Rotation of the crescent image of M87* and polarization of its ESE hotspot

TL;DR

This paper analyzes the rotation and polarization features of M87*'s crescent image, revealing a distinct ESE hotspot with implications for magnetic fields, accretion flow orientation, and plasma dynamics near the black hole.

Contribution

It provides new insights into the polarization and rotation behavior of M87*'s crescent image, suggesting a possible retrograde accretion flow and the nature of the ESE hotspot.

Findings

ESE hotspot is unpolarized, indicating distinct origin.

Observed rotation rate of the crescent aligns with Faraday-corrected polarization angle.

Counterclockwise image rotation suggests retrograde accretion.

Abstract

The first image of the black hole (BH) M87* obtained by the Event Horizon Telescope (EHT) has the shape of a crescent extending from the E to WSW position angles, with a tentative 'ESE hotspot'. Assuming that the BH spin vector is aligned with both the inner accretion axis and the projected direction of the kpc-scale relativistic jet, the position of the ESE hotspot is inconsistent with the axisymmetric accretion flow. Recent polarimetric EHT images of M87* show that the ESE hotspot is essentially unpolarized, which strongly supports its distinct origin. If the hotspot emission is due to the synchrotron radiation, its depolarization requires either isotropically tangled magnetic fields or an additional Faraday dispersion measure. The 6-day EHT observing campaign in April 2017 allowed in principle to detect orbital motions advancing by up to ~60 deg. The apparent rotation rate of the major axis of the EHT crescent image is consistent with the rotation rate of the Faraday-corrected polarization angle measured by the ALMA. However, the counterclockwise (CCW) sense of these rotations is opposite to the clockwise (CW) rotation of the plasma flows implied by the N-S brightness asymmetry, which might indicate that accretion in M87 is retrograde.