Orientation of the crescent image of M 87*

Abstract

The first image of the black hole (BH) M 87* obtained by the Event Horizon Telescope (EHT) has the shape of a crescent extending from the E to WSW position angles, while the observed direction of the large-scale jet is WNW. Images based on numerical simulations of BH accretion flows suggest that on average the projected BH spin axis should be oriented SSW. We explore highly simplified toy models for geometric distribution and kinematics of emitting regions in the Kerr metric, perform ray tracing to calculate the corresponding images, and simulate their observation by the EHT to calculate the corresponding visibilities and closure phases. We strictly assume that (1) the BH spin vector is fixed to the jet axis, (2) the emitting regions are stationary and symmetric with respect to the BH spin, and that (3) the emissivities are isotropic in the local rest frames. Emission from the crescent sector between SSE and WSW can be readily explained in terms of an equatorial ring with either circular or plunging geodesic flows, regardless of the value of BH spin. In the case of plane-symmetric polar caps with plunging geodesic flows, the dominant image of the cap located behind the BH is sensitive to the angular momentum of the emitter. Within the constraints of our model, we have not found a viable explanation for the observed brightness of the ESE sector. Most likely, the ESE "hotspot" has been produced by a non-stationary localised perturbation in the inner accretion flow. Alternatively, it could result from locally anisotropic synchrotron emissivities. Multi-epoch and polarimetric results from the EHT will be essential to verify the theoretically expected alignment of the BH spin with the large-scale jet.

7 pages, 7 figures, published in Astronomy & Astrophysics, matching the publication title

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• Silhouettes of invisible black holes
• Modeling the motion of a bright spot in jets from black holes M87* and SgrA*
• Strong gravitational lens image of the M87 black hole with a simple accreting matter model
• Spins of supermassive black holes M87* and SgrA* revealed from the size of dark spots in Event Horizon Telescope Images
• Event-horizon-scale Imaging of M87* under Different Assumptions via Deep Generative Image Priors
• Chaotic magnetic disconnections trigger flux eruptions in accretion flows channeled onto magnetically saturated Kerr black holes
• Forgotten treasures in the HST/FOC UV imaging polarimetric archives of active galactic nuclei III. Five years monitoring of M87
• Visible shapes of black holes M87* and SgrA*