How narrow is the M87* ring? I. The choice of closure likelihood function

Understand

Event Horizon Telescope (EHT) observations of the core of the galaxy M87 suggest an observational appearance dominated by a ring of approximately 40$\mu$as in diameter.

• The thickness of the ring is less certain: imaging efforts constrained it to be less than half the diameter (consistent with an imaging resolution of 20$\mu$as), while visibility-domain modeling suggested a variety of fractional widths, including as low as $10\%$ on some days.
• The fractional width is very interesting as it has the potential to discriminate between different astrophysical scenarios for the source; in fact, the $10$--$20\%$ range is so narrow as to be in tension with theoretical expectations.
• In the first of a series of papers on the width of the observed ring, we reproduce a subset of EHT visibility-domain modeling results and we explore whether alternative data analysis methods might favor thicker rings.