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Table 1.

Parameters and prior ranges used for the fitting.

Parameter Range Description
i [90° ,180° ] Viewing angle (inclination) of the observer (i = 0° is face-on, i = 90° is edge-on)
a* [0.0, 0.9] Dimensionless BH spin
B0, s [10, 103] Magnetic field strength (G)
rs [7, 15] Radius of the hot-spot orbit (M)
ϕcam [0, 360° ] Location of the camera at the first point of observation (shifts light curves left-right)
LPfrac [0, 1] Dimensionless scaling of the model fractional linear polarization
PA [ − 180° ,180° ] Position angle of the BH spin projected on the observer’s screen, measured east of north
Kcoef [0.5, 1.5] Dimensionless coefficient defining the orbital velocity with respect to the Keplerian case
ss [1, 4] The size (1σ radius) of the hot spot (M)
IBG [0, 5] Dimensionless scaling of the background emission w.r.t. max observed brightness of the hot spot
θs [45° ,135° ] The θ angle of the spot orbit, by default θ = 90° (equatorial orbit)
Xoff [ − 20, 20] Astrometric offset of the BH in the X direction (μas)
Yoff [ − 20, 20] Astrometric offset of the BH in the Y direction (μas)

Notes. All priors are flat, except for B0, s, for which we used a truncated log-normal prior.

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