Synthetic Images of Magnetospheric Reconnection-Powered Radiation around Supermassive Black Holes

Accreting supermassive black holes can now be observed at the event-horizon scale at millimeter wavelengths. Current predictions for the image rely on hypotheses (fluid modeling, thermal electrons) which might not always hold in the vicinity of the black hole, so that a full kinetic treatment is in order. In this Letter, we describe the first 3D global general-relativistic particle-in-cell simulation of a black-hole magnetosphere. The system displays a persistent equatorial current sheet. Synthetic radio images are computed by ray-tracing synchrotron emission from nonthermal particles accelerated in this current sheet by magnetic reconnection. We identify several time-dependent features of the image at moderate viewing angles: a variable radius of the ring, and hot spots moving along it. In this regime, our model predicts that most of the flux of the image lies inside the critical curve. These results could help promote understanding of future observations of black-hole magnetospheres at improved temporal and spatial resolution.

Figure 1

Snapshot of the total plasma density, normalized by the fiducial Goldreich-Julian density, at $t=40r_g/c$. Magnetic field lines are represented as blue solid lines. The black surface marks the event horizon.

Figure 2

Time-averaged synchrotron images from the 2D simulation. The flux is in arbitrary units. The three columns show three different viewing angles ${\alpha }_{\mathrm{obs}}$, measured with respect to the spin axis of the black hole. The white cross marks the position of the black hole. The blue dashed line denotes the critical curve for $a=0.99$ at each viewing angle.

Figure 3

Top row: Snapshot of the image from the 2D simulation (leftmost panel) and differential maps illustrating the time evolution. The three maps show the difference between successive snapshots at later times and the first one. Bottom row: Image from the 3D simulation at a given azimuthal viewing angle ${\phi }_{\mathrm{obs}}$ (leftmost panel) and differential maps illustrating the dependence on ${\phi }_{\mathrm{obs}}$. The three maps show the difference between images at higher ${\phi }_{\mathrm{obs}}$ and the first one.