Possible imprints of cosmic strings in the shadows of galactic black holes

Abstract

We examine the shadow cast by a Kerr black hole pierced by a cosmic string. The observable images depend not only on the black hole spin parameter and the angle of inclination, but also on the deficit angle yielded by the cosmic string. The dependence of the observable characteristics of the shadow on the deficit angle is explored. The imprints in the black hole shadow left by the presence of a cosmic string can serve as a method for observational detection of such strings.It's well known that the shadow (or the apparent shape) of a compact relativistic object encodes information about the nature of this object [1]. That is why the apparent shapes of various black holes and other compact objects, such as wormholes and naked singularities, have been intensively studied in the last years. The shadows of the black holes and naked singularities from the Kerr-Newmann family of solutions of the EinsteinMaxwell equations have been thoroughly investigated in [2]- [8]. The shadow of a black hole with a NUT-charge has been obtained in [9]. The black hole shadows in EinsteinMaxwell-dilaton gravity, Chern-Simons modified gravity and braneworld gravity have been examined in [10], [11], [12]. The apparent shape of the Sen black hole has been studied in [13]. The wormhole shadows have been recently investigated in [14], [15].With the advance of technology the experimental observation of the shadows of compact objects is now possible. Experiments that allow such observations include the Event Horizon Telescope [16], which is a system of earth-based telescopes measuring in the (sub)millimeter wavelength, the space-based radio telescopes RadioAstron and Millimetron [17], [18], and the space-based X-ray interferometer MAXIM [19]. In the next few years these missions are expected to reach resolution high enough to observe the shadow of the supermassive compact object at the center of our galaxy or those located at nearby galaxies [18]. The results of these experiments should be compared with the theoretical models. In this way the observations will reject some of the models or will make it possible to distinguish between different types of compact objects [20]. Even more, the mentioned observations can be used for detection of theoretically predicted objects and effects not observed so far. Such a theoretical prediction is the possible existence * E-mail: tintschev@phys.uni-sofia.bg † E-mail: yazad@phys.uni-sofia.bg 1 of cosmic strings. These objects are expected to have formed during phase transitions in the early universe through spontaneous symmetry breakings [21]. It has also been shown that cosmic strings generally form at the end of inflation within the framework of various supersymmetric grand unification theories [22]. A cosmic string makes the space-time around it a conical space-time with a deficit angle δ = 8πGµ/c 4 where µ is the string tension and G and c are the gravitational constant and speed of light, respectively. The deficit angle manifests itself physically by giving rise to interesting phenomena and eff...

Horizon structure of rotating Einstein–Born–Infeld black holes and shadow

“…a dark area in front of a luminous background [56,59]. Hence, their is a significant effort to study black hole shadows and this has become a quite active research field [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] (for a review, see [60]). For the Schwarzschild black hole the shadow of the black hole is a perfect circle [60], and it is enlarged in the case of a Reissner-Nordström black hole [61].…”

“…The photons that cross the event horizon, due to strong gravity, are removed from the observable universe which lead to a shadow (silhouette) imprinted by a black hole on the bright emission that exists in its vicinity. So far the shadows of the compact gravitational objects in the different cases have been extensively studied, see, e.g., [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. Furthermore a new general formalism to describe the shadow of black hole as an arbitrary polar curve expressed in terms of a Legendre expansion was developed in a recent paper [49].…”

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].…”

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].…”