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

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 $a_{*}\simeq 0.85$ for magnetic dipoles; the magnetic charge parameter $g$ of a magnetic monopole can mimic spins up to $a_{*}\simeq 0.8$. 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 $a_{*}\simeq 0.99$. 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 $Q_m/M\le 0.4118$.

Figure 1

Trajectories of test particles with positive electric charge for different values of the black hole electric charge $Q$. The top row is for a black hole with electric charge $Q/M=-0.5$, the central row is for an electrically neutral Schwarzschild black hole, and the bottom row is for a black hole with electric charge $Q/M=0.5$.

Figure 2

Radial profile of the effective potential of charged particles around static and electrically charged stringy black holes in the equatorial plane $\theta =\pi /2$. The left panel is for different values of the black hole charge. The right panel is for different values of the particle charge.

Figure 3

Left panel: ISCO radius as a function of the black hole electric charge $Q$ for different values of the particle electric charge $q$. Right panel: ISCO radius as a function of the particle electric charge $q$ for different values of the black hole electric charge $Q$.

Figure 4

Energy of an electrically charged particle in a circular orbit as a function of the orbital radius for different values of the black hole electric charge $Q$ (left panel) and of the particle electric charge $q$ (right panel).

Figure 5

Angular momentum of an electrically charged particle in a circular orbit as a function of the orbital radius for different values of the black hole electric charge $Q$ (left panel) and of the particle electric charge $q$ (right panel).

Figure 6

Relation between the electric charge $Q$ of a nonrotating stringy black hole and the rotation parameter $a$ of an electrically uncharged Kerr black hole when we require the same value of the ISCO radius (in units of $M$). This plot is obtained in the case of electrically neutral test particles and suggests that a nonrotating stringy black hole can mimic an electrically uncharged Kerr black hole.

Figure 7

Radial profile of the effective potential for massive magnetic monopoles orbiting a magnetically charged black hole. Left and central panels: $V_{\mathrm{eff}}$ versus $r/M$ for different values of $g$ in the case of $Q_m=1$. Right panel: $V_{\mathrm{eff}}$ versus $r/M$ for different values of $Q_m$ in the case of $g=0.1$.

Figure 8

Radial profile of the specific angular momentum for magnetic monopoles orbiting a magnetically charged black hole. Left panel: $L^2$ versus $r/M$ for different values of magnetic charge parameter $g$ for $Q_m=1$. Right panel: $L^2$ versus $r/M$ for different values of the black hole magnetic charge $Q_m$ for $g=0.1$.

Figure 9

Rotation parameter $a$ of a Kerr black hole versus magnetic charge $g$ of a test particle orbiting a stringy black hole with magnetic charge $Q_m$ leading to the same ISCO radius for different values of $Q_m$. However, the stringy scenario can mimic the standard scenario of a Kerr black hole only for a Kerr black hole spin parameter up to 0.8.

Figure 10

Radial profile of the specific angular momentum in circular orbits of a magnetic dipole for different values of the magnetic charge $Q_m$ and of the parameter $\beta$.

Figure 11

Radial profile of the specific energy in circular orbits of a magnetic dipole for different values of the magnetic charge $Q_m$ and of the parameter $\beta$.

Figure 12

ISCO radius of a magnetic dipole around magnetically charged stringy black hole for different values of the parameter $\beta$ and the black hole magnetic charge. The top panel shows the impact of the parameter $\beta$, and the bottom panel shows the impact of the black hole magnetic charge.

Figure 13

ISCO radius of a magnetic dipole orbiting magnetically charged stringy black holes, Kerr black holes, and Schwarzschild black holes immersed in external magnetic fields for different values of the parameter $\beta$.

Figure 14

Relations between the spin of a rotating Kerr black hole and the magnetic charge of a stringy black hole providing the same ISCO radius for different values of the parameter $\beta$.

Figure 15

Relation between the parameters of magnetic coupling and magnetic charge for the same ISCO radius for different values of the parameter $\beta$.