Observational signature of a near-extremal Kerr-Sen black hole in the heterotic string theory

Abstract

We analytically study the optical appearance of an isotropically emitter orbiting near the horizon of a near-extremely rotating Kerr-Sen (KS) black hole which is an electrically charged black hole arising in heterotic string theory. We study the influence of the Sen charge on the observational quantities, including the image position, flux and redshift factor. Moreover, we compare the results with those for a near-extremal Kerr-Newman (KN) black hole, which is the charged rotating black hole in general relativity. We find quantitative corrections of the signatures of these charged black holes (both KS and KN) compare to that of a neutral Kerr black hole. This may serve as distinctive features of different black holes for future tests by the Event Horizon Telescope.

Photon emissions from near-horizon extremal and near-extremal Kerr equatorial emitters

“…Next, by analyzing the root structures of the radial potential (2.16), the equatorial orbits are classified in the phase space (L, E) regarding to their radial motions [34,36]. The classifications 6 for the NHEK orbits and for the near-NHEK orbits are collected in Table 1 and Table 2, respectively, where R ± and R 0 are the root(s) for the corresponding cases…”

“…Besides the silhouette of a black hole shadow, there are other possible observational signatures with interesting features, for examples, the image of an hot spot surrounding a black hole [3][4][5][6], light rings (LRs) [7][8][9] and photon rings [10][11][12][13][14][15]. Theoretically, as the first step, one has to understand clearly the photon emissions around the black hole, especially near the black hole horizon.…”

Observability of Zero-angular-momentum Sources Near Kerr Black Holes

“…Therefore, not only do the high-spin black holes attract theoretical interests, but also they are astrophysically relevant. Indeed, the nice (near-)NHEK properties have been extensively applied to the analytical studies on gravitational waves [11][12][13], relativistic jets [14,15], electromagnetic emission 1 and observational signatures [16][17][18][28][29][30]. Note that the ISCO of a high-spin black hole is in the NHEK region, a region although appearing to approach the horizon (or "coincides" with the horizon in the extreme case), is however only in the subnear-horizon region of a near-extremal black hole, and inside it there is still a near-NHEK region residing in the deepest portion of the near-horizon throat.…”

QED Effects on Kerr Black Hole Shadows immersed in uniform magnetic fields

“…With the confidence and motivation given by the observational images of black holes, theoretical research has been widely carried out in recently years. Inspired by the pioneer works [3,4], people have studied the observational signatures of supermassive objects on many aspects including the shadows [5][6][7][8][9][10][11][12][13][14][15][16], the images of companion stars [17][18][19][20], the photon rings [21][22][23][24][25][26][27][28] and etc. [29][30][31][32][33][34][35][36][37][38].…”