Astronomy & Astrophysics, vol. 699, pp. A279(18pp)

Authors:

  • Rohan Dahale
  • Ilje Cho
  • Kotaro Moriyama
  • Kaj Wiik
  • Paul Tiede
  • José L. Gómez
  • Chi-Kwan Chan
  • Roman Gold
  • Vadim Y. Bernshteyn
  • Marianna Foschi
  • Britton Jeter
  • Hung-Yi Pu
  • Boris Georgiev
  • Abhishek V. Joshi
  • Alejandro Cruz-Osorio
  • Iniyan Natarajan
  • Avery E. Broderick
  • León D. S. Salas
  • Koushik Chatterjee
  • The Event Horizon Telescope Collaboration

Keywords:

  • accretion
  • accretion disks
  • black hole physics
  • gravitation
  • galaxies: active

URL:

Abstract:

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)’s 2018 array, including improved $(u,v)$-coverage from the Greenland Telescope, we measured 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 of $\xi = 50.1_-7.6^+6.2$ degrees. To interpret this measurement, we compared it to general relativistic magnetohydrodynamic (GRMHD) simulations spanning a wide range of physical parameters including the thermal or nonthermal 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 nonthermal 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.