets. However, an X-ray interferometer with $\mu$as resolution is needed to be able to probe the dust sublimation radii of the entire sample, and would also open up a window to observe the outer edge of the accretion disk, and even the X-ray corona in a handful of sources.
• Figure 4: Simulated images of an AGN accretion disk and hard X-ray corona observed with micro-arcsecond resolution. These simulated disks were made assuming an accreting $10^{8}\mathrm{M}_{\odot}$ SMBH at a distance of 5 Mpc from Earth, consistent with the closest objects where we can spatially resolve the corona with $\mu$as resolution (Figure \ref{['Fig:resolution']}). Left: Accretion disk and corona system where the corona is in the form of a torus surrounding the SMBH. The torus has an inner (cross-sectional) radius of $10r_{\rm g}$. Right: Same as Left, but with a spherical corona with radius of $10r_{\rm g}$. Images were made using the X-ray ray-tracing code RefleXreflex2017.
• Figure 5: High spatial resolution X-ray imaging is crucial for the dissecting regions dominated by starburst/AGN feedback. This figure shows different views of the Seyfert galaxy NGC 1365. The Chandra emission-line image (A) resolves scales of $\sim90$ pc on the sky (Wang_2009, reproduced with permission). MUSE observations (Venturi2018, reproduced with permission) (B) show shock-like structures related to the AGN driven outflow in the map of [O III] line-of-sight velocities minus the stellar velocities. JWST NIRCam (C) and ALMA (D) resolve the central 2.5 kpc region on $<9$ pc scales, highlighting prominent starburst-driven wind signatures (Liu2023, reproduced with permission). While Chandra shows X-ray emission spatially aligned with the AGN-driven outflow seen with VLT/MUSE, it lacks the angular resolution to resolve the potential X-ray counterparts to these finer features.