Can the dynamics of test particles around charged stringy black holes mimic the spin of Kerr black holes?

Abstract

We study the motion of electrically charged particles, magnetic monopoles, and magnetic dipoles around electrically and magnetically charged stringy black holes. From the analysis of the radius of the innermost stable circular orbit of electrically charged particles, we show that the electric charge $Q$ of stringy black holes can mimic well the spin of Kerr black holes; the black hole magnetic charge ${Q}_{m}$ can mimic spins up to ${a}_{*}\ensuremath{\simeq}0.85$ for magnetic dipoles; the magnetic charge parameter $g$ of a magnetic monopole can mimic spins up to ${a}_{*}\ensuremath{\simeq}0.8$. This is due to the destructive character of the magnetic field, and such a result excludes the existence of an appreciable black hole magnetic charge in astrophysical black holes from the observation of rapidly rotating objects with dimensionless spin up to ${a}_{*}\ensuremath{\simeq}0.99$. We then consider the magnetar SGR (PSR) J1745-2900 as a magnetic dipole orbiting the supermassive black hole Sagittarius A* (Sgr A*). We show that Sgr A* may be interpreted as a stringy black hole with magnetic charge ${Q}_{m}/M\ensuremath{\le}0.4118$.