Orientation of the crescent image of M87*

Krzysztof Nalewajko, Agata Różańska

The first image of the black hole (BH) M87* obtained by the Event Horizon Telescope (EHT; EHT Collaboration et al. 2019a) has the shape of a crescent extending from the E to SWW position angles, while the observed direction of the large-scale jet is NWW (Walker et al., 2018). Comparing the EHT observed image with images based on numerical simulations of BH accretion flows suggests that on average the projected BH spin axis should be oriented SSW (EHT Collaboration et al., 2019b). Alternatively, if the spin axis is matched with the jet direction, emission from the SEE sector of the photon ring can only be accounted for as a temporary fluctuation. We explore highly simplified toy models for geometric distribution and kinematics of emitting regions in the Kerr metric, and perform ray tracing to calculate images. We strictly assume that: (1) the BH spin vector is fixed to the jet axis (position angle PA_jet = 288^◦, inclination i = 162^◦), (2) the emitting regions are stationary and symmetric with respect to the BH spin, (3) the emissivities are isotropic in the local rest frames. Emission from the crescent sector SSE - SWW 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 SEE sector. We conclude that the SEE emission should indeed be interpreted as a temporary departure from intrinsic axial symmetry of the inner accretion flow.

Proceedings of the Polish Astronomical Society, vol. 10, 282-284 (2020)

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