The Event Horizon Telescope (EHT) has recently delivered the first
resolved images of M87*, the supermassive black hole in the center of
the M87 galaxy. These images were produced using 230 GHz observations
performed in 2017 April. Additional observations are required to
investigate the persistence of the primary image feature—a ring
with azimuthal brightness asymmetry—and to quantify the image
variability on event horizon scales. To address this need, we analyze
M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and
2013. While these observations do not contain enough information to
produce images, they are sufficient to constrain simple geometric
models. We develop a modeling approach based on the framework utilized
for the 2017 EHT data analysis and validate our procedures using
synthetic data. Applying the same approach to the observational data
sets, we find the M87* morphology in 2009-2017 to be consistent with a
persistent asymmetric ring of ∼40 μas diameter. The position
angle of the peak intensity varies in time. In particular, we find a
significant difference between the position angle measured in 2013 and
2017. These variations are in broad agreement with predictions of a
subset of general relativistic magnetohydrodynamic simulations. We show
that quantifying the variability across multiple observational epochs
has the potential to constrain the physical properties of the source,
such as the accretion state or the black hole spin.
