Zack Gelles and Jayke Nguyen

From the calendar

Supermassive black holes launch luminous, relativistic jets that are observed at parsec-level scales. While black hole spin is believed to play an important role in powering these jets, the spin parameter has proven difficult to measure directly. In this talk, I will discuss how black hole spin imprints itself on surrounding magnetic fields, yielding a new observable signature of spin. Specifically, I will show how black hole spin “winds up” the magnetic field at critical surfaces called light cylinders, where plasma begins to accelerate into an inflow or an outflow. Using a combination of semi-analytic models and numerical simulations, I will then demonstrate that this wind-up translates into spatial polarization swings in the emission, which can be observed through radio interferometry. This technique would allow us to constrain supermassive black hole spins and jet Lorentz factors with forthcoming observations of several nearby AGN.

Based on arXiv:2410.00954, arXiv:2601.13307, arXiv:2606.12518

Aperture masking interferometry (AMI) is a powerful high contrast direct imaging technique enabling detections at angular separations beyond the typical λ/D diffraction limit. Combining that with the extremely stable environment of JWST, a new regime of precision AMI is possible. Post-launch, JWST AMI was limited by detector systematics (such as the brighter-fatter effect) and aberrations originating from optical telescope element (OTE) limiting its sensitivity. However, a new pipeline, AMIGO, has recently shown significant improvements in forward modeling the full optical system from photons-to-pixels, allowing us to reach photon-noise limited contrasts of ~1e-4 at sub-λ/D angular resolution. Currently, we are applying AMIGO to reduce a backlog of archival JWST AMI data originally collected under GTO and GO programs, but unable to be reduced to their full potential until now. Applying AMIGO to archival observations of HR 8799, we recover the four known planets and find evidence for a candidate fifth, innermost planet, HR 8799 f, at a projected separation of ~7 au and an F380M contrast of ~2e-4. Follow-up observations are needed to confirm its planetary nature. The breadth of AMI data and robustness of AMIGO may also eventually allow us to perform calibrator-free interferometry, reducing required observing time by 50%. By unlocking the full precision of JWST AMI, these archival observations may reveal new planetary companions at solar system scales while establishing a foundation for future JWST interferometric imaging. This study also has design implications for HWO, where the extreme contrast requirements needed to image an Earth-like planet around a Sun-like star will rely on engineered observatory stability and post-processing with forward modeling.