Brightness Asymmetry of Black Hole Images as a Probe of Observer Inclination. (arXiv:2105.03424v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Medeiros_L/0/1/0/all/0/1">Lia Medeiros</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chan_C/0/1/0/all/0/1">Chi-Kwan Chan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Narayan_R/0/1/0/all/0/1">Ramesh Narayan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ozel_F/0/1/0/all/0/1">Feryal Ozel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Psaltis_D/0/1/0/all/0/1">Dimitrios Psaltis</a>
The Event Horizon Telescope recently captured images of the supermassive
black hole in the center of the M87 galaxy, which show a ring-like emission
structure with the South side only slightly brighter than the North side. This
relatively weak asymmetry in the brightness profile along the ring has been
interpreted as a consequence of the low inclination of the observer (around 17
deg for M87), which suppresses the Doppler beaming and boosting effects that
might otherwise be expected due to the nearly relativistic velocities of the
orbiting plasma. In this work, we use a large suite of general relativistic
magnetohydrodynamic simulations to reassess the validity of this argument. By
constructing explicit counter examples, we show that low-inclination is a
sufficient but not necessary condition for images to have low brightness
asymmetry. Accretion flow models with high accumulated magnetic flux close to
the black hole horizon (the so-called magnetically arrested disks) and low
black-hole spins have angular velocities that are substantially smaller than
the orbital velocities of test particles at the same location. As a result,
such models can produce images with low brightness asymmetry even when viewed
edge on.
The Event Horizon Telescope recently captured images of the supermassive
black hole in the center of the M87 galaxy, which show a ring-like emission
structure with the South side only slightly brighter than the North side. This
relatively weak asymmetry in the brightness profile along the ring has been
interpreted as a consequence of the low inclination of the observer (around 17
deg for M87), which suppresses the Doppler beaming and boosting effects that
might otherwise be expected due to the nearly relativistic velocities of the
orbiting plasma. In this work, we use a large suite of general relativistic
magnetohydrodynamic simulations to reassess the validity of this argument. By
constructing explicit counter examples, we show that low-inclination is a
sufficient but not necessary condition for images to have low brightness
asymmetry. Accretion flow models with high accumulated magnetic flux close to
the black hole horizon (the so-called magnetically arrested disks) and low
black-hole spins have angular velocities that are substantially smaller than
the orbital velocities of test particles at the same location. As a result,
such models can produce images with low brightness asymmetry even when viewed
edge on.
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