Black Hole Spin Signature in the Black Hole Shadow of M87 in the Flaring State

Abstract

Imaging the immediate vicinity of supermassive black holes (SMBH) and extracting a BH-spin signature is one of the grand challenges in astrophysics. M87 is known as one of the best targets for imaging the BH shadow and it can be partially thick against synchrotron self-absorption (SSA), particularly in a flaring state with high mass accretion rate. However, little is known about influences of the SSA-thick region on BH shadow images. Here we investigate BH shadow images of M87 at 230 GHz properly taking into account the SSA-thick region. When the BH has a high spin value, the corresponding BH shadow image shows the positional offset between the center of the photon ring and that of the SSA-thick ring at the innermost stable circular orbit (ISCO) due to the frame-dragging effect in the Kerr spacetime. As a result, we find that a dark-crescent structure is generally produced between the photon ring and the SSA-thick ISCO ring in the BH shadow image. The scale size of the dark-crescent increase with BH spin: its width reaches up to $\sim 2$ gravitational radius when the BH spin is 99.8% of its maximum value. The dark crescent is regarded as a new signature of a highly spinning BH. This feature is expected to appear in flaring states with relatively high mass accretion rate rather than the quiescent states. We have simulated the image reconstruction of our theoretical image by assuming the current and future Event Horizon Telescope (EHT) array, and have found that the future EHT including space-very long baseline interferometry in 2020s can detect the dark crescent.

12 pages, 7 figures, accepted for publication in ApJ

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• Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign
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• The shadow of M87* black hole within rational nonlinear electrodynamics
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• A Jet-Bases Emission Model of the EHT 2017 Image of M87*
• Implications from the velocity profile of the M87 jet: a possibility of a slowly rotating black hole magnetosphere
• Orientation of the crescent image of M 87*
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• The evolutions of the innermost stable circular orbits in dynamical spacetimes
• RAIKOU: A General Relativistic, Multi-wavelength Radiative Transfer Code
• Black Hole Shadow Observations with Space-Ground Interferometers
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• Survey of Non-thermal Electron around Supermassive Black Holes through Polarization Flips
• Monitoring the Size and Flux Density of Sgr A* during the Active State in 2019 with East Asian VLBI Network
• The Japanese Vision for the Black Hole Explorer Mission
• Investigating the Disk-Jet Structure in M87 through Flux Separation in the Linear and Circular Polarization Images
• Visible shapes of black holes M87* and SgrA*
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