Resolving Nearby Supermassive Black Holes with the Black Hole Explorer

Recent Event Horizon Telescope results have demonstrated unique and transformative science in gravitational physics and black hole astrophysics enabled by event-horizon-scale imaging of supermassive black holes. The new analysis still awaits peer review, but it already lays out the central claim clearly.

This matters because astrophysics becomes persuasive only when an observed signal can be tied to a physically defensible explanation. Compact objects such as neutron stars and black holes are natural laboratories for extreme physics, but the distance and complexity of these systems make interpretation difficult without multi-wavelength coverage and careful modeling. A detection without a mechanism is only half a result. the other half comes from showing that the signal fits quantitatively inside a coherent physical picture rather than merely being consistent with a broad family of models. Nevertheless, the angular resolution of current ground-based very long baseline interferometry (VLBI) arrays limits such studies to only two sources, precluding systematic. The proposed Black Hole Explorer (BHEX), a millimeter/submillimeter space VLBI mission, would overcome this limitation by delivering substantially higher angular resolution.

Here, we present a series of simulated observations to assess a population of nearby horizon-scale targets accessible with BHEX. Based on a recently developed SMBH number density model, we find that BHEX could infer black hole masses for ~70-90 sources from size measurements, constrain magnetic field.

Targeted observations of ~50 nearby SMBHs are expected to yield measurements for ~30 source sizes and ~10 shadows and linear-polarization patterns. These projections are supported by detailed imaging simulations of general relativistic magnetohydrodynamic (GRMHD) models for eleven nearby SMBHs.

Together, our results highlight BHEX as a powerful facility for revealing the demographics of SMBH properties across diverse accretion states, radio loudness, host galaxy. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy.

The broader interest lies in turning an observational clue into something that can be weighed against competing models of the underlying physics. Astrophysics does not have the luxury of controlled experiments; everything is inferred from radiation that traveled across cosmic distances under conditions that cannot be reproduced in a terrestrial laboratory. This makes the interpretation chain longer and more uncertain than in bench science, but it also means that a well-constrained measurement of an extreme object carries theoretical information that no earthbound experiment can provide.

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Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether independent datasets and physical modeling converge on the same interpretation. Multi-wavelength follow-up, combining X-ray, radio and optical data where possible, is typically what separates a compelling detection from a robust physical characterization. In high-energy astrophysics, results that initially looked definitive have been revised when data from a second messenger arrived; the current result should be read with that history in mind. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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