Kavli Affiliate: John E. Carlstrom
| First 5 Authors: Rohan Dahale, Ilje Cho, Kotaro Moriyama, Kaj Wiik, Paul Tiede
| Summary:
We investigate the origin of the elliptical ring structure observed in the
images of the supermassive black hole M87*, aiming to disentangle contributions
from gravitational, astrophysical, and imaging effects. Leveraging the enhanced
capabilities of the Event Horizon Telescope (EHT) 2018 array, including
improved $(u,v)$-coverage from the Greenland Telescope, we measure the ring’s
ellipticity using five independent imaging methods, obtaining a consistent
average value of $tau = 0.08_{-0.02}^{+0.03}$ with a position angle $xi =
50.1_{-7.6}^{+6.2}$ degrees. To interpret this measurement, we compare against
General Relativistic Magnetohydrodynamic (GRMHD) simulations spanning a wide
range of physical parameters including thermal or non-thermal electron
distribution function, spins, and ion-to-electron temperature ratios in both
low and high-density regions. We find no statistically significant correlation
between spin and ellipticity in GRMHD images. Instead, we identify a
correlation between ellipticity and the fraction of non-ring emission,
particularly in non-thermal models and models with higher jet emission. These
results indicate that the ellipticity measured from the m87 emission structure
is consistent with that expected from simulations of turbulent accretion flows
around black holes, where it is dominated by astrophysical effects rather than
gravitational ones. Future high-resolution imaging, including space very long
baseline interferometry and long-term monitoring, will be essential to isolate
gravitational signatures from astrophysical effects.
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