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

Understand

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.