The Variability of the Black-Hole Image in M87 at the Dynamical Time Scale

TL;DR

This study investigates the variability of the M87 black-hole image at the dynamical timescale using EHT data and simulations, revealing that certain image features and configurations align with observed low variability, supporting lensing-dominated models.

Contribution

It demonstrates that the observed low variability constrains black-hole and accretion flow models, favoring thin, lensing-dominated ring structures over turbulent ones.

Findings

Three baseline triangles show low variability (~3-5°).

Triangles crossing amplitude minima exhibit high variability (~90-180°).

Models with thin, lensing-dominated rings best match observed variability.

Abstract

The black-hole images obtained with the Event Horizon Telescope (EHT) are expected to be variable at the dynamical timescale near their horizons. For the black hole at the center of the M87 galaxy, this timescale (5-61 days) is comparable to the 6-day extent of the 2017 EHT observations. Closure phases along baseline triangles are robust interferometric observables that are sensitive to the expected structural changes of the images but are free of station-based atmospheric and instrumental errors. We explored the day-to-day variability in closure phase measurements on all six linearly independent non-trivial baseline triangles that can be formed from the 2017 observations. We showed that three triangles exhibit very low day-to-day variability, with a dispersion of $\sim3-5^\circ$. The only triangles that exhibit substantially higher variability ($\sim90-180^\circ$) are the ones with baselines that cross visibility amplitude minima on the $u-v$ plane, as expected from theoretical modeling. We used two sets of General Relativistic magnetohydrodynamic simulations to explore the dependence of the predicted variability on various black-hole and accretion-flow parameters. We found that changing the magnetic field configuration, electron temperature model, or black-hole spin has a marginal effect on the model consistency with the observed level of variability. On the other hand, the most discriminating image characteristic of models is the fractional width of the bright ring of emission. Models that best reproduce the observed small level of variability are characterized by thin ring-like images with structures dominated by gravitational lensing effects and thus least affected by turbulence in the accreting plasmas.