Black Hole Shadows of Charged Spinning Black Holes

We propose a method for measuring the black hole charge by imaging a blackhole shadow in a galactic center by future interferometers. Even when the blackhole is uncharged, it is possible to confirm the charge neutrality by thismethod. We first derive the analytic formulae of the black hole shadow in anoptically thin medium around a charged spinning black hole, and theninvestigate how contours of the black hole shadow depend on the spin and thecharge of the black hole for several inclination angles between the rotationaxis of the black hole and the observer. This method only assumes stationaryblack hole and general relativity. By fitting the formula of the contours ofthe shadow to the observed image of the shadow, in addition to the black holecharge, one can also determine the black hole spin and the inclination anglewithout any degeneracy among the charge, the spin, and the inclination angleunless the inclination angle is null.Comment: Accepted version of the published paper, Publ. Astron. Soc. Japan 57, 27