Influence of a plasma on the shadow of a spherically symmetric black hole

Abstract

We analytically calculate the influence of a plasma on the shadow of a black hole (or of another compact object). We restrict to spherically symmetric and static situations, where the shadow is circular. The plasma is assumed to be non-magnetized and pressure-less. We derive the general formulas for a spherically symmetric plasma density on an unspecified spherically symmetric and static spacetime. Our main result is an analytical formula for the angular size of the shadow. As a plasma is a dispersive medium, the radius of the shadow depends on the photon frequency. The effect of the plasma is significant only in the radio regime. The formalism applies not only to black holes but also, e.g., to wormholes. As examples for the underlying spacetime model, we consider the Schwarzschild spacetime and the Ellis wormhole. In particular, we treat the case that the plasma is in radial free fall from infinity onto a Schwarzschild black hole. We find that for an observer far away from a Schwarzschild black hole the plasma has a decreasing effect on the size of the shadow. The perspectives of actually observing the influence of a plasma on the shadows of supermassive black holes are discussed. PACS numbers: 04.20.

On the shadow of rotating traversable wormholes

“…Motivated by present observational missions [5], the shadows of a variety of black holes were studied, both in general relativity and alternative theories of gravity [6][7][8][9][10][11][12][13][14][15][16]. The specific features of the images can be used to extract information about the physical properties of the compact objects, and to a e-mail: gyulchev@phys.uni-sofia.bg b e-mail: pnedkova@phys.uni-sofia.bg c e-mail: yazad@phys.uni-sofia.bg differentiate between gravitational theories [17][18][19].…”

Strong deflection limit analysis and gravitational lensing of an Ellis wormhole

“…[22,23,[39][40][41][42][43]. Micro lens [44][45][46][47], astrometric image centroid displacements [48,49], the Einstein rings [43], the time delay of light rays [50], the signed magnification sum [51], the gravitational lensing shear [52], constraints of the number density from gravitational lensing observations [19,53], a wave effect of gravitational lenses [53], shadows surrounded by a plasma [54] and optically thin dust [55], binary gravitational lenses [56], a particle collision [57], and several observables such as rotation curves [58] in the Ellis wormhole spacetime and in general spacetimes that are coincident with the Ellis wormhole spacetime under the weak-field approximation have been investigated. The visualization of the Ellis wormhole was studied by Muller [59].…”

Double shadow of a regular phantom black hole as photons couple to the Weyl tensor

“…For the rotating case, the shadow has an elongated shape in the direction of the rotation axis due to the dragging effect [2,3]. Motivated by that the investigation of the shadow is very useful for measuring the nature of the black hole and the corresponding observations may be obtained in the near future, a lot of attention have been attracted on this subject in the last few years [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].…”